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diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl01.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl01.rst new file mode 100644 index 00000000000..8ee0d922386 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl01.rst @@ -0,0 +1,194 @@ +===================================================== +Kaleidoscope: Kaleidoscope Introduction and the Lexer +===================================================== + +.. contents:: + :local: + +The Kaleidoscope Language +========================= + +This tutorial will be illustrated with a toy language that we'll call +"`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived +from "meaning beautiful, form, and view"). Kaleidoscope is a procedural +language that allows you to define functions, use conditionals, math, +etc. Over the course of the tutorial, we'll extend Kaleidoscope to +support the if/then/else construct, a for loop, user defined operators, +JIT compilation with a simple command line interface, etc. + +Because we want to keep things simple, the only datatype in Kaleidoscope +is a 64-bit floating point type (aka 'double' in C parlance). As such, +all values are implicitly double precision and the language doesn't +require type declarations. This gives the language a very nice and +simple syntax. For example, the following simple example computes +`Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_ + +:: + + # Compute the x'th fibonacci number. + def fib(x) + if x < 3 then + 1 + else + fib(x-1)+fib(x-2) + + # This expression will compute the 40th number. + fib(40) + +We also allow Kaleidoscope to call into standard library functions (the +LLVM JIT makes this completely trivial). This means that you can use the +'extern' keyword to define a function before you use it (this is also +useful for mutually recursive functions). For example: + +:: + + extern sin(arg); + extern cos(arg); + extern atan2(arg1 arg2); + + atan2(sin(.4), cos(42)) + +A more interesting example is included in Chapter 6 where we write a +little Kaleidoscope application that `displays a Mandelbrot +Set <LangImpl06.html#kicking-the-tires>`_ at various levels of magnification. + +Lets dive into the implementation of this language! + +The Lexer +========= + +When it comes to implementing a language, the first thing needed is the +ability to process a text file and recognize what it says. The +traditional way to do this is to use a +"`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka +'scanner') to break the input up into "tokens". Each token returned by +the lexer includes a token code and potentially some metadata (e.g. the +numeric value of a number). First, we define the possibilities: + +.. code-block:: c++ + + // The lexer returns tokens [0-255] if it is an unknown character, otherwise one + // of these for known things. + enum Token { + tok_eof = -1, + + // commands + tok_def = -2, + tok_extern = -3, + + // primary + tok_identifier = -4, + tok_number = -5, + }; + + static std::string IdentifierStr; // Filled in if tok_identifier + static double NumVal; // Filled in if tok_number + +Each token returned by our lexer will either be one of the Token enum +values or it will be an 'unknown' character like '+', which is returned +as its ASCII value. If the current token is an identifier, the +``IdentifierStr`` global variable holds the name of the identifier. If +the current token is a numeric literal (like 1.0), ``NumVal`` holds its +value. Note that we use global variables for simplicity, this is not the +best choice for a real language implementation :). + +The actual implementation of the lexer is a single function named +``gettok``. The ``gettok`` function is called to return the next token +from standard input. Its definition starts as: + +.. code-block:: c++ + + /// gettok - Return the next token from standard input. + static int gettok() { + static int LastChar = ' '; + + // Skip any whitespace. + while (isspace(LastChar)) + LastChar = getchar(); + +``gettok`` works by calling the C ``getchar()`` function to read +characters one at a time from standard input. It eats them as it +recognizes them and stores the last character read, but not processed, +in LastChar. The first thing that it has to do is ignore whitespace +between tokens. This is accomplished with the loop above. + +The next thing ``gettok`` needs to do is recognize identifiers and +specific keywords like "def". Kaleidoscope does this with this simple +loop: + +.. code-block:: c++ + + if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]* + IdentifierStr = LastChar; + while (isalnum((LastChar = getchar()))) + IdentifierStr += LastChar; + + if (IdentifierStr == "def") + return tok_def; + if (IdentifierStr == "extern") + return tok_extern; + return tok_identifier; + } + +Note that this code sets the '``IdentifierStr``' global whenever it +lexes an identifier. Also, since language keywords are matched by the +same loop, we handle them here inline. Numeric values are similar: + +.. code-block:: c++ + + if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+ + std::string NumStr; + do { + NumStr += LastChar; + LastChar = getchar(); + } while (isdigit(LastChar) || LastChar == '.'); + + NumVal = strtod(NumStr.c_str(), 0); + return tok_number; + } + +This is all pretty straight-forward code for processing input. When +reading a numeric value from input, we use the C ``strtod`` function to +convert it to a numeric value that we store in ``NumVal``. Note that +this isn't doing sufficient error checking: it will incorrectly read +"1.23.45.67" and handle it as if you typed in "1.23". Feel free to +extend it :). Next we handle comments: + +.. code-block:: c++ + + if (LastChar == '#') { + // Comment until end of line. + do + LastChar = getchar(); + while (LastChar != EOF && LastChar != '\n' && LastChar != '\r'); + + if (LastChar != EOF) + return gettok(); + } + +We handle comments by skipping to the end of the line and then return +the next token. Finally, if the input doesn't match one of the above +cases, it is either an operator character like '+' or the end of the +file. These are handled with this code: + +.. code-block:: c++ + + // Check for end of file. Don't eat the EOF. + if (LastChar == EOF) + return tok_eof; + + // Otherwise, just return the character as its ascii value. + int ThisChar = LastChar; + LastChar = getchar(); + return ThisChar; + } + +With this, we have the complete lexer for the basic Kaleidoscope +language (the `full code listing <LangImpl02.html#full-code-listing>`_ for the Lexer +is available in the `next chapter <LangImpl02.html>`_ of the tutorial). +Next we'll `build a simple parser that uses this to build an Abstract +Syntax Tree <LangImpl02.html>`_. When we have that, we'll include a +driver so that you can use the lexer and parser together. + +`Next: Implementing a Parser and AST <LangImpl02.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl02.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl02.rst new file mode 100644 index 00000000000..6982e969c8a --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl02.rst @@ -0,0 +1,737 @@ +=========================================== +Kaleidoscope: Implementing a Parser and AST +=========================================== + +.. contents:: + :local: + +Chapter 2 Introduction +====================== + +Welcome to Chapter 2 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. This chapter shows you how to use the +lexer, built in `Chapter 1 <LangImpl01.html>`_, to build a full +`parser <http://en.wikipedia.org/wiki/Parsing>`_ for our Kaleidoscope +language. Once we have a parser, we'll define and build an `Abstract +Syntax Tree <http://en.wikipedia.org/wiki/Abstract_syntax_tree>`_ (AST). + +The parser we will build uses a combination of `Recursive Descent +Parsing <http://en.wikipedia.org/wiki/Recursive_descent_parser>`_ and +`Operator-Precedence +Parsing <http://en.wikipedia.org/wiki/Operator-precedence_parser>`_ to +parse the Kaleidoscope language (the latter for binary expressions and +the former for everything else). Before we get to parsing though, let's +talk about the output of the parser: the Abstract Syntax Tree. + +The Abstract Syntax Tree (AST) +============================== + +The AST for a program captures its behavior in such a way that it is +easy for later stages of the compiler (e.g. code generation) to +interpret. We basically want one object for each construct in the +language, and the AST should closely model the language. In +Kaleidoscope, we have expressions, a prototype, and a function object. +We'll start with expressions first: + +.. code-block:: c++ + + /// ExprAST - Base class for all expression nodes. + class ExprAST { + public: + virtual ~ExprAST() {} + }; + + /// NumberExprAST - Expression class for numeric literals like "1.0". + class NumberExprAST : public ExprAST { + double Val; + + public: + NumberExprAST(double Val) : Val(Val) {} + }; + +The code above shows the definition of the base ExprAST class and one +subclass which we use for numeric literals. The important thing to note +about this code is that the NumberExprAST class captures the numeric +value of the literal as an instance variable. This allows later phases +of the compiler to know what the stored numeric value is. + +Right now we only create the AST, so there are no useful accessor +methods on them. It would be very easy to add a virtual method to pretty +print the code, for example. Here are the other expression AST node +definitions that we'll use in the basic form of the Kaleidoscope +language: + +.. code-block:: c++ + + /// VariableExprAST - Expression class for referencing a variable, like "a". + class VariableExprAST : public ExprAST { + std::string Name; + + public: + VariableExprAST(const std::string &Name) : Name(Name) {} + }; + + /// BinaryExprAST - Expression class for a binary operator. + class BinaryExprAST : public ExprAST { + char Op; + std::unique_ptr<ExprAST> LHS, RHS; + + public: + BinaryExprAST(char op, std::unique_ptr<ExprAST> LHS, + std::unique_ptr<ExprAST> RHS) + : Op(op), LHS(std::move(LHS)), RHS(std::move(RHS)) {} + }; + + /// CallExprAST - Expression class for function calls. + class CallExprAST : public ExprAST { + std::string Callee; + std::vector<std::unique_ptr<ExprAST>> Args; + + public: + CallExprAST(const std::string &Callee, + std::vector<std::unique_ptr<ExprAST>> Args) + : Callee(Callee), Args(std::move(Args)) {} + }; + +This is all (intentionally) rather straight-forward: variables capture +the variable name, binary operators capture their opcode (e.g. '+'), and +calls capture a function name as well as a list of any argument +expressions. One thing that is nice about our AST is that it captures +the language features without talking about the syntax of the language. +Note that there is no discussion about precedence of binary operators, +lexical structure, etc. + +For our basic language, these are all of the expression nodes we'll +define. Because it doesn't have conditional control flow, it isn't +Turing-complete; we'll fix that in a later installment. The two things +we need next are a way to talk about the interface to a function, and a +way to talk about functions themselves: + +.. code-block:: c++ + + /// PrototypeAST - This class represents the "prototype" for a function, + /// which captures its name, and its argument names (thus implicitly the number + /// of arguments the function takes). + class PrototypeAST { + std::string Name; + std::vector<std::string> Args; + + public: + PrototypeAST(const std::string &name, std::vector<std::string> Args) + : Name(name), Args(std::move(Args)) {} + + const std::string &getName() const { return Name; } + }; + + /// FunctionAST - This class represents a function definition itself. + class FunctionAST { + std::unique_ptr<PrototypeAST> Proto; + std::unique_ptr<ExprAST> Body; + + public: + FunctionAST(std::unique_ptr<PrototypeAST> Proto, + std::unique_ptr<ExprAST> Body) + : Proto(std::move(Proto)), Body(std::move(Body)) {} + }; + +In Kaleidoscope, functions are typed with just a count of their +arguments. Since all values are double precision floating point, the +type of each argument doesn't need to be stored anywhere. In a more +aggressive and realistic language, the "ExprAST" class would probably +have a type field. + +With this scaffolding, we can now talk about parsing expressions and +function bodies in Kaleidoscope. + +Parser Basics +============= + +Now that we have an AST to build, we need to define the parser code to +build it. The idea here is that we want to parse something like "x+y" +(which is returned as three tokens by the lexer) into an AST that could +be generated with calls like this: + +.. code-block:: c++ + + auto LHS = llvm::make_unique<VariableExprAST>("x"); + auto RHS = llvm::make_unique<VariableExprAST>("y"); + auto Result = std::make_unique<BinaryExprAST>('+', std::move(LHS), + std::move(RHS)); + +In order to do this, we'll start by defining some basic helper routines: + +.. code-block:: c++ + + /// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current + /// token the parser is looking at. getNextToken reads another token from the + /// lexer and updates CurTok with its results. + static int CurTok; + static int getNextToken() { + return CurTok = gettok(); + } + +This implements a simple token buffer around the lexer. This allows us +to look one token ahead at what the lexer is returning. Every function +in our parser will assume that CurTok is the current token that needs to +be parsed. + +.. code-block:: c++ + + + /// LogError* - These are little helper functions for error handling. + std::unique_ptr<ExprAST> LogError(const char *Str) { + fprintf(stderr, "LogError: %s\n", Str); + return nullptr; + } + std::unique_ptr<PrototypeAST> LogErrorP(const char *Str) { + LogError(Str); + return nullptr; + } + +The ``LogError`` routines are simple helper routines that our parser will +use to handle errors. The error recovery in our parser will not be the +best and is not particular user-friendly, but it will be enough for our +tutorial. These routines make it easier to handle errors in routines +that have various return types: they always return null. + +With these basic helper functions, we can implement the first piece of +our grammar: numeric literals. + +Basic Expression Parsing +======================== + +We start with numeric literals, because they are the simplest to +process. For each production in our grammar, we'll define a function +which parses that production. For numeric literals, we have: + +.. code-block:: c++ + + /// numberexpr ::= number + static std::unique_ptr<ExprAST> ParseNumberExpr() { + auto Result = llvm::make_unique<NumberExprAST>(NumVal); + getNextToken(); // consume the number + return std::move(Result); + } + +This routine is very simple: it expects to be called when the current +token is a ``tok_number`` token. It takes the current number value, +creates a ``NumberExprAST`` node, advances the lexer to the next token, +and finally returns. + +There are some interesting aspects to this. The most important one is +that this routine eats all of the tokens that correspond to the +production and returns the lexer buffer with the next token (which is +not part of the grammar production) ready to go. This is a fairly +standard way to go for recursive descent parsers. For a better example, +the parenthesis operator is defined like this: + +.. code-block:: c++ + + /// parenexpr ::= '(' expression ')' + static std::unique_ptr<ExprAST> ParseParenExpr() { + getNextToken(); // eat (. + auto V = ParseExpression(); + if (!V) + return nullptr; + + if (CurTok != ')') + return LogError("expected ')'"); + getNextToken(); // eat ). + return V; + } + +This function illustrates a number of interesting things about the +parser: + +1) It shows how we use the LogError routines. When called, this function +expects that the current token is a '(' token, but after parsing the +subexpression, it is possible that there is no ')' waiting. For example, +if the user types in "(4 x" instead of "(4)", the parser should emit an +error. Because errors can occur, the parser needs a way to indicate that +they happened: in our parser, we return null on an error. + +2) Another interesting aspect of this function is that it uses recursion +by calling ``ParseExpression`` (we will soon see that +``ParseExpression`` can call ``ParseParenExpr``). This is powerful +because it allows us to handle recursive grammars, and keeps each +production very simple. Note that parentheses do not cause construction +of AST nodes themselves. While we could do it this way, the most +important role of parentheses are to guide the parser and provide +grouping. Once the parser constructs the AST, parentheses are not +needed. + +The next simple production is for handling variable references and +function calls: + +.. code-block:: c++ + + /// identifierexpr + /// ::= identifier + /// ::= identifier '(' expression* ')' + static std::unique_ptr<ExprAST> ParseIdentifierExpr() { + std::string IdName = IdentifierStr; + + getNextToken(); // eat identifier. + + if (CurTok != '(') // Simple variable ref. + return llvm::make_unique<VariableExprAST>(IdName); + + // Call. + getNextToken(); // eat ( + std::vector<std::unique_ptr<ExprAST>> Args; + if (CurTok != ')') { + while (1) { + if (auto Arg = ParseExpression()) + Args.push_back(std::move(Arg)); + else + return nullptr; + + if (CurTok == ')') + break; + + if (CurTok != ',') + return LogError("Expected ')' or ',' in argument list"); + getNextToken(); + } + } + + // Eat the ')'. + getNextToken(); + + return llvm::make_unique<CallExprAST>(IdName, std::move(Args)); + } + +This routine follows the same style as the other routines. (It expects +to be called if the current token is a ``tok_identifier`` token). It +also has recursion and error handling. One interesting aspect of this is +that it uses *look-ahead* to determine if the current identifier is a +stand alone variable reference or if it is a function call expression. +It handles this by checking to see if the token after the identifier is +a '(' token, constructing either a ``VariableExprAST`` or +``CallExprAST`` node as appropriate. + +Now that we have all of our simple expression-parsing logic in place, we +can define a helper function to wrap it together into one entry point. +We call this class of expressions "primary" expressions, for reasons +that will become more clear `later in the +tutorial <LangImpl6.html#user-defined-unary-operators>`_. In order to parse an arbitrary +primary expression, we need to determine what sort of expression it is: + +.. code-block:: c++ + + /// primary + /// ::= identifierexpr + /// ::= numberexpr + /// ::= parenexpr + static std::unique_ptr<ExprAST> ParsePrimary() { + switch (CurTok) { + default: + return LogError("unknown token when expecting an expression"); + case tok_identifier: + return ParseIdentifierExpr(); + case tok_number: + return ParseNumberExpr(); + case '(': + return ParseParenExpr(); + } + } + +Now that you see the definition of this function, it is more obvious why +we can assume the state of CurTok in the various functions. This uses +look-ahead to determine which sort of expression is being inspected, and +then parses it with a function call. + +Now that basic expressions are handled, we need to handle binary +expressions. They are a bit more complex. + +Binary Expression Parsing +========================= + +Binary expressions are significantly harder to parse because they are +often ambiguous. For example, when given the string "x+y\*z", the parser +can choose to parse it as either "(x+y)\*z" or "x+(y\*z)". With common +definitions from mathematics, we expect the later parse, because "\*" +(multiplication) has higher *precedence* than "+" (addition). + +There are many ways to handle this, but an elegant and efficient way is +to use `Operator-Precedence +Parsing <http://en.wikipedia.org/wiki/Operator-precedence_parser>`_. +This parsing technique uses the precedence of binary operators to guide +recursion. To start with, we need a table of precedences: + +.. code-block:: c++ + + /// BinopPrecedence - This holds the precedence for each binary operator that is + /// defined. + static std::map<char, int> BinopPrecedence; + + /// GetTokPrecedence - Get the precedence of the pending binary operator token. + static int GetTokPrecedence() { + if (!isascii(CurTok)) + return -1; + + // Make sure it's a declared binop. + int TokPrec = BinopPrecedence[CurTok]; + if (TokPrec <= 0) return -1; + return TokPrec; + } + + int main() { + // Install standard binary operators. + // 1 is lowest precedence. + BinopPrecedence['<'] = 10; + BinopPrecedence['+'] = 20; + BinopPrecedence['-'] = 20; + BinopPrecedence['*'] = 40; // highest. + ... + } + +For the basic form of Kaleidoscope, we will only support 4 binary +operators (this can obviously be extended by you, our brave and intrepid +reader). The ``GetTokPrecedence`` function returns the precedence for +the current token, or -1 if the token is not a binary operator. Having a +map makes it easy to add new operators and makes it clear that the +algorithm doesn't depend on the specific operators involved, but it +would be easy enough to eliminate the map and do the comparisons in the +``GetTokPrecedence`` function. (Or just use a fixed-size array). + +With the helper above defined, we can now start parsing binary +expressions. The basic idea of operator precedence parsing is to break +down an expression with potentially ambiguous binary operators into +pieces. Consider, for example, the expression "a+b+(c+d)\*e\*f+g". +Operator precedence parsing considers this as a stream of primary +expressions separated by binary operators. As such, it will first parse +the leading primary expression "a", then it will see the pairs [+, b] +[+, (c+d)] [\*, e] [\*, f] and [+, g]. Note that because parentheses are +primary expressions, the binary expression parser doesn't need to worry +about nested subexpressions like (c+d) at all. + +To start, an expression is a primary expression potentially followed by +a sequence of [binop,primaryexpr] pairs: + +.. code-block:: c++ + + /// expression + /// ::= primary binoprhs + /// + static std::unique_ptr<ExprAST> ParseExpression() { + auto LHS = ParsePrimary(); + if (!LHS) + return nullptr; + + return ParseBinOpRHS(0, std::move(LHS)); + } + +``ParseBinOpRHS`` is the function that parses the sequence of pairs for +us. It takes a precedence and a pointer to an expression for the part +that has been parsed so far. Note that "x" is a perfectly valid +expression: As such, "binoprhs" is allowed to be empty, in which case it +returns the expression that is passed into it. In our example above, the +code passes the expression for "a" into ``ParseBinOpRHS`` and the +current token is "+". + +The precedence value passed into ``ParseBinOpRHS`` indicates the +*minimal operator precedence* that the function is allowed to eat. For +example, if the current pair stream is [+, x] and ``ParseBinOpRHS`` is +passed in a precedence of 40, it will not consume any tokens (because +the precedence of '+' is only 20). With this in mind, ``ParseBinOpRHS`` +starts with: + +.. code-block:: c++ + + /// binoprhs + /// ::= ('+' primary)* + static std::unique_ptr<ExprAST> ParseBinOpRHS(int ExprPrec, + std::unique_ptr<ExprAST> LHS) { + // If this is a binop, find its precedence. + while (1) { + int TokPrec = GetTokPrecedence(); + + // If this is a binop that binds at least as tightly as the current binop, + // consume it, otherwise we are done. + if (TokPrec < ExprPrec) + return LHS; + +This code gets the precedence of the current token and checks to see if +if is too low. Because we defined invalid tokens to have a precedence of +-1, this check implicitly knows that the pair-stream ends when the token +stream runs out of binary operators. If this check succeeds, we know +that the token is a binary operator and that it will be included in this +expression: + +.. code-block:: c++ + + // Okay, we know this is a binop. + int BinOp = CurTok; + getNextToken(); // eat binop + + // Parse the primary expression after the binary operator. + auto RHS = ParsePrimary(); + if (!RHS) + return nullptr; + +As such, this code eats (and remembers) the binary operator and then +parses the primary expression that follows. This builds up the whole +pair, the first of which is [+, b] for the running example. + +Now that we parsed the left-hand side of an expression and one pair of +the RHS sequence, we have to decide which way the expression associates. +In particular, we could have "(a+b) binop unparsed" or "a + (b binop +unparsed)". To determine this, we look ahead at "binop" to determine its +precedence and compare it to BinOp's precedence (which is '+' in this +case): + +.. code-block:: c++ + + // If BinOp binds less tightly with RHS than the operator after RHS, let + // the pending operator take RHS as its LHS. + int NextPrec = GetTokPrecedence(); + if (TokPrec < NextPrec) { + +If the precedence of the binop to the right of "RHS" is lower or equal +to the precedence of our current operator, then we know that the +parentheses associate as "(a+b) binop ...". In our example, the current +operator is "+" and the next operator is "+", we know that they have the +same precedence. In this case we'll create the AST node for "a+b", and +then continue parsing: + +.. code-block:: c++ + + ... if body omitted ... + } + + // Merge LHS/RHS. + LHS = llvm::make_unique<BinaryExprAST>(BinOp, std::move(LHS), + std::move(RHS)); + } // loop around to the top of the while loop. + } + +In our example above, this will turn "a+b+" into "(a+b)" and execute the +next iteration of the loop, with "+" as the current token. The code +above will eat, remember, and parse "(c+d)" as the primary expression, +which makes the current pair equal to [+, (c+d)]. It will then evaluate +the 'if' conditional above with "\*" as the binop to the right of the +primary. In this case, the precedence of "\*" is higher than the +precedence of "+" so the if condition will be entered. + +The critical question left here is "how can the if condition parse the +right hand side in full"? In particular, to build the AST correctly for +our example, it needs to get all of "(c+d)\*e\*f" as the RHS expression +variable. The code to do this is surprisingly simple (code from the +above two blocks duplicated for context): + +.. code-block:: c++ + + // If BinOp binds less tightly with RHS than the operator after RHS, let + // the pending operator take RHS as its LHS. + int NextPrec = GetTokPrecedence(); + if (TokPrec < NextPrec) { + RHS = ParseBinOpRHS(TokPrec+1, std::move(RHS)); + if (!RHS) + return nullptr; + } + // Merge LHS/RHS. + LHS = llvm::make_unique<BinaryExprAST>(BinOp, std::move(LHS), + std::move(RHS)); + } // loop around to the top of the while loop. + } + +At this point, we know that the binary operator to the RHS of our +primary has higher precedence than the binop we are currently parsing. +As such, we know that any sequence of pairs whose operators are all +higher precedence than "+" should be parsed together and returned as +"RHS". To do this, we recursively invoke the ``ParseBinOpRHS`` function +specifying "TokPrec+1" as the minimum precedence required for it to +continue. In our example above, this will cause it to return the AST +node for "(c+d)\*e\*f" as RHS, which is then set as the RHS of the '+' +expression. + +Finally, on the next iteration of the while loop, the "+g" piece is +parsed and added to the AST. With this little bit of code (14 +non-trivial lines), we correctly handle fully general binary expression +parsing in a very elegant way. This was a whirlwind tour of this code, +and it is somewhat subtle. I recommend running through it with a few +tough examples to see how it works. + +This wraps up handling of expressions. At this point, we can point the +parser at an arbitrary token stream and build an expression from it, +stopping at the first token that is not part of the expression. Next up +we need to handle function definitions, etc. + +Parsing the Rest +================ + +The next thing missing is handling of function prototypes. In +Kaleidoscope, these are used both for 'extern' function declarations as +well as function body definitions. The code to do this is +straight-forward and not very interesting (once you've survived +expressions): + +.. code-block:: c++ + + /// prototype + /// ::= id '(' id* ')' + static std::unique_ptr<PrototypeAST> ParsePrototype() { + if (CurTok != tok_identifier) + return LogErrorP("Expected function name in prototype"); + + std::string FnName = IdentifierStr; + getNextToken(); + + if (CurTok != '(') + return LogErrorP("Expected '(' in prototype"); + + // Read the list of argument names. + std::vector<std::string> ArgNames; + while (getNextToken() == tok_identifier) + ArgNames.push_back(IdentifierStr); + if (CurTok != ')') + return LogErrorP("Expected ')' in prototype"); + + // success. + getNextToken(); // eat ')'. + + return llvm::make_unique<PrototypeAST>(FnName, std::move(ArgNames)); + } + +Given this, a function definition is very simple, just a prototype plus +an expression to implement the body: + +.. code-block:: c++ + + /// definition ::= 'def' prototype expression + static std::unique_ptr<FunctionAST> ParseDefinition() { + getNextToken(); // eat def. + auto Proto = ParsePrototype(); + if (!Proto) return nullptr; + + if (auto E = ParseExpression()) + return llvm::make_unique<FunctionAST>(std::move(Proto), std::move(E)); + return nullptr; + } + +In addition, we support 'extern' to declare functions like 'sin' and +'cos' as well as to support forward declaration of user functions. These +'extern's are just prototypes with no body: + +.. code-block:: c++ + + /// external ::= 'extern' prototype + static std::unique_ptr<PrototypeAST> ParseExtern() { + getNextToken(); // eat extern. + return ParsePrototype(); + } + +Finally, we'll also let the user type in arbitrary top-level expressions +and evaluate them on the fly. We will handle this by defining anonymous +nullary (zero argument) functions for them: + +.. code-block:: c++ + + /// toplevelexpr ::= expression + static std::unique_ptr<FunctionAST> ParseTopLevelExpr() { + if (auto E = ParseExpression()) { + // Make an anonymous proto. + auto Proto = llvm::make_unique<PrototypeAST>("", std::vector<std::string>()); + return llvm::make_unique<FunctionAST>(std::move(Proto), std::move(E)); + } + return nullptr; + } + +Now that we have all the pieces, let's build a little driver that will +let us actually *execute* this code we've built! + +The Driver +========== + +The driver for this simply invokes all of the parsing pieces with a +top-level dispatch loop. There isn't much interesting here, so I'll just +include the top-level loop. See `below <#full-code-listing>`_ for full code in the +"Top-Level Parsing" section. + +.. code-block:: c++ + + /// top ::= definition | external | expression | ';' + static void MainLoop() { + while (1) { + fprintf(stderr, "ready> "); + switch (CurTok) { + case tok_eof: + return; + case ';': // ignore top-level semicolons. + getNextToken(); + break; + case tok_def: + HandleDefinition(); + break; + case tok_extern: + HandleExtern(); + break; + default: + HandleTopLevelExpression(); + break; + } + } + } + +The most interesting part of this is that we ignore top-level +semicolons. Why is this, you ask? The basic reason is that if you type +"4 + 5" at the command line, the parser doesn't know whether that is the +end of what you will type or not. For example, on the next line you +could type "def foo..." in which case 4+5 is the end of a top-level +expression. Alternatively you could type "\* 6", which would continue +the expression. Having top-level semicolons allows you to type "4+5;", +and the parser will know you are done. + +Conclusions +=========== + +With just under 400 lines of commented code (240 lines of non-comment, +non-blank code), we fully defined our minimal language, including a +lexer, parser, and AST builder. With this done, the executable will +validate Kaleidoscope code and tell us if it is grammatically invalid. +For example, here is a sample interaction: + +.. code-block:: bash + + $ ./a.out + ready> def foo(x y) x+foo(y, 4.0); + Parsed a function definition. + ready> def foo(x y) x+y y; + Parsed a function definition. + Parsed a top-level expr + ready> def foo(x y) x+y ); + Parsed a function definition. + Error: unknown token when expecting an expression + ready> extern sin(a); + ready> Parsed an extern + ready> ^D + $ + +There is a lot of room for extension here. You can define new AST nodes, +extend the language in many ways, etc. In the `next +installment <LangImpl03.html>`_, we will describe how to generate LLVM +Intermediate Representation (IR) from the AST. + +Full Code Listing +================= + +Here is the complete code listing for our running example. Because this +uses the LLVM libraries, we need to link them in. To do this, we use the +`llvm-config <http://llvm.org/cmds/llvm-config.html>`_ tool to inform +our makefile/command line about which options to use: + +.. code-block:: bash + + # Compile + clang++ -g -O3 toy.cpp `llvm-config --cxxflags` + # Run + ./a.out + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter2/toy.cpp + :language: c++ + +`Next: Implementing Code Generation to LLVM IR <LangImpl03.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl03.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl03.rst new file mode 100644 index 00000000000..da465ef7061 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl03.rst @@ -0,0 +1,568 @@ +======================================== +Kaleidoscope: Code generation to LLVM IR +======================================== + +.. contents:: + :local: + +Chapter 3 Introduction +====================== + +Welcome to Chapter 3 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. This chapter shows you how to transform +the `Abstract Syntax Tree <LangImpl02.html>`_, built in Chapter 2, into +LLVM IR. This will teach you a little bit about how LLVM does things, as +well as demonstrate how easy it is to use. It's much more work to build +a lexer and parser than it is to generate LLVM IR code. :) + +**Please note**: the code in this chapter and later require LLVM 3.7 or +later. LLVM 3.6 and before will not work with it. Also note that you +need to use a version of this tutorial that matches your LLVM release: +If you are using an official LLVM release, use the version of the +documentation included with your release or on the `llvm.org releases +page <http://llvm.org/releases/>`_. + +Code Generation Setup +===================== + +In order to generate LLVM IR, we want some simple setup to get started. +First we define virtual code generation (codegen) methods in each AST +class: + +.. code-block:: c++ + + /// ExprAST - Base class for all expression nodes. + class ExprAST { + public: + virtual ~ExprAST() {} + virtual Value *codegen() = 0; + }; + + /// NumberExprAST - Expression class for numeric literals like "1.0". + class NumberExprAST : public ExprAST { + double Val; + + public: + NumberExprAST(double Val) : Val(Val) {} + virtual Value *codegen(); + }; + ... + +The codegen() method says to emit IR for that AST node along with all +the things it depends on, and they all return an LLVM Value object. +"Value" is the class used to represent a "`Static Single Assignment +(SSA) <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_ +register" or "SSA value" in LLVM. The most distinct aspect of SSA values +is that their value is computed as the related instruction executes, and +it does not get a new value until (and if) the instruction re-executes. +In other words, there is no way to "change" an SSA value. For more +information, please read up on `Static Single +Assignment <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_ +- the concepts are really quite natural once you grok them. + +Note that instead of adding virtual methods to the ExprAST class +hierarchy, it could also make sense to use a `visitor +pattern <http://en.wikipedia.org/wiki/Visitor_pattern>`_ or some other +way to model this. Again, this tutorial won't dwell on good software +engineering practices: for our purposes, adding a virtual method is +simplest. + +The second thing we want is an "LogError" method like we used for the +parser, which will be used to report errors found during code generation +(for example, use of an undeclared parameter): + +.. code-block:: c++ + + static LLVMContext TheContext; + static IRBuilder<> Builder(TheContext); + static std::unique_ptr<Module> TheModule; + static std::map<std::string, Value *> NamedValues; + + Value *LogErrorV(const char *Str) { + LogError(Str); + return nullptr; + } + +The static variables will be used during code generation. ``TheContext`` +is an opaque object that owns a lot of core LLVM data structures, such as +the type and constant value tables. We don't need to understand it in +detail, we just need a single instance to pass into APIs that require it. + +The ``Builder`` object is a helper object that makes it easy to generate +LLVM instructions. Instances of the +`IRBuilder <http://llvm.org/doxygen/IRBuilder_8h-source.html>`_ +class template keep track of the current place to insert instructions +and has methods to create new instructions. + +``TheModule`` is an LLVM construct that contains functions and global +variables. In many ways, it is the top-level structure that the LLVM IR +uses to contain code. It will own the memory for all of the IR that we +generate, which is why the codegen() method returns a raw Value\*, +rather than a unique_ptr<Value>. + +The ``NamedValues`` map keeps track of which values are defined in the +current scope and what their LLVM representation is. (In other words, it +is a symbol table for the code). In this form of Kaleidoscope, the only +things that can be referenced are function parameters. As such, function +parameters will be in this map when generating code for their function +body. + +With these basics in place, we can start talking about how to generate +code for each expression. Note that this assumes that the ``Builder`` +has been set up to generate code *into* something. For now, we'll assume +that this has already been done, and we'll just use it to emit code. + +Expression Code Generation +========================== + +Generating LLVM code for expression nodes is very straightforward: less +than 45 lines of commented code for all four of our expression nodes. +First we'll do numeric literals: + +.. code-block:: c++ + + Value *NumberExprAST::codegen() { + return ConstantFP::get(TheContext, APFloat(Val)); + } + +In the LLVM IR, numeric constants are represented with the +``ConstantFP`` class, which holds the numeric value in an ``APFloat`` +internally (``APFloat`` has the capability of holding floating point +constants of Arbitrary Precision). This code basically just creates +and returns a ``ConstantFP``. Note that in the LLVM IR that constants +are all uniqued together and shared. For this reason, the API uses the +"foo::get(...)" idiom instead of "new foo(..)" or "foo::Create(..)". + +.. code-block:: c++ + + Value *VariableExprAST::codegen() { + // Look this variable up in the function. + Value *V = NamedValues[Name]; + if (!V) + LogErrorV("Unknown variable name"); + return V; + } + +References to variables are also quite simple using LLVM. In the simple +version of Kaleidoscope, we assume that the variable has already been +emitted somewhere and its value is available. In practice, the only +values that can be in the ``NamedValues`` map are function arguments. +This code simply checks to see that the specified name is in the map (if +not, an unknown variable is being referenced) and returns the value for +it. In future chapters, we'll add support for `loop induction +variables <LangImpl5.html#for-loop-expression>`_ in the symbol table, and for `local +variables <LangImpl7.html#user-defined-local-variables>`_. + +.. code-block:: c++ + + Value *BinaryExprAST::codegen() { + Value *L = LHS->codegen(); + Value *R = RHS->codegen(); + if (!L || !R) + return nullptr; + + switch (Op) { + case '+': + return Builder.CreateFAdd(L, R, "addtmp"); + case '-': + return Builder.CreateFSub(L, R, "subtmp"); + case '*': + return Builder.CreateFMul(L, R, "multmp"); + case '<': + L = Builder.CreateFCmpULT(L, R, "cmptmp"); + // Convert bool 0/1 to double 0.0 or 1.0 + return Builder.CreateUIToFP(L, Type::getDoubleTy(TheContext), + "booltmp"); + default: + return LogErrorV("invalid binary operator"); + } + } + +Binary operators start to get more interesting. The basic idea here is +that we recursively emit code for the left-hand side of the expression, +then the right-hand side, then we compute the result of the binary +expression. In this code, we do a simple switch on the opcode to create +the right LLVM instruction. + +In the example above, the LLVM builder class is starting to show its +value. IRBuilder knows where to insert the newly created instruction, +all you have to do is specify what instruction to create (e.g. with +``CreateFAdd``), which operands to use (``L`` and ``R`` here) and +optionally provide a name for the generated instruction. + +One nice thing about LLVM is that the name is just a hint. For instance, +if the code above emits multiple "addtmp" variables, LLVM will +automatically provide each one with an increasing, unique numeric +suffix. Local value names for instructions are purely optional, but it +makes it much easier to read the IR dumps. + +`LLVM instructions <../LangRef.html#instruction-reference>`_ are constrained by strict +rules: for example, the Left and Right operators of an `add +instruction <../LangRef.html#add-instruction>`_ must have the same type, and the +result type of the add must match the operand types. Because all values +in Kaleidoscope are doubles, this makes for very simple code for add, +sub and mul. + +On the other hand, LLVM specifies that the `fcmp +instruction <../LangRef.html#fcmp-instruction>`_ always returns an 'i1' value (a +one bit integer). The problem with this is that Kaleidoscope wants the +value to be a 0.0 or 1.0 value. In order to get these semantics, we +combine the fcmp instruction with a `uitofp +instruction <../LangRef.html#uitofp-to-instruction>`_. This instruction converts its +input integer into a floating point value by treating the input as an +unsigned value. In contrast, if we used the `sitofp +instruction <../LangRef.html#sitofp-to-instruction>`_, the Kaleidoscope '<' operator +would return 0.0 and -1.0, depending on the input value. + +.. code-block:: c++ + + Value *CallExprAST::codegen() { + // Look up the name in the global module table. + Function *CalleeF = TheModule->getFunction(Callee); + if (!CalleeF) + return LogErrorV("Unknown function referenced"); + + // If argument mismatch error. + if (CalleeF->arg_size() != Args.size()) + return LogErrorV("Incorrect # arguments passed"); + + std::vector<Value *> ArgsV; + for (unsigned i = 0, e = Args.size(); i != e; ++i) { + ArgsV.push_back(Args[i]->codegen()); + if (!ArgsV.back()) + return nullptr; + } + + return Builder.CreateCall(CalleeF, ArgsV, "calltmp"); + } + +Code generation for function calls is quite straightforward with LLVM. The code +above initially does a function name lookup in the LLVM Module's symbol table. +Recall that the LLVM Module is the container that holds the functions we are +JIT'ing. By giving each function the same name as what the user specifies, we +can use the LLVM symbol table to resolve function names for us. + +Once we have the function to call, we recursively codegen each argument +that is to be passed in, and create an LLVM `call +instruction <../LangRef.html#call-instruction>`_. Note that LLVM uses the native C +calling conventions by default, allowing these calls to also call into +standard library functions like "sin" and "cos", with no additional +effort. + +This wraps up our handling of the four basic expressions that we have so +far in Kaleidoscope. Feel free to go in and add some more. For example, +by browsing the `LLVM language reference <../LangRef.html>`_ you'll find +several other interesting instructions that are really easy to plug into +our basic framework. + +Function Code Generation +======================== + +Code generation for prototypes and functions must handle a number of +details, which make their code less beautiful than expression code +generation, but allows us to illustrate some important points. First, +let's talk about code generation for prototypes: they are used both for +function bodies and external function declarations. The code starts +with: + +.. code-block:: c++ + + Function *PrototypeAST::codegen() { + // Make the function type: double(double,double) etc. + std::vector<Type*> Doubles(Args.size(), + Type::getDoubleTy(TheContext)); + FunctionType *FT = + FunctionType::get(Type::getDoubleTy(TheContext), Doubles, false); + + Function *F = + Function::Create(FT, Function::ExternalLinkage, Name, TheModule.get()); + +This code packs a lot of power into a few lines. Note first that this +function returns a "Function\*" instead of a "Value\*". Because a +"prototype" really talks about the external interface for a function +(not the value computed by an expression), it makes sense for it to +return the LLVM Function it corresponds to when codegen'd. + +The call to ``FunctionType::get`` creates the ``FunctionType`` that +should be used for a given Prototype. Since all function arguments in +Kaleidoscope are of type double, the first line creates a vector of "N" +LLVM double types. It then uses the ``Functiontype::get`` method to +create a function type that takes "N" doubles as arguments, returns one +double as a result, and that is not vararg (the false parameter +indicates this). Note that Types in LLVM are uniqued just like Constants +are, so you don't "new" a type, you "get" it. + +The final line above actually creates the IR Function corresponding to +the Prototype. This indicates the type, linkage and name to use, as +well as which module to insert into. "`external +linkage <../LangRef.html#linkage>`_" means that the function may be +defined outside the current module and/or that it is callable by +functions outside the module. The Name passed in is the name the user +specified: since "``TheModule``" is specified, this name is registered +in "``TheModule``"s symbol table. + +.. code-block:: c++ + + // Set names for all arguments. + unsigned Idx = 0; + for (auto &Arg : F->args()) + Arg.setName(Args[Idx++]); + + return F; + +Finally, we set the name of each of the function's arguments according to the +names given in the Prototype. This step isn't strictly necessary, but keeping +the names consistent makes the IR more readable, and allows subsequent code to +refer directly to the arguments for their names, rather than having to look up +them up in the Prototype AST. + +At this point we have a function prototype with no body. This is how LLVM IR +represents function declarations. For extern statements in Kaleidoscope, this +is as far as we need to go. For function definitions however, we need to +codegen and attach a function body. + +.. code-block:: c++ + + Function *FunctionAST::codegen() { + // First, check for an existing function from a previous 'extern' declaration. + Function *TheFunction = TheModule->getFunction(Proto->getName()); + + if (!TheFunction) + TheFunction = Proto->codegen(); + + if (!TheFunction) + return nullptr; + + if (!TheFunction->empty()) + return (Function*)LogErrorV("Function cannot be redefined."); + + +For function definitions, we start by searching TheModule's symbol table for an +existing version of this function, in case one has already been created using an +'extern' statement. If Module::getFunction returns null then no previous version +exists, so we'll codegen one from the Prototype. In either case, we want to +assert that the function is empty (i.e. has no body yet) before we start. + +.. code-block:: c++ + + // Create a new basic block to start insertion into. + BasicBlock *BB = BasicBlock::Create(TheContext, "entry", TheFunction); + Builder.SetInsertPoint(BB); + + // Record the function arguments in the NamedValues map. + NamedValues.clear(); + for (auto &Arg : TheFunction->args()) + NamedValues[Arg.getName()] = &Arg; + +Now we get to the point where the ``Builder`` is set up. The first line +creates a new `basic block <http://en.wikipedia.org/wiki/Basic_block>`_ +(named "entry"), which is inserted into ``TheFunction``. The second line +then tells the builder that new instructions should be inserted into the +end of the new basic block. Basic blocks in LLVM are an important part +of functions that define the `Control Flow +Graph <http://en.wikipedia.org/wiki/Control_flow_graph>`_. Since we +don't have any control flow, our functions will only contain one block +at this point. We'll fix this in `Chapter 5 <LangImpl05.html>`_ :). + +Next we add the function arguments to the NamedValues map (after first clearing +it out) so that they're accessible to ``VariableExprAST`` nodes. + +.. code-block:: c++ + + if (Value *RetVal = Body->codegen()) { + // Finish off the function. + Builder.CreateRet(RetVal); + + // Validate the generated code, checking for consistency. + verifyFunction(*TheFunction); + + return TheFunction; + } + +Once the insertion point has been set up and the NamedValues map populated, +we call the ``codegen()`` method for the root expression of the function. If no +error happens, this emits code to compute the expression into the entry block +and returns the value that was computed. Assuming no error, we then create an +LLVM `ret instruction <../LangRef.html#ret-instruction>`_, which completes the function. +Once the function is built, we call ``verifyFunction``, which is +provided by LLVM. This function does a variety of consistency checks on +the generated code, to determine if our compiler is doing everything +right. Using this is important: it can catch a lot of bugs. Once the +function is finished and validated, we return it. + +.. code-block:: c++ + + // Error reading body, remove function. + TheFunction->eraseFromParent(); + return nullptr; + } + +The only piece left here is handling of the error case. For simplicity, +we handle this by merely deleting the function we produced with the +``eraseFromParent`` method. This allows the user to redefine a function +that they incorrectly typed in before: if we didn't delete it, it would +live in the symbol table, with a body, preventing future redefinition. + +This code does have a bug, though: If the ``FunctionAST::codegen()`` method +finds an existing IR Function, it does not validate its signature against the +definition's own prototype. This means that an earlier 'extern' declaration will +take precedence over the function definition's signature, which can cause +codegen to fail, for instance if the function arguments are named differently. +There are a number of ways to fix this bug, see what you can come up with! Here +is a testcase: + +:: + + extern foo(a); # ok, defines foo. + def foo(b) b; # Error: Unknown variable name. (decl using 'a' takes precedence). + +Driver Changes and Closing Thoughts +=================================== + +For now, code generation to LLVM doesn't really get us much, except that +we can look at the pretty IR calls. The sample code inserts calls to +codegen into the "``HandleDefinition``", "``HandleExtern``" etc +functions, and then dumps out the LLVM IR. This gives a nice way to look +at the LLVM IR for simple functions. For example: + +:: + + ready> 4+5; + Read top-level expression: + define double @0() { + entry: + ret double 9.000000e+00 + } + +Note how the parser turns the top-level expression into anonymous +functions for us. This will be handy when we add `JIT +support <LangImpl4.html#adding-a-jit-compiler>`_ in the next chapter. Also note that the +code is very literally transcribed, no optimizations are being performed +except simple constant folding done by IRBuilder. We will `add +optimizations <LangImpl4.html#trivial-constant-folding>`_ explicitly in the next +chapter. + +:: + + ready> def foo(a b) a*a + 2*a*b + b*b; + Read function definition: + define double @foo(double %a, double %b) { + entry: + %multmp = fmul double %a, %a + %multmp1 = fmul double 2.000000e+00, %a + %multmp2 = fmul double %multmp1, %b + %addtmp = fadd double %multmp, %multmp2 + %multmp3 = fmul double %b, %b + %addtmp4 = fadd double %addtmp, %multmp3 + ret double %addtmp4 + } + +This shows some simple arithmetic. Notice the striking similarity to the +LLVM builder calls that we use to create the instructions. + +:: + + ready> def bar(a) foo(a, 4.0) + bar(31337); + Read function definition: + define double @bar(double %a) { + entry: + %calltmp = call double @foo(double %a, double 4.000000e+00) + %calltmp1 = call double @bar(double 3.133700e+04) + %addtmp = fadd double %calltmp, %calltmp1 + ret double %addtmp + } + +This shows some function calls. Note that this function will take a long +time to execute if you call it. In the future we'll add conditional +control flow to actually make recursion useful :). + +:: + + ready> extern cos(x); + Read extern: + declare double @cos(double) + + ready> cos(1.234); + Read top-level expression: + define double @1() { + entry: + %calltmp = call double @cos(double 1.234000e+00) + ret double %calltmp + } + +This shows an extern for the libm "cos" function, and a call to it. + +.. TODO:: Abandon Pygments' horrible `llvm` lexer. It just totally gives up + on highlighting this due to the first line. + +:: + + ready> ^D + ; ModuleID = 'my cool jit' + + define double @0() { + entry: + %addtmp = fadd double 4.000000e+00, 5.000000e+00 + ret double %addtmp + } + + define double @foo(double %a, double %b) { + entry: + %multmp = fmul double %a, %a + %multmp1 = fmul double 2.000000e+00, %a + %multmp2 = fmul double %multmp1, %b + %addtmp = fadd double %multmp, %multmp2 + %multmp3 = fmul double %b, %b + %addtmp4 = fadd double %addtmp, %multmp3 + ret double %addtmp4 + } + + define double @bar(double %a) { + entry: + %calltmp = call double @foo(double %a, double 4.000000e+00) + %calltmp1 = call double @bar(double 3.133700e+04) + %addtmp = fadd double %calltmp, %calltmp1 + ret double %addtmp + } + + declare double @cos(double) + + define double @1() { + entry: + %calltmp = call double @cos(double 1.234000e+00) + ret double %calltmp + } + +When you quit the current demo (by sending an EOF via CTRL+D on Linux +or CTRL+Z and ENTER on Windows), it dumps out the IR for the entire +module generated. Here you can see the big picture with all the +functions referencing each other. + +This wraps up the third chapter of the Kaleidoscope tutorial. Up next, +we'll describe how to `add JIT codegen and optimizer +support <LangImpl04.html>`_ to this so we can actually start running +code! + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +the LLVM code generator. Because this uses the LLVM libraries, we need +to link them in. To do this, we use the +`llvm-config <http://llvm.org/cmds/llvm-config.html>`_ tool to inform +our makefile/command line about which options to use: + +.. code-block:: bash + + # Compile + clang++ -g -O3 toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core` -o toy + # Run + ./toy + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter3/toy.cpp + :language: c++ + +`Next: Adding JIT and Optimizer Support <LangImpl04.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl04.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl04.rst new file mode 100644 index 00000000000..bdd21d6cd4a --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl04.rst @@ -0,0 +1,659 @@ +============================================== +Kaleidoscope: Adding JIT and Optimizer Support +============================================== + +.. contents:: + :local: + +Chapter 4 Introduction +====================== + +Welcome to Chapter 4 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. Chapters 1-3 described the implementation +of a simple language and added support for generating LLVM IR. This +chapter describes two new techniques: adding optimizer support to your +language, and adding JIT compiler support. These additions will +demonstrate how to get nice, efficient code for the Kaleidoscope +language. + +Trivial Constant Folding +======================== + +Our demonstration for Chapter 3 is elegant and easy to extend. +Unfortunately, it does not produce wonderful code. The IRBuilder, +however, does give us obvious optimizations when compiling simple code: + +:: + + ready> def test(x) 1+2+x; + Read function definition: + define double @test(double %x) { + entry: + %addtmp = fadd double 3.000000e+00, %x + ret double %addtmp + } + +This code is not a literal transcription of the AST built by parsing the +input. That would be: + +:: + + ready> def test(x) 1+2+x; + Read function definition: + define double @test(double %x) { + entry: + %addtmp = fadd double 2.000000e+00, 1.000000e+00 + %addtmp1 = fadd double %addtmp, %x + ret double %addtmp1 + } + +Constant folding, as seen above, in particular, is a very common and +very important optimization: so much so that many language implementors +implement constant folding support in their AST representation. + +With LLVM, you don't need this support in the AST. Since all calls to +build LLVM IR go through the LLVM IR builder, the builder itself checked +to see if there was a constant folding opportunity when you call it. If +so, it just does the constant fold and return the constant instead of +creating an instruction. + +Well, that was easy :). In practice, we recommend always using +``IRBuilder`` when generating code like this. It has no "syntactic +overhead" for its use (you don't have to uglify your compiler with +constant checks everywhere) and it can dramatically reduce the amount of +LLVM IR that is generated in some cases (particular for languages with a +macro preprocessor or that use a lot of constants). + +On the other hand, the ``IRBuilder`` is limited by the fact that it does +all of its analysis inline with the code as it is built. If you take a +slightly more complex example: + +:: + + ready> def test(x) (1+2+x)*(x+(1+2)); + ready> Read function definition: + define double @test(double %x) { + entry: + %addtmp = fadd double 3.000000e+00, %x + %addtmp1 = fadd double %x, 3.000000e+00 + %multmp = fmul double %addtmp, %addtmp1 + ret double %multmp + } + +In this case, the LHS and RHS of the multiplication are the same value. +We'd really like to see this generate "``tmp = x+3; result = tmp*tmp;``" +instead of computing "``x+3``" twice. + +Unfortunately, no amount of local analysis will be able to detect and +correct this. This requires two transformations: reassociation of +expressions (to make the add's lexically identical) and Common +Subexpression Elimination (CSE) to delete the redundant add instruction. +Fortunately, LLVM provides a broad range of optimizations that you can +use, in the form of "passes". + +LLVM Optimization Passes +======================== + +.. warning:: + + Due to the transition to the new PassManager infrastructure this tutorial + is based on ``llvm::legacy::FunctionPassManager`` which can be found in + `LegacyPassManager.h <http://llvm.org/doxygen/classllvm_1_1legacy_1_1FunctionPassManager.html>`_. + For the purpose of the this tutorial the above should be used until + the pass manager transition is complete. + +LLVM provides many optimization passes, which do many different sorts of +things and have different tradeoffs. Unlike other systems, LLVM doesn't +hold to the mistaken notion that one set of optimizations is right for +all languages and for all situations. LLVM allows a compiler implementor +to make complete decisions about what optimizations to use, in which +order, and in what situation. + +As a concrete example, LLVM supports both "whole module" passes, which +look across as large of body of code as they can (often a whole file, +but if run at link time, this can be a substantial portion of the whole +program). It also supports and includes "per-function" passes which just +operate on a single function at a time, without looking at other +functions. For more information on passes and how they are run, see the +`How to Write a Pass <../WritingAnLLVMPass.html>`_ document and the +`List of LLVM Passes <../Passes.html>`_. + +For Kaleidoscope, we are currently generating functions on the fly, one +at a time, as the user types them in. We aren't shooting for the +ultimate optimization experience in this setting, but we also want to +catch the easy and quick stuff where possible. As such, we will choose +to run a few per-function optimizations as the user types the function +in. If we wanted to make a "static Kaleidoscope compiler", we would use +exactly the code we have now, except that we would defer running the +optimizer until the entire file has been parsed. + +In order to get per-function optimizations going, we need to set up a +`FunctionPassManager <../WritingAnLLVMPass.html#what-passmanager-doesr>`_ to hold +and organize the LLVM optimizations that we want to run. Once we have +that, we can add a set of optimizations to run. We'll need a new +FunctionPassManager for each module that we want to optimize, so we'll +write a function to create and initialize both the module and pass manager +for us: + +.. code-block:: c++ + + void InitializeModuleAndPassManager(void) { + // Open a new module. + TheModule = llvm::make_unique<Module>("my cool jit", TheContext); + + // Create a new pass manager attached to it. + TheFPM = llvm::make_unique<FunctionPassManager>(TheModule.get()); + + // Do simple "peephole" optimizations and bit-twiddling optzns. + TheFPM->add(createInstructionCombiningPass()); + // Reassociate expressions. + TheFPM->add(createReassociatePass()); + // Eliminate Common SubExpressions. + TheFPM->add(createGVNPass()); + // Simplify the control flow graph (deleting unreachable blocks, etc). + TheFPM->add(createCFGSimplificationPass()); + + TheFPM->doInitialization(); + } + +This code initializes the global module ``TheModule``, and the function pass +manager ``TheFPM``, which is attached to ``TheModule``. Once the pass manager is +set up, we use a series of "add" calls to add a bunch of LLVM passes. + +In this case, we choose to add four optimization passes. +The passes we choose here are a pretty standard set +of "cleanup" optimizations that are useful for a wide variety of code. I won't +delve into what they do but, believe me, they are a good starting place :). + +Once the PassManager is set up, we need to make use of it. We do this by +running it after our newly created function is constructed (in +``FunctionAST::codegen()``), but before it is returned to the client: + +.. code-block:: c++ + + if (Value *RetVal = Body->codegen()) { + // Finish off the function. + Builder.CreateRet(RetVal); + + // Validate the generated code, checking for consistency. + verifyFunction(*TheFunction); + + // Optimize the function. + TheFPM->run(*TheFunction); + + return TheFunction; + } + +As you can see, this is pretty straightforward. The +``FunctionPassManager`` optimizes and updates the LLVM Function\* in +place, improving (hopefully) its body. With this in place, we can try +our test above again: + +:: + + ready> def test(x) (1+2+x)*(x+(1+2)); + ready> Read function definition: + define double @test(double %x) { + entry: + %addtmp = fadd double %x, 3.000000e+00 + %multmp = fmul double %addtmp, %addtmp + ret double %multmp + } + +As expected, we now get our nicely optimized code, saving a floating +point add instruction from every execution of this function. + +LLVM provides a wide variety of optimizations that can be used in +certain circumstances. Some `documentation about the various +passes <../Passes.html>`_ is available, but it isn't very complete. +Another good source of ideas can come from looking at the passes that +``Clang`` runs to get started. The "``opt``" tool allows you to +experiment with passes from the command line, so you can see if they do +anything. + +Now that we have reasonable code coming out of our front-end, let's talk +about executing it! + +Adding a JIT Compiler +===================== + +Code that is available in LLVM IR can have a wide variety of tools +applied to it. For example, you can run optimizations on it (as we did +above), you can dump it out in textual or binary forms, you can compile +the code to an assembly file (.s) for some target, or you can JIT +compile it. The nice thing about the LLVM IR representation is that it +is the "common currency" between many different parts of the compiler. + +In this section, we'll add JIT compiler support to our interpreter. The +basic idea that we want for Kaleidoscope is to have the user enter +function bodies as they do now, but immediately evaluate the top-level +expressions they type in. For example, if they type in "1 + 2;", we +should evaluate and print out 3. If they define a function, they should +be able to call it from the command line. + +In order to do this, we first prepare the environment to create code for +the current native target and declare and initialize the JIT. This is +done by calling some ``InitializeNativeTarget\*`` functions and +adding a global variable ``TheJIT``, and initializing it in +``main``: + +.. code-block:: c++ + + static std::unique_ptr<KaleidoscopeJIT> TheJIT; + ... + int main() { + InitializeNativeTarget(); + InitializeNativeTargetAsmPrinter(); + InitializeNativeTargetAsmParser(); + + // Install standard binary operators. + // 1 is lowest precedence. + BinopPrecedence['<'] = 10; + BinopPrecedence['+'] = 20; + BinopPrecedence['-'] = 20; + BinopPrecedence['*'] = 40; // highest. + + // Prime the first token. + fprintf(stderr, "ready> "); + getNextToken(); + + TheJIT = llvm::make_unique<KaleidoscopeJIT>(); + + // Run the main "interpreter loop" now. + MainLoop(); + + return 0; + } + +We also need to setup the data layout for the JIT: + +.. code-block:: c++ + + void InitializeModuleAndPassManager(void) { + // Open a new module. + TheModule = llvm::make_unique<Module>("my cool jit", TheContext); + TheModule->setDataLayout(TheJIT->getTargetMachine().createDataLayout()); + + // Create a new pass manager attached to it. + TheFPM = llvm::make_unique<FunctionPassManager>(TheModule.get()); + ... + +The KaleidoscopeJIT class is a simple JIT built specifically for these +tutorials, available inside the LLVM source code +at llvm-src/examples/Kaleidoscope/include/KaleidoscopeJIT.h. +In later chapters we will look at how it works and extend it with +new features, but for now we will take it as given. Its API is very simple: +``addModule`` adds an LLVM IR module to the JIT, making its functions +available for execution; ``removeModule`` removes a module, freeing any +memory associated with the code in that module; and ``findSymbol`` allows us +to look up pointers to the compiled code. + +We can take this simple API and change our code that parses top-level expressions to +look like this: + +.. code-block:: c++ + + static void HandleTopLevelExpression() { + // Evaluate a top-level expression into an anonymous function. + if (auto FnAST = ParseTopLevelExpr()) { + if (FnAST->codegen()) { + + // JIT the module containing the anonymous expression, keeping a handle so + // we can free it later. + auto H = TheJIT->addModule(std::move(TheModule)); + InitializeModuleAndPassManager(); + + // Search the JIT for the __anon_expr symbol. + auto ExprSymbol = TheJIT->findSymbol("__anon_expr"); + assert(ExprSymbol && "Function not found"); + + // Get the symbol's address and cast it to the right type (takes no + // arguments, returns a double) so we can call it as a native function. + double (*FP)() = (double (*)())(intptr_t)ExprSymbol.getAddress(); + fprintf(stderr, "Evaluated to %f\n", FP()); + + // Delete the anonymous expression module from the JIT. + TheJIT->removeModule(H); + } + +If parsing and codegen succeeed, the next step is to add the module containing +the top-level expression to the JIT. We do this by calling addModule, which +triggers code generation for all the functions in the module, and returns a +handle that can be used to remove the module from the JIT later. Once the module +has been added to the JIT it can no longer be modified, so we also open a new +module to hold subsequent code by calling ``InitializeModuleAndPassManager()``. + +Once we've added the module to the JIT we need to get a pointer to the final +generated code. We do this by calling the JIT's findSymbol method, and passing +the name of the top-level expression function: ``__anon_expr``. Since we just +added this function, we assert that findSymbol returned a result. + +Next, we get the in-memory address of the ``__anon_expr`` function by calling +``getAddress()`` on the symbol. Recall that we compile top-level expressions +into a self-contained LLVM function that takes no arguments and returns the +computed double. Because the LLVM JIT compiler matches the native platform ABI, +this means that you can just cast the result pointer to a function pointer of +that type and call it directly. This means, there is no difference between JIT +compiled code and native machine code that is statically linked into your +application. + +Finally, since we don't support re-evaluation of top-level expressions, we +remove the module from the JIT when we're done to free the associated memory. +Recall, however, that the module we created a few lines earlier (via +``InitializeModuleAndPassManager``) is still open and waiting for new code to be +added. + +With just these two changes, let's see how Kaleidoscope works now! + +:: + + ready> 4+5; + Read top-level expression: + define double @0() { + entry: + ret double 9.000000e+00 + } + + Evaluated to 9.000000 + +Well this looks like it is basically working. The dump of the function +shows the "no argument function that always returns double" that we +synthesize for each top-level expression that is typed in. This +demonstrates very basic functionality, but can we do more? + +:: + + ready> def testfunc(x y) x + y*2; + Read function definition: + define double @testfunc(double %x, double %y) { + entry: + %multmp = fmul double %y, 2.000000e+00 + %addtmp = fadd double %multmp, %x + ret double %addtmp + } + + ready> testfunc(4, 10); + Read top-level expression: + define double @1() { + entry: + %calltmp = call double @testfunc(double 4.000000e+00, double 1.000000e+01) + ret double %calltmp + } + + Evaluated to 24.000000 + + ready> testfunc(5, 10); + ready> LLVM ERROR: Program used external function 'testfunc' which could not be resolved! + + +Function definitions and calls also work, but something went very wrong on that +last line. The call looks valid, so what happened? As you may have guessed from +the API a Module is a unit of allocation for the JIT, and testfunc was part +of the same module that contained anonymous expression. When we removed that +module from the JIT to free the memory for the anonymous expression, we deleted +the definition of ``testfunc`` along with it. Then, when we tried to call +testfunc a second time, the JIT could no longer find it. + +The easiest way to fix this is to put the anonymous expression in a separate +module from the rest of the function definitions. The JIT will happily resolve +function calls across module boundaries, as long as each of the functions called +has a prototype, and is added to the JIT before it is called. By putting the +anonymous expression in a different module we can delete it without affecting +the rest of the functions. + +In fact, we're going to go a step further and put every function in its own +module. Doing so allows us to exploit a useful property of the KaleidoscopeJIT +that will make our environment more REPL-like: Functions can be added to the +JIT more than once (unlike a module where every function must have a unique +definition). When you look up a symbol in KaleidoscopeJIT it will always return +the most recent definition: + +:: + + ready> def foo(x) x + 1; + Read function definition: + define double @foo(double %x) { + entry: + %addtmp = fadd double %x, 1.000000e+00 + ret double %addtmp + } + + ready> foo(2); + Evaluated to 3.000000 + + ready> def foo(x) x + 2; + define double @foo(double %x) { + entry: + %addtmp = fadd double %x, 2.000000e+00 + ret double %addtmp + } + + ready> foo(2); + Evaluated to 4.000000 + + +To allow each function to live in its own module we'll need a way to +re-generate previous function declarations into each new module we open: + +.. code-block:: c++ + + static std::unique_ptr<KaleidoscopeJIT> TheJIT; + + ... + + Function *getFunction(std::string Name) { + // First, see if the function has already been added to the current module. + if (auto *F = TheModule->getFunction(Name)) + return F; + + // If not, check whether we can codegen the declaration from some existing + // prototype. + auto FI = FunctionProtos.find(Name); + if (FI != FunctionProtos.end()) + return FI->second->codegen(); + + // If no existing prototype exists, return null. + return nullptr; + } + + ... + + Value *CallExprAST::codegen() { + // Look up the name in the global module table. + Function *CalleeF = getFunction(Callee); + + ... + + Function *FunctionAST::codegen() { + // Transfer ownership of the prototype to the FunctionProtos map, but keep a + // reference to it for use below. + auto &P = *Proto; + FunctionProtos[Proto->getName()] = std::move(Proto); + Function *TheFunction = getFunction(P.getName()); + if (!TheFunction) + return nullptr; + + +To enable this, we'll start by adding a new global, ``FunctionProtos``, that +holds the most recent prototype for each function. We'll also add a convenience +method, ``getFunction()``, to replace calls to ``TheModule->getFunction()``. +Our convenience method searches ``TheModule`` for an existing function +declaration, falling back to generating a new declaration from FunctionProtos if +it doesn't find one. In ``CallExprAST::codegen()`` we just need to replace the +call to ``TheModule->getFunction()``. In ``FunctionAST::codegen()`` we need to +update the FunctionProtos map first, then call ``getFunction()``. With this +done, we can always obtain a function declaration in the current module for any +previously declared function. + +We also need to update HandleDefinition and HandleExtern: + +.. code-block:: c++ + + static void HandleDefinition() { + if (auto FnAST = ParseDefinition()) { + if (auto *FnIR = FnAST->codegen()) { + fprintf(stderr, "Read function definition:"); + FnIR->print(errs()); + fprintf(stderr, "\n"); + TheJIT->addModule(std::move(TheModule)); + InitializeModuleAndPassManager(); + } + } else { + // Skip token for error recovery. + getNextToken(); + } + } + + static void HandleExtern() { + if (auto ProtoAST = ParseExtern()) { + if (auto *FnIR = ProtoAST->codegen()) { + fprintf(stderr, "Read extern: "); + FnIR->print(errs()); + fprintf(stderr, "\n"); + FunctionProtos[ProtoAST->getName()] = std::move(ProtoAST); + } + } else { + // Skip token for error recovery. + getNextToken(); + } + } + +In HandleDefinition, we add two lines to transfer the newly defined function to +the JIT and open a new module. In HandleExtern, we just need to add one line to +add the prototype to FunctionProtos. + +With these changes made, let's try our REPL again (I removed the dump of the +anonymous functions this time, you should get the idea by now :) : + +:: + + ready> def foo(x) x + 1; + ready> foo(2); + Evaluated to 3.000000 + + ready> def foo(x) x + 2; + ready> foo(2); + Evaluated to 4.000000 + +It works! + +Even with this simple code, we get some surprisingly powerful capabilities - +check this out: + +:: + + ready> extern sin(x); + Read extern: + declare double @sin(double) + + ready> extern cos(x); + Read extern: + declare double @cos(double) + + ready> sin(1.0); + Read top-level expression: + define double @2() { + entry: + ret double 0x3FEAED548F090CEE + } + + Evaluated to 0.841471 + + ready> def foo(x) sin(x)*sin(x) + cos(x)*cos(x); + Read function definition: + define double @foo(double %x) { + entry: + %calltmp = call double @sin(double %x) + %multmp = fmul double %calltmp, %calltmp + %calltmp2 = call double @cos(double %x) + %multmp4 = fmul double %calltmp2, %calltmp2 + %addtmp = fadd double %multmp, %multmp4 + ret double %addtmp + } + + ready> foo(4.0); + Read top-level expression: + define double @3() { + entry: + %calltmp = call double @foo(double 4.000000e+00) + ret double %calltmp + } + + Evaluated to 1.000000 + +Whoa, how does the JIT know about sin and cos? The answer is surprisingly +simple: The KaleidoscopeJIT has a straightforward symbol resolution rule that +it uses to find symbols that aren't available in any given module: First +it searches all the modules that have already been added to the JIT, from the +most recent to the oldest, to find the newest definition. If no definition is +found inside the JIT, it falls back to calling "``dlsym("sin")``" on the +Kaleidoscope process itself. Since "``sin``" is defined within the JIT's +address space, it simply patches up calls in the module to call the libm +version of ``sin`` directly. But in some cases this even goes further: +as sin and cos are names of standard math functions, the constant folder +will directly evaluate the function calls to the correct result when called +with constants like in the "``sin(1.0)``" above. + +In the future we'll see how tweaking this symbol resolution rule can be used to +enable all sorts of useful features, from security (restricting the set of +symbols available to JIT'd code), to dynamic code generation based on symbol +names, and even lazy compilation. + +One immediate benefit of the symbol resolution rule is that we can now extend +the language by writing arbitrary C++ code to implement operations. For example, +if we add: + +.. code-block:: c++ + + #ifdef _WIN32 + #define DLLEXPORT __declspec(dllexport) + #else + #define DLLEXPORT + #endif + + /// putchard - putchar that takes a double and returns 0. + extern "C" DLLEXPORT double putchard(double X) { + fputc((char)X, stderr); + return 0; + } + +Note, that for Windows we need to actually export the functions because +the dynamic symbol loader will use GetProcAddress to find the symbols. + +Now we can produce simple output to the console by using things like: +"``extern putchard(x); putchard(120);``", which prints a lowercase 'x' +on the console (120 is the ASCII code for 'x'). Similar code could be +used to implement file I/O, console input, and many other capabilities +in Kaleidoscope. + +This completes the JIT and optimizer chapter of the Kaleidoscope +tutorial. At this point, we can compile a non-Turing-complete +programming language, optimize and JIT compile it in a user-driven way. +Next up we'll look into `extending the language with control flow +constructs <LangImpl05.html>`_, tackling some interesting LLVM IR issues +along the way. + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +the LLVM JIT and optimizer. To build this example, use: + +.. code-block:: bash + + # Compile + clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy + # Run + ./toy + +If you are compiling this on Linux, make sure to add the "-rdynamic" +option as well. This makes sure that the external functions are resolved +properly at runtime. + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter4/toy.cpp + :language: c++ + +`Next: Extending the language: control flow <LangImpl05.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl05-cfg.png b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl05-cfg.png Binary files differnew file mode 100644 index 00000000000..cdba92ff6c5 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl05-cfg.png diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl05.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl05.rst new file mode 100644 index 00000000000..dad24890e12 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl05.rst @@ -0,0 +1,814 @@ +================================================== +Kaleidoscope: Extending the Language: Control Flow +================================================== + +.. contents:: + :local: + +Chapter 5 Introduction +====================== + +Welcome to Chapter 5 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. Parts 1-4 described the implementation of +the simple Kaleidoscope language and included support for generating +LLVM IR, followed by optimizations and a JIT compiler. Unfortunately, as +presented, Kaleidoscope is mostly useless: it has no control flow other +than call and return. This means that you can't have conditional +branches in the code, significantly limiting its power. In this episode +of "build that compiler", we'll extend Kaleidoscope to have an +if/then/else expression plus a simple 'for' loop. + +If/Then/Else +============ + +Extending Kaleidoscope to support if/then/else is quite straightforward. +It basically requires adding support for this "new" concept to the +lexer, parser, AST, and LLVM code emitter. This example is nice, because +it shows how easy it is to "grow" a language over time, incrementally +extending it as new ideas are discovered. + +Before we get going on "how" we add this extension, let's talk about +"what" we want. The basic idea is that we want to be able to write this +sort of thing: + +:: + + def fib(x) + if x < 3 then + 1 + else + fib(x-1)+fib(x-2); + +In Kaleidoscope, every construct is an expression: there are no +statements. As such, the if/then/else expression needs to return a value +like any other. Since we're using a mostly functional form, we'll have +it evaluate its conditional, then return the 'then' or 'else' value +based on how the condition was resolved. This is very similar to the C +"?:" expression. + +The semantics of the if/then/else expression is that it evaluates the +condition to a boolean equality value: 0.0 is considered to be false and +everything else is considered to be true. If the condition is true, the +first subexpression is evaluated and returned, if the condition is +false, the second subexpression is evaluated and returned. Since +Kaleidoscope allows side-effects, this behavior is important to nail +down. + +Now that we know what we "want", let's break this down into its +constituent pieces. + +Lexer Extensions for If/Then/Else +--------------------------------- + +The lexer extensions are straightforward. First we add new enum values +for the relevant tokens: + +.. code-block:: c++ + + // control + tok_if = -6, + tok_then = -7, + tok_else = -8, + +Once we have that, we recognize the new keywords in the lexer. This is +pretty simple stuff: + +.. code-block:: c++ + + ... + if (IdentifierStr == "def") + return tok_def; + if (IdentifierStr == "extern") + return tok_extern; + if (IdentifierStr == "if") + return tok_if; + if (IdentifierStr == "then") + return tok_then; + if (IdentifierStr == "else") + return tok_else; + return tok_identifier; + +AST Extensions for If/Then/Else +------------------------------- + +To represent the new expression we add a new AST node for it: + +.. code-block:: c++ + + /// IfExprAST - Expression class for if/then/else. + class IfExprAST : public ExprAST { + std::unique_ptr<ExprAST> Cond, Then, Else; + + public: + IfExprAST(std::unique_ptr<ExprAST> Cond, std::unique_ptr<ExprAST> Then, + std::unique_ptr<ExprAST> Else) + : Cond(std::move(Cond)), Then(std::move(Then)), Else(std::move(Else)) {} + + Value *codegen() override; + }; + +The AST node just has pointers to the various subexpressions. + +Parser Extensions for If/Then/Else +---------------------------------- + +Now that we have the relevant tokens coming from the lexer and we have +the AST node to build, our parsing logic is relatively straightforward. +First we define a new parsing function: + +.. code-block:: c++ + + /// ifexpr ::= 'if' expression 'then' expression 'else' expression + static std::unique_ptr<ExprAST> ParseIfExpr() { + getNextToken(); // eat the if. + + // condition. + auto Cond = ParseExpression(); + if (!Cond) + return nullptr; + + if (CurTok != tok_then) + return LogError("expected then"); + getNextToken(); // eat the then + + auto Then = ParseExpression(); + if (!Then) + return nullptr; + + if (CurTok != tok_else) + return LogError("expected else"); + + getNextToken(); + + auto Else = ParseExpression(); + if (!Else) + return nullptr; + + return llvm::make_unique<IfExprAST>(std::move(Cond), std::move(Then), + std::move(Else)); + } + +Next we hook it up as a primary expression: + +.. code-block:: c++ + + static std::unique_ptr<ExprAST> ParsePrimary() { + switch (CurTok) { + default: + return LogError("unknown token when expecting an expression"); + case tok_identifier: + return ParseIdentifierExpr(); + case tok_number: + return ParseNumberExpr(); + case '(': + return ParseParenExpr(); + case tok_if: + return ParseIfExpr(); + } + } + +LLVM IR for If/Then/Else +------------------------ + +Now that we have it parsing and building the AST, the final piece is +adding LLVM code generation support. This is the most interesting part +of the if/then/else example, because this is where it starts to +introduce new concepts. All of the code above has been thoroughly +described in previous chapters. + +To motivate the code we want to produce, let's take a look at a simple +example. Consider: + +:: + + extern foo(); + extern bar(); + def baz(x) if x then foo() else bar(); + +If you disable optimizations, the code you'll (soon) get from +Kaleidoscope looks like this: + +.. code-block:: llvm + + declare double @foo() + + declare double @bar() + + define double @baz(double %x) { + entry: + %ifcond = fcmp one double %x, 0.000000e+00 + br i1 %ifcond, label %then, label %else + + then: ; preds = %entry + %calltmp = call double @foo() + br label %ifcont + + else: ; preds = %entry + %calltmp1 = call double @bar() + br label %ifcont + + ifcont: ; preds = %else, %then + %iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ] + ret double %iftmp + } + +To visualize the control flow graph, you can use a nifty feature of the +LLVM '`opt <http://llvm.org/cmds/opt.html>`_' tool. If you put this LLVM +IR into "t.ll" and run "``llvm-as < t.ll | opt -analyze -view-cfg``", `a +window will pop up <../ProgrammersManual.html#viewing-graphs-while-debugging-code>`_ and you'll +see this graph: + +.. figure:: LangImpl05-cfg.png + :align: center + :alt: Example CFG + + Example CFG + +Another way to get this is to call "``F->viewCFG()``" or +"``F->viewCFGOnly()``" (where F is a "``Function*``") either by +inserting actual calls into the code and recompiling or by calling these +in the debugger. LLVM has many nice features for visualizing various +graphs. + +Getting back to the generated code, it is fairly simple: the entry block +evaluates the conditional expression ("x" in our case here) and compares +the result to 0.0 with the "``fcmp one``" instruction ('one' is "Ordered +and Not Equal"). Based on the result of this expression, the code jumps +to either the "then" or "else" blocks, which contain the expressions for +the true/false cases. + +Once the then/else blocks are finished executing, they both branch back +to the 'ifcont' block to execute the code that happens after the +if/then/else. In this case the only thing left to do is to return to the +caller of the function. The question then becomes: how does the code +know which expression to return? + +The answer to this question involves an important SSA operation: the +`Phi +operation <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_. +If you're not familiar with SSA, `the wikipedia +article <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_ +is a good introduction and there are various other introductions to it +available on your favorite search engine. The short version is that +"execution" of the Phi operation requires "remembering" which block +control came from. The Phi operation takes on the value corresponding to +the input control block. In this case, if control comes in from the +"then" block, it gets the value of "calltmp". If control comes from the +"else" block, it gets the value of "calltmp1". + +At this point, you are probably starting to think "Oh no! This means my +simple and elegant front-end will have to start generating SSA form in +order to use LLVM!". Fortunately, this is not the case, and we strongly +advise *not* implementing an SSA construction algorithm in your +front-end unless there is an amazingly good reason to do so. In +practice, there are two sorts of values that float around in code +written for your average imperative programming language that might need +Phi nodes: + +#. Code that involves user variables: ``x = 1; x = x + 1;`` +#. Values that are implicit in the structure of your AST, such as the + Phi node in this case. + +In `Chapter 7 <LangImpl07.html>`_ of this tutorial ("mutable variables"), +we'll talk about #1 in depth. For now, just believe me that you don't +need SSA construction to handle this case. For #2, you have the choice +of using the techniques that we will describe for #1, or you can insert +Phi nodes directly, if convenient. In this case, it is really +easy to generate the Phi node, so we choose to do it directly. + +Okay, enough of the motivation and overview, let's generate code! + +Code Generation for If/Then/Else +-------------------------------- + +In order to generate code for this, we implement the ``codegen`` method +for ``IfExprAST``: + +.. code-block:: c++ + + Value *IfExprAST::codegen() { + Value *CondV = Cond->codegen(); + if (!CondV) + return nullptr; + + // Convert condition to a bool by comparing non-equal to 0.0. + CondV = Builder.CreateFCmpONE( + CondV, ConstantFP::get(TheContext, APFloat(0.0)), "ifcond"); + +This code is straightforward and similar to what we saw before. We emit +the expression for the condition, then compare that value to zero to get +a truth value as a 1-bit (bool) value. + +.. code-block:: c++ + + Function *TheFunction = Builder.GetInsertBlock()->getParent(); + + // Create blocks for the then and else cases. Insert the 'then' block at the + // end of the function. + BasicBlock *ThenBB = + BasicBlock::Create(TheContext, "then", TheFunction); + BasicBlock *ElseBB = BasicBlock::Create(TheContext, "else"); + BasicBlock *MergeBB = BasicBlock::Create(TheContext, "ifcont"); + + Builder.CreateCondBr(CondV, ThenBB, ElseBB); + +This code creates the basic blocks that are related to the if/then/else +statement, and correspond directly to the blocks in the example above. +The first line gets the current Function object that is being built. It +gets this by asking the builder for the current BasicBlock, and asking +that block for its "parent" (the function it is currently embedded +into). + +Once it has that, it creates three blocks. Note that it passes +"TheFunction" into the constructor for the "then" block. This causes the +constructor to automatically insert the new block into the end of the +specified function. The other two blocks are created, but aren't yet +inserted into the function. + +Once the blocks are created, we can emit the conditional branch that +chooses between them. Note that creating new blocks does not implicitly +affect the IRBuilder, so it is still inserting into the block that the +condition went into. Also note that it is creating a branch to the +"then" block and the "else" block, even though the "else" block isn't +inserted into the function yet. This is all ok: it is the standard way +that LLVM supports forward references. + +.. code-block:: c++ + + // Emit then value. + Builder.SetInsertPoint(ThenBB); + + Value *ThenV = Then->codegen(); + if (!ThenV) + return nullptr; + + Builder.CreateBr(MergeBB); + // Codegen of 'Then' can change the current block, update ThenBB for the PHI. + ThenBB = Builder.GetInsertBlock(); + +After the conditional branch is inserted, we move the builder to start +inserting into the "then" block. Strictly speaking, this call moves the +insertion point to be at the end of the specified block. However, since +the "then" block is empty, it also starts out by inserting at the +beginning of the block. :) + +Once the insertion point is set, we recursively codegen the "then" +expression from the AST. To finish off the "then" block, we create an +unconditional branch to the merge block. One interesting (and very +important) aspect of the LLVM IR is that it `requires all basic blocks +to be "terminated" <../LangRef.html#functionstructure>`_ with a `control +flow instruction <../LangRef.html#terminators>`_ such as return or +branch. This means that all control flow, *including fall throughs* must +be made explicit in the LLVM IR. If you violate this rule, the verifier +will emit an error. + +The final line here is quite subtle, but is very important. The basic +issue is that when we create the Phi node in the merge block, we need to +set up the block/value pairs that indicate how the Phi will work. +Importantly, the Phi node expects to have an entry for each predecessor +of the block in the CFG. Why then, are we getting the current block when +we just set it to ThenBB 5 lines above? The problem is that the "Then" +expression may actually itself change the block that the Builder is +emitting into if, for example, it contains a nested "if/then/else" +expression. Because calling ``codegen()`` recursively could arbitrarily change +the notion of the current block, we are required to get an up-to-date +value for code that will set up the Phi node. + +.. code-block:: c++ + + // Emit else block. + TheFunction->getBasicBlockList().push_back(ElseBB); + Builder.SetInsertPoint(ElseBB); + + Value *ElseV = Else->codegen(); + if (!ElseV) + return nullptr; + + Builder.CreateBr(MergeBB); + // codegen of 'Else' can change the current block, update ElseBB for the PHI. + ElseBB = Builder.GetInsertBlock(); + +Code generation for the 'else' block is basically identical to codegen +for the 'then' block. The only significant difference is the first line, +which adds the 'else' block to the function. Recall previously that the +'else' block was created, but not added to the function. Now that the +'then' and 'else' blocks are emitted, we can finish up with the merge +code: + +.. code-block:: c++ + + // Emit merge block. + TheFunction->getBasicBlockList().push_back(MergeBB); + Builder.SetInsertPoint(MergeBB); + PHINode *PN = + Builder.CreatePHI(Type::getDoubleTy(TheContext), 2, "iftmp"); + + PN->addIncoming(ThenV, ThenBB); + PN->addIncoming(ElseV, ElseBB); + return PN; + } + +The first two lines here are now familiar: the first adds the "merge" +block to the Function object (it was previously floating, like the else +block above). The second changes the insertion point so that newly +created code will go into the "merge" block. Once that is done, we need +to create the PHI node and set up the block/value pairs for the PHI. + +Finally, the CodeGen function returns the phi node as the value computed +by the if/then/else expression. In our example above, this returned +value will feed into the code for the top-level function, which will +create the return instruction. + +Overall, we now have the ability to execute conditional code in +Kaleidoscope. With this extension, Kaleidoscope is a fairly complete +language that can calculate a wide variety of numeric functions. Next up +we'll add another useful expression that is familiar from non-functional +languages... + +'for' Loop Expression +===================== + +Now that we know how to add basic control flow constructs to the +language, we have the tools to add more powerful things. Let's add +something more aggressive, a 'for' expression: + +:: + + extern putchard(char); + def printstar(n) + for i = 1, i < n, 1.0 in + putchard(42); # ascii 42 = '*' + + # print 100 '*' characters + printstar(100); + +This expression defines a new variable ("i" in this case) which iterates +from a starting value, while the condition ("i < n" in this case) is +true, incrementing by an optional step value ("1.0" in this case). If +the step value is omitted, it defaults to 1.0. While the loop is true, +it executes its body expression. Because we don't have anything better +to return, we'll just define the loop as always returning 0.0. In the +future when we have mutable variables, it will get more useful. + +As before, let's talk about the changes that we need to Kaleidoscope to +support this. + +Lexer Extensions for the 'for' Loop +----------------------------------- + +The lexer extensions are the same sort of thing as for if/then/else: + +.. code-block:: c++ + + ... in enum Token ... + // control + tok_if = -6, tok_then = -7, tok_else = -8, + tok_for = -9, tok_in = -10 + + ... in gettok ... + if (IdentifierStr == "def") + return tok_def; + if (IdentifierStr == "extern") + return tok_extern; + if (IdentifierStr == "if") + return tok_if; + if (IdentifierStr == "then") + return tok_then; + if (IdentifierStr == "else") + return tok_else; + if (IdentifierStr == "for") + return tok_for; + if (IdentifierStr == "in") + return tok_in; + return tok_identifier; + +AST Extensions for the 'for' Loop +--------------------------------- + +The AST node is just as simple. It basically boils down to capturing the +variable name and the constituent expressions in the node. + +.. code-block:: c++ + + /// ForExprAST - Expression class for for/in. + class ForExprAST : public ExprAST { + std::string VarName; + std::unique_ptr<ExprAST> Start, End, Step, Body; + + public: + ForExprAST(const std::string &VarName, std::unique_ptr<ExprAST> Start, + std::unique_ptr<ExprAST> End, std::unique_ptr<ExprAST> Step, + std::unique_ptr<ExprAST> Body) + : VarName(VarName), Start(std::move(Start)), End(std::move(End)), + Step(std::move(Step)), Body(std::move(Body)) {} + + Value *codegen() override; + }; + +Parser Extensions for the 'for' Loop +------------------------------------ + +The parser code is also fairly standard. The only interesting thing here +is handling of the optional step value. The parser code handles it by +checking to see if the second comma is present. If not, it sets the step +value to null in the AST node: + +.. code-block:: c++ + + /// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression + static std::unique_ptr<ExprAST> ParseForExpr() { + getNextToken(); // eat the for. + + if (CurTok != tok_identifier) + return LogError("expected identifier after for"); + + std::string IdName = IdentifierStr; + getNextToken(); // eat identifier. + + if (CurTok != '=') + return LogError("expected '=' after for"); + getNextToken(); // eat '='. + + + auto Start = ParseExpression(); + if (!Start) + return nullptr; + if (CurTok != ',') + return LogError("expected ',' after for start value"); + getNextToken(); + + auto End = ParseExpression(); + if (!End) + return nullptr; + + // The step value is optional. + std::unique_ptr<ExprAST> Step; + if (CurTok == ',') { + getNextToken(); + Step = ParseExpression(); + if (!Step) + return nullptr; + } + + if (CurTok != tok_in) + return LogError("expected 'in' after for"); + getNextToken(); // eat 'in'. + + auto Body = ParseExpression(); + if (!Body) + return nullptr; + + return llvm::make_unique<ForExprAST>(IdName, std::move(Start), + std::move(End), std::move(Step), + std::move(Body)); + } + +And again we hook it up as a primary expression: + +.. code-block:: c++ + + static std::unique_ptr<ExprAST> ParsePrimary() { + switch (CurTok) { + default: + return LogError("unknown token when expecting an expression"); + case tok_identifier: + return ParseIdentifierExpr(); + case tok_number: + return ParseNumberExpr(); + case '(': + return ParseParenExpr(); + case tok_if: + return ParseIfExpr(); + case tok_for: + return ParseForExpr(); + } + } + +LLVM IR for the 'for' Loop +-------------------------- + +Now we get to the good part: the LLVM IR we want to generate for this +thing. With the simple example above, we get this LLVM IR (note that +this dump is generated with optimizations disabled for clarity): + +.. code-block:: llvm + + declare double @putchard(double) + + define double @printstar(double %n) { + entry: + ; initial value = 1.0 (inlined into phi) + br label %loop + + loop: ; preds = %loop, %entry + %i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ] + ; body + %calltmp = call double @putchard(double 4.200000e+01) + ; increment + %nextvar = fadd double %i, 1.000000e+00 + + ; termination test + %cmptmp = fcmp ult double %i, %n + %booltmp = uitofp i1 %cmptmp to double + %loopcond = fcmp one double %booltmp, 0.000000e+00 + br i1 %loopcond, label %loop, label %afterloop + + afterloop: ; preds = %loop + ; loop always returns 0.0 + ret double 0.000000e+00 + } + +This loop contains all the same constructs we saw before: a phi node, +several expressions, and some basic blocks. Let's see how this fits +together. + +Code Generation for the 'for' Loop +---------------------------------- + +The first part of codegen is very simple: we just output the start +expression for the loop value: + +.. code-block:: c++ + + Value *ForExprAST::codegen() { + // Emit the start code first, without 'variable' in scope. + Value *StartVal = Start->codegen(); + if (!StartVal) + return nullptr; + +With this out of the way, the next step is to set up the LLVM basic +block for the start of the loop body. In the case above, the whole loop +body is one block, but remember that the body code itself could consist +of multiple blocks (e.g. if it contains an if/then/else or a for/in +expression). + +.. code-block:: c++ + + // Make the new basic block for the loop header, inserting after current + // block. + Function *TheFunction = Builder.GetInsertBlock()->getParent(); + BasicBlock *PreheaderBB = Builder.GetInsertBlock(); + BasicBlock *LoopBB = + BasicBlock::Create(TheContext, "loop", TheFunction); + + // Insert an explicit fall through from the current block to the LoopBB. + Builder.CreateBr(LoopBB); + +This code is similar to what we saw for if/then/else. Because we will +need it to create the Phi node, we remember the block that falls through +into the loop. Once we have that, we create the actual block that starts +the loop and create an unconditional branch for the fall-through between +the two blocks. + +.. code-block:: c++ + + // Start insertion in LoopBB. + Builder.SetInsertPoint(LoopBB); + + // Start the PHI node with an entry for Start. + PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(TheContext), + 2, VarName.c_str()); + Variable->addIncoming(StartVal, PreheaderBB); + +Now that the "preheader" for the loop is set up, we switch to emitting +code for the loop body. To begin with, we move the insertion point and +create the PHI node for the loop induction variable. Since we already +know the incoming value for the starting value, we add it to the Phi +node. Note that the Phi will eventually get a second value for the +backedge, but we can't set it up yet (because it doesn't exist!). + +.. code-block:: c++ + + // Within the loop, the variable is defined equal to the PHI node. If it + // shadows an existing variable, we have to restore it, so save it now. + Value *OldVal = NamedValues[VarName]; + NamedValues[VarName] = Variable; + + // Emit the body of the loop. This, like any other expr, can change the + // current BB. Note that we ignore the value computed by the body, but don't + // allow an error. + if (!Body->codegen()) + return nullptr; + +Now the code starts to get more interesting. Our 'for' loop introduces a +new variable to the symbol table. This means that our symbol table can +now contain either function arguments or loop variables. To handle this, +before we codegen the body of the loop, we add the loop variable as the +current value for its name. Note that it is possible that there is a +variable of the same name in the outer scope. It would be easy to make +this an error (emit an error and return null if there is already an +entry for VarName) but we choose to allow shadowing of variables. In +order to handle this correctly, we remember the Value that we are +potentially shadowing in ``OldVal`` (which will be null if there is no +shadowed variable). + +Once the loop variable is set into the symbol table, the code +recursively codegen's the body. This allows the body to use the loop +variable: any references to it will naturally find it in the symbol +table. + +.. code-block:: c++ + + // Emit the step value. + Value *StepVal = nullptr; + if (Step) { + StepVal = Step->codegen(); + if (!StepVal) + return nullptr; + } else { + // If not specified, use 1.0. + StepVal = ConstantFP::get(TheContext, APFloat(1.0)); + } + + Value *NextVar = Builder.CreateFAdd(Variable, StepVal, "nextvar"); + +Now that the body is emitted, we compute the next value of the iteration +variable by adding the step value, or 1.0 if it isn't present. +'``NextVar``' will be the value of the loop variable on the next +iteration of the loop. + +.. code-block:: c++ + + // Compute the end condition. + Value *EndCond = End->codegen(); + if (!EndCond) + return nullptr; + + // Convert condition to a bool by comparing non-equal to 0.0. + EndCond = Builder.CreateFCmpONE( + EndCond, ConstantFP::get(TheContext, APFloat(0.0)), "loopcond"); + +Finally, we evaluate the exit value of the loop, to determine whether +the loop should exit. This mirrors the condition evaluation for the +if/then/else statement. + +.. code-block:: c++ + + // Create the "after loop" block and insert it. + BasicBlock *LoopEndBB = Builder.GetInsertBlock(); + BasicBlock *AfterBB = + BasicBlock::Create(TheContext, "afterloop", TheFunction); + + // Insert the conditional branch into the end of LoopEndBB. + Builder.CreateCondBr(EndCond, LoopBB, AfterBB); + + // Any new code will be inserted in AfterBB. + Builder.SetInsertPoint(AfterBB); + +With the code for the body of the loop complete, we just need to finish +up the control flow for it. This code remembers the end block (for the +phi node), then creates the block for the loop exit ("afterloop"). Based +on the value of the exit condition, it creates a conditional branch that +chooses between executing the loop again and exiting the loop. Any +future code is emitted in the "afterloop" block, so it sets the +insertion position to it. + +.. code-block:: c++ + + // Add a new entry to the PHI node for the backedge. + Variable->addIncoming(NextVar, LoopEndBB); + + // Restore the unshadowed variable. + if (OldVal) + NamedValues[VarName] = OldVal; + else + NamedValues.erase(VarName); + + // for expr always returns 0.0. + return Constant::getNullValue(Type::getDoubleTy(TheContext)); + } + +The final code handles various cleanups: now that we have the "NextVar" +value, we can add the incoming value to the loop PHI node. After that, +we remove the loop variable from the symbol table, so that it isn't in +scope after the for loop. Finally, code generation of the for loop +always returns 0.0, so that is what we return from +``ForExprAST::codegen()``. + +With this, we conclude the "adding control flow to Kaleidoscope" chapter +of the tutorial. In this chapter we added two control flow constructs, +and used them to motivate a couple of aspects of the LLVM IR that are +important for front-end implementors to know. In the next chapter of our +saga, we will get a bit crazier and add `user-defined +operators <LangImpl06.html>`_ to our poor innocent language. + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +the if/then/else and for expressions. To build this example, use: + +.. code-block:: bash + + # Compile + clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy + # Run + ./toy + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter5/toy.cpp + :language: c++ + +`Next: Extending the language: user-defined operators <LangImpl06.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl06.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl06.rst new file mode 100644 index 00000000000..2a9f4c6b609 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl06.rst @@ -0,0 +1,768 @@ +============================================================ +Kaleidoscope: Extending the Language: User-defined Operators +============================================================ + +.. contents:: + :local: + +Chapter 6 Introduction +====================== + +Welcome to Chapter 6 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. At this point in our tutorial, we now +have a fully functional language that is fairly minimal, but also +useful. There is still one big problem with it, however. Our language +doesn't have many useful operators (like division, logical negation, or +even any comparisons besides less-than). + +This chapter of the tutorial takes a wild digression into adding +user-defined operators to the simple and beautiful Kaleidoscope +language. This digression now gives us a simple and ugly language in +some ways, but also a powerful one at the same time. One of the great +things about creating your own language is that you get to decide what +is good or bad. In this tutorial we'll assume that it is okay to use +this as a way to show some interesting parsing techniques. + +At the end of this tutorial, we'll run through an example Kaleidoscope +application that `renders the Mandelbrot set <#kicking-the-tires>`_. This gives an +example of what you can build with Kaleidoscope and its feature set. + +User-defined Operators: the Idea +================================ + +The "operator overloading" that we will add to Kaleidoscope is more +general than in languages like C++. In C++, you are only allowed to +redefine existing operators: you can't programmatically change the +grammar, introduce new operators, change precedence levels, etc. In this +chapter, we will add this capability to Kaleidoscope, which will let the +user round out the set of operators that are supported. + +The point of going into user-defined operators in a tutorial like this +is to show the power and flexibility of using a hand-written parser. +Thus far, the parser we have been implementing uses recursive descent +for most parts of the grammar and operator precedence parsing for the +expressions. See `Chapter 2 <LangImpl02.html>`_ for details. By +using operator precedence parsing, it is very easy to allow +the programmer to introduce new operators into the grammar: the grammar +is dynamically extensible as the JIT runs. + +The two specific features we'll add are programmable unary operators +(right now, Kaleidoscope has no unary operators at all) as well as +binary operators. An example of this is: + +:: + + # Logical unary not. + def unary!(v) + if v then + 0 + else + 1; + + # Define > with the same precedence as <. + def binary> 10 (LHS RHS) + RHS < LHS; + + # Binary "logical or", (note that it does not "short circuit") + def binary| 5 (LHS RHS) + if LHS then + 1 + else if RHS then + 1 + else + 0; + + # Define = with slightly lower precedence than relationals. + def binary= 9 (LHS RHS) + !(LHS < RHS | LHS > RHS); + +Many languages aspire to being able to implement their standard runtime +library in the language itself. In Kaleidoscope, we can implement +significant parts of the language in the library! + +We will break down implementation of these features into two parts: +implementing support for user-defined binary operators and adding unary +operators. + +User-defined Binary Operators +============================= + +Adding support for user-defined binary operators is pretty simple with +our current framework. We'll first add support for the unary/binary +keywords: + +.. code-block:: c++ + + enum Token { + ... + // operators + tok_binary = -11, + tok_unary = -12 + }; + ... + static int gettok() { + ... + if (IdentifierStr == "for") + return tok_for; + if (IdentifierStr == "in") + return tok_in; + if (IdentifierStr == "binary") + return tok_binary; + if (IdentifierStr == "unary") + return tok_unary; + return tok_identifier; + +This just adds lexer support for the unary and binary keywords, like we +did in `previous chapters <LangImpl5.html#lexer-extensions-for-if-then-else>`_. One nice thing +about our current AST, is that we represent binary operators with full +generalisation by using their ASCII code as the opcode. For our extended +operators, we'll use this same representation, so we don't need any new +AST or parser support. + +On the other hand, we have to be able to represent the definitions of +these new operators, in the "def binary\| 5" part of the function +definition. In our grammar so far, the "name" for the function +definition is parsed as the "prototype" production and into the +``PrototypeAST`` AST node. To represent our new user-defined operators +as prototypes, we have to extend the ``PrototypeAST`` AST node like +this: + +.. code-block:: c++ + + /// PrototypeAST - This class represents the "prototype" for a function, + /// which captures its argument names as well as if it is an operator. + class PrototypeAST { + std::string Name; + std::vector<std::string> Args; + bool IsOperator; + unsigned Precedence; // Precedence if a binary op. + + public: + PrototypeAST(const std::string &name, std::vector<std::string> Args, + bool IsOperator = false, unsigned Prec = 0) + : Name(name), Args(std::move(Args)), IsOperator(IsOperator), + Precedence(Prec) {} + + Function *codegen(); + const std::string &getName() const { return Name; } + + bool isUnaryOp() const { return IsOperator && Args.size() == 1; } + bool isBinaryOp() const { return IsOperator && Args.size() == 2; } + + char getOperatorName() const { + assert(isUnaryOp() || isBinaryOp()); + return Name[Name.size() - 1]; + } + + unsigned getBinaryPrecedence() const { return Precedence; } + }; + +Basically, in addition to knowing a name for the prototype, we now keep +track of whether it was an operator, and if it was, what precedence +level the operator is at. The precedence is only used for binary +operators (as you'll see below, it just doesn't apply for unary +operators). Now that we have a way to represent the prototype for a +user-defined operator, we need to parse it: + +.. code-block:: c++ + + /// prototype + /// ::= id '(' id* ')' + /// ::= binary LETTER number? (id, id) + static std::unique_ptr<PrototypeAST> ParsePrototype() { + std::string FnName; + + unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary. + unsigned BinaryPrecedence = 30; + + switch (CurTok) { + default: + return LogErrorP("Expected function name in prototype"); + case tok_identifier: + FnName = IdentifierStr; + Kind = 0; + getNextToken(); + break; + case tok_binary: + getNextToken(); + if (!isascii(CurTok)) + return LogErrorP("Expected binary operator"); + FnName = "binary"; + FnName += (char)CurTok; + Kind = 2; + getNextToken(); + + // Read the precedence if present. + if (CurTok == tok_number) { + if (NumVal < 1 || NumVal > 100) + return LogErrorP("Invalid precedence: must be 1..100"); + BinaryPrecedence = (unsigned)NumVal; + getNextToken(); + } + break; + } + + if (CurTok != '(') + return LogErrorP("Expected '(' in prototype"); + + std::vector<std::string> ArgNames; + while (getNextToken() == tok_identifier) + ArgNames.push_back(IdentifierStr); + if (CurTok != ')') + return LogErrorP("Expected ')' in prototype"); + + // success. + getNextToken(); // eat ')'. + + // Verify right number of names for operator. + if (Kind && ArgNames.size() != Kind) + return LogErrorP("Invalid number of operands for operator"); + + return llvm::make_unique<PrototypeAST>(FnName, std::move(ArgNames), Kind != 0, + BinaryPrecedence); + } + +This is all fairly straightforward parsing code, and we have already +seen a lot of similar code in the past. One interesting part about the +code above is the couple lines that set up ``FnName`` for binary +operators. This builds names like "binary@" for a newly defined "@" +operator. It then takes advantage of the fact that symbol names in the +LLVM symbol table are allowed to have any character in them, including +embedded nul characters. + +The next interesting thing to add, is codegen support for these binary +operators. Given our current structure, this is a simple addition of a +default case for our existing binary operator node: + +.. code-block:: c++ + + Value *BinaryExprAST::codegen() { + Value *L = LHS->codegen(); + Value *R = RHS->codegen(); + if (!L || !R) + return nullptr; + + switch (Op) { + case '+': + return Builder.CreateFAdd(L, R, "addtmp"); + case '-': + return Builder.CreateFSub(L, R, "subtmp"); + case '*': + return Builder.CreateFMul(L, R, "multmp"); + case '<': + L = Builder.CreateFCmpULT(L, R, "cmptmp"); + // Convert bool 0/1 to double 0.0 or 1.0 + return Builder.CreateUIToFP(L, Type::getDoubleTy(TheContext), + "booltmp"); + default: + break; + } + + // If it wasn't a builtin binary operator, it must be a user defined one. Emit + // a call to it. + Function *F = getFunction(std::string("binary") + Op); + assert(F && "binary operator not found!"); + + Value *Ops[2] = { L, R }; + return Builder.CreateCall(F, Ops, "binop"); + } + +As you can see above, the new code is actually really simple. It just +does a lookup for the appropriate operator in the symbol table and +generates a function call to it. Since user-defined operators are just +built as normal functions (because the "prototype" boils down to a +function with the right name) everything falls into place. + +The final piece of code we are missing, is a bit of top-level magic: + +.. code-block:: c++ + + Function *FunctionAST::codegen() { + // Transfer ownership of the prototype to the FunctionProtos map, but keep a + // reference to it for use below. + auto &P = *Proto; + FunctionProtos[Proto->getName()] = std::move(Proto); + Function *TheFunction = getFunction(P.getName()); + if (!TheFunction) + return nullptr; + + // If this is an operator, install it. + if (P.isBinaryOp()) + BinopPrecedence[P.getOperatorName()] = P.getBinaryPrecedence(); + + // Create a new basic block to start insertion into. + BasicBlock *BB = BasicBlock::Create(TheContext, "entry", TheFunction); + ... + +Basically, before codegening a function, if it is a user-defined +operator, we register it in the precedence table. This allows the binary +operator parsing logic we already have in place to handle it. Since we +are working on a fully-general operator precedence parser, this is all +we need to do to "extend the grammar". + +Now we have useful user-defined binary operators. This builds a lot on +the previous framework we built for other operators. Adding unary +operators is a bit more challenging, because we don't have any framework +for it yet - let's see what it takes. + +User-defined Unary Operators +============================ + +Since we don't currently support unary operators in the Kaleidoscope +language, we'll need to add everything to support them. Above, we added +simple support for the 'unary' keyword to the lexer. In addition to +that, we need an AST node: + +.. code-block:: c++ + + /// UnaryExprAST - Expression class for a unary operator. + class UnaryExprAST : public ExprAST { + char Opcode; + std::unique_ptr<ExprAST> Operand; + + public: + UnaryExprAST(char Opcode, std::unique_ptr<ExprAST> Operand) + : Opcode(Opcode), Operand(std::move(Operand)) {} + + Value *codegen() override; + }; + +This AST node is very simple and obvious by now. It directly mirrors the +binary operator AST node, except that it only has one child. With this, +we need to add the parsing logic. Parsing a unary operator is pretty +simple: we'll add a new function to do it: + +.. code-block:: c++ + + /// unary + /// ::= primary + /// ::= '!' unary + static std::unique_ptr<ExprAST> ParseUnary() { + // If the current token is not an operator, it must be a primary expr. + if (!isascii(CurTok) || CurTok == '(' || CurTok == ',') + return ParsePrimary(); + + // If this is a unary operator, read it. + int Opc = CurTok; + getNextToken(); + if (auto Operand = ParseUnary()) + return llvm::make_unique<UnaryExprAST>(Opc, std::move(Operand)); + return nullptr; + } + +The grammar we add is pretty straightforward here. If we see a unary +operator when parsing a primary operator, we eat the operator as a +prefix and parse the remaining piece as another unary operator. This +allows us to handle multiple unary operators (e.g. "!!x"). Note that +unary operators can't have ambiguous parses like binary operators can, +so there is no need for precedence information. + +The problem with this function, is that we need to call ParseUnary from +somewhere. To do this, we change previous callers of ParsePrimary to +call ParseUnary instead: + +.. code-block:: c++ + + /// binoprhs + /// ::= ('+' unary)* + static std::unique_ptr<ExprAST> ParseBinOpRHS(int ExprPrec, + std::unique_ptr<ExprAST> LHS) { + ... + // Parse the unary expression after the binary operator. + auto RHS = ParseUnary(); + if (!RHS) + return nullptr; + ... + } + /// expression + /// ::= unary binoprhs + /// + static std::unique_ptr<ExprAST> ParseExpression() { + auto LHS = ParseUnary(); + if (!LHS) + return nullptr; + + return ParseBinOpRHS(0, std::move(LHS)); + } + +With these two simple changes, we are now able to parse unary operators +and build the AST for them. Next up, we need to add parser support for +prototypes, to parse the unary operator prototype. We extend the binary +operator code above with: + +.. code-block:: c++ + + /// prototype + /// ::= id '(' id* ')' + /// ::= binary LETTER number? (id, id) + /// ::= unary LETTER (id) + static std::unique_ptr<PrototypeAST> ParsePrototype() { + std::string FnName; + + unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary. + unsigned BinaryPrecedence = 30; + + switch (CurTok) { + default: + return LogErrorP("Expected function name in prototype"); + case tok_identifier: + FnName = IdentifierStr; + Kind = 0; + getNextToken(); + break; + case tok_unary: + getNextToken(); + if (!isascii(CurTok)) + return LogErrorP("Expected unary operator"); + FnName = "unary"; + FnName += (char)CurTok; + Kind = 1; + getNextToken(); + break; + case tok_binary: + ... + +As with binary operators, we name unary operators with a name that +includes the operator character. This assists us at code generation +time. Speaking of, the final piece we need to add is codegen support for +unary operators. It looks like this: + +.. code-block:: c++ + + Value *UnaryExprAST::codegen() { + Value *OperandV = Operand->codegen(); + if (!OperandV) + return nullptr; + + Function *F = getFunction(std::string("unary") + Opcode); + if (!F) + return LogErrorV("Unknown unary operator"); + + return Builder.CreateCall(F, OperandV, "unop"); + } + +This code is similar to, but simpler than, the code for binary +operators. It is simpler primarily because it doesn't need to handle any +predefined operators. + +Kicking the Tires +================= + +It is somewhat hard to believe, but with a few simple extensions we've +covered in the last chapters, we have grown a real-ish language. With +this, we can do a lot of interesting things, including I/O, math, and a +bunch of other things. For example, we can now add a nice sequencing +operator (printd is defined to print out the specified value and a +newline): + +:: + + ready> extern printd(x); + Read extern: + declare double @printd(double) + + ready> def binary : 1 (x y) 0; # Low-precedence operator that ignores operands. + ... + ready> printd(123) : printd(456) : printd(789); + 123.000000 + 456.000000 + 789.000000 + Evaluated to 0.000000 + +We can also define a bunch of other "primitive" operations, such as: + +:: + + # Logical unary not. + def unary!(v) + if v then + 0 + else + 1; + + # Unary negate. + def unary-(v) + 0-v; + + # Define > with the same precedence as <. + def binary> 10 (LHS RHS) + RHS < LHS; + + # Binary logical or, which does not short circuit. + def binary| 5 (LHS RHS) + if LHS then + 1 + else if RHS then + 1 + else + 0; + + # Binary logical and, which does not short circuit. + def binary& 6 (LHS RHS) + if !LHS then + 0 + else + !!RHS; + + # Define = with slightly lower precedence than relationals. + def binary = 9 (LHS RHS) + !(LHS < RHS | LHS > RHS); + + # Define ':' for sequencing: as a low-precedence operator that ignores operands + # and just returns the RHS. + def binary : 1 (x y) y; + +Given the previous if/then/else support, we can also define interesting +functions for I/O. For example, the following prints out a character +whose "density" reflects the value passed in: the lower the value, the +denser the character: + +:: + + ready> extern putchard(char); + ... + ready> def printdensity(d) + if d > 8 then + putchard(32) # ' ' + else if d > 4 then + putchard(46) # '.' + else if d > 2 then + putchard(43) # '+' + else + putchard(42); # '*' + ... + ready> printdensity(1): printdensity(2): printdensity(3): + printdensity(4): printdensity(5): printdensity(9): + putchard(10); + **++. + Evaluated to 0.000000 + +Based on these simple primitive operations, we can start to define more +interesting things. For example, here's a little function that determines +the number of iterations it takes for a certain function in the complex +plane to diverge: + +:: + + # Determine whether the specific location diverges. + # Solve for z = z^2 + c in the complex plane. + def mandelconverger(real imag iters creal cimag) + if iters > 255 | (real*real + imag*imag > 4) then + iters + else + mandelconverger(real*real - imag*imag + creal, + 2*real*imag + cimag, + iters+1, creal, cimag); + + # Return the number of iterations required for the iteration to escape + def mandelconverge(real imag) + mandelconverger(real, imag, 0, real, imag); + +This "``z = z2 + c``" function is a beautiful little creature that is +the basis for computation of the `Mandelbrot +Set <http://en.wikipedia.org/wiki/Mandelbrot_set>`_. Our +``mandelconverge`` function returns the number of iterations that it +takes for a complex orbit to escape, saturating to 255. This is not a +very useful function by itself, but if you plot its value over a +two-dimensional plane, you can see the Mandelbrot set. Given that we are +limited to using putchard here, our amazing graphical output is limited, +but we can whip together something using the density plotter above: + +:: + + # Compute and plot the mandelbrot set with the specified 2 dimensional range + # info. + def mandelhelp(xmin xmax xstep ymin ymax ystep) + for y = ymin, y < ymax, ystep in ( + (for x = xmin, x < xmax, xstep in + printdensity(mandelconverge(x,y))) + : putchard(10) + ) + + # mandel - This is a convenient helper function for plotting the mandelbrot set + # from the specified position with the specified Magnification. + def mandel(realstart imagstart realmag imagmag) + mandelhelp(realstart, realstart+realmag*78, realmag, + imagstart, imagstart+imagmag*40, imagmag); + +Given this, we can try plotting out the mandelbrot set! Lets try it out: + +:: + + ready> mandel(-2.3, -1.3, 0.05, 0.07); + *******************************+++++++++++************************************* + *************************+++++++++++++++++++++++******************************* + **********************+++++++++++++++++++++++++++++**************************** + *******************+++++++++++++++++++++.. ...++++++++************************* + *****************++++++++++++++++++++++.... ...+++++++++*********************** + ***************+++++++++++++++++++++++..... ...+++++++++********************* + **************+++++++++++++++++++++++.... ....+++++++++******************** + *************++++++++++++++++++++++...... .....++++++++******************* + ************+++++++++++++++++++++....... .......+++++++****************** + ***********+++++++++++++++++++.... ... .+++++++***************** + **********+++++++++++++++++....... .+++++++**************** + *********++++++++++++++........... ...+++++++*************** + ********++++++++++++............ ...++++++++************** + ********++++++++++... .......... .++++++++************** + *******+++++++++..... .+++++++++************* + *******++++++++...... ..+++++++++************* + *******++++++....... ..+++++++++************* + *******+++++...... ..+++++++++************* + *******.... .... ...+++++++++************* + *******.... . ...+++++++++************* + *******+++++...... ...+++++++++************* + *******++++++....... ..+++++++++************* + *******++++++++...... .+++++++++************* + *******+++++++++..... ..+++++++++************* + ********++++++++++... .......... .++++++++************** + ********++++++++++++............ ...++++++++************** + *********++++++++++++++.......... ...+++++++*************** + **********++++++++++++++++........ .+++++++**************** + **********++++++++++++++++++++.... ... ..+++++++**************** + ***********++++++++++++++++++++++....... .......++++++++***************** + ************+++++++++++++++++++++++...... ......++++++++****************** + **************+++++++++++++++++++++++.... ....++++++++******************** + ***************+++++++++++++++++++++++..... ...+++++++++********************* + *****************++++++++++++++++++++++.... ...++++++++*********************** + *******************+++++++++++++++++++++......++++++++************************* + *********************++++++++++++++++++++++.++++++++*************************** + *************************+++++++++++++++++++++++******************************* + ******************************+++++++++++++************************************ + ******************************************************************************* + ******************************************************************************* + ******************************************************************************* + Evaluated to 0.000000 + ready> mandel(-2, -1, 0.02, 0.04); + **************************+++++++++++++++++++++++++++++++++++++++++++++++++++++ + ***********************++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + *********************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++. + *******************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++... + *****************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++..... + ***************++++++++++++++++++++++++++++++++++++++++++++++++++++++++........ + **************++++++++++++++++++++++++++++++++++++++++++++++++++++++........... + ************+++++++++++++++++++++++++++++++++++++++++++++++++++++.............. + ***********++++++++++++++++++++++++++++++++++++++++++++++++++........ . + **********++++++++++++++++++++++++++++++++++++++++++++++............. + ********+++++++++++++++++++++++++++++++++++++++++++.................. + *******+++++++++++++++++++++++++++++++++++++++....................... + ******+++++++++++++++++++++++++++++++++++........................... + *****++++++++++++++++++++++++++++++++............................ + *****++++++++++++++++++++++++++++............................... + ****++++++++++++++++++++++++++...... ......................... + ***++++++++++++++++++++++++......... ...... ........... + ***++++++++++++++++++++++............ + **+++++++++++++++++++++.............. + **+++++++++++++++++++................ + *++++++++++++++++++................. + *++++++++++++++++............ ... + *++++++++++++++.............. + *+++....++++................ + *.......... ........... + * + *.......... ........... + *+++....++++................ + *++++++++++++++.............. + *++++++++++++++++............ ... + *++++++++++++++++++................. + **+++++++++++++++++++................ + **+++++++++++++++++++++.............. + ***++++++++++++++++++++++............ + ***++++++++++++++++++++++++......... ...... ........... + ****++++++++++++++++++++++++++...... ......................... + *****++++++++++++++++++++++++++++............................... + *****++++++++++++++++++++++++++++++++............................ + ******+++++++++++++++++++++++++++++++++++........................... + *******+++++++++++++++++++++++++++++++++++++++....................... + ********+++++++++++++++++++++++++++++++++++++++++++.................. + Evaluated to 0.000000 + ready> mandel(-0.9, -1.4, 0.02, 0.03); + ******************************************************************************* + ******************************************************************************* + ******************************************************************************* + **********+++++++++++++++++++++************************************************ + *+++++++++++++++++++++++++++++++++++++++*************************************** + +++++++++++++++++++++++++++++++++++++++++++++********************************** + ++++++++++++++++++++++++++++++++++++++++++++++++++***************************** + ++++++++++++++++++++++++++++++++++++++++++++++++++++++************************* + +++++++++++++++++++++++++++++++++++++++++++++++++++++++++********************** + +++++++++++++++++++++++++++++++++.........++++++++++++++++++******************* + +++++++++++++++++++++++++++++++.... ......+++++++++++++++++++**************** + +++++++++++++++++++++++++++++....... ........+++++++++++++++++++************** + ++++++++++++++++++++++++++++........ ........++++++++++++++++++++************ + +++++++++++++++++++++++++++......... .. ...+++++++++++++++++++++********** + ++++++++++++++++++++++++++........... ....++++++++++++++++++++++******** + ++++++++++++++++++++++++............. .......++++++++++++++++++++++****** + +++++++++++++++++++++++............. ........+++++++++++++++++++++++**** + ++++++++++++++++++++++........... ..........++++++++++++++++++++++*** + ++++++++++++++++++++........... .........++++++++++++++++++++++* + ++++++++++++++++++............ ...........++++++++++++++++++++ + ++++++++++++++++............... .............++++++++++++++++++ + ++++++++++++++................. ...............++++++++++++++++ + ++++++++++++.................. .................++++++++++++++ + +++++++++.................. .................+++++++++++++ + ++++++........ . ......... ..++++++++++++ + ++............ ...... ....++++++++++ + .............. ...++++++++++ + .............. ....+++++++++ + .............. .....++++++++ + ............. ......++++++++ + ........... .......++++++++ + ......... ........+++++++ + ......... ........+++++++ + ......... ....+++++++ + ........ ...+++++++ + ....... ...+++++++ + ....+++++++ + .....+++++++ + ....+++++++ + ....+++++++ + ....+++++++ + Evaluated to 0.000000 + ready> ^D + +At this point, you may be starting to realize that Kaleidoscope is a +real and powerful language. It may not be self-similar :), but it can be +used to plot things that are! + +With this, we conclude the "adding user-defined operators" chapter of +the tutorial. We have successfully augmented our language, adding the +ability to extend the language in the library, and we have shown how +this can be used to build a simple but interesting end-user application +in Kaleidoscope. At this point, Kaleidoscope can build a variety of +applications that are functional and can call functions with +side-effects, but it can't actually define and mutate a variable itself. + +Strikingly, variable mutation is an important feature of some languages, +and it is not at all obvious how to `add support for mutable +variables <LangImpl07.html>`_ without having to add an "SSA construction" +phase to your front-end. In the next chapter, we will describe how you +can add variable mutation without building SSA in your front-end. + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +the support for user-defined operators. To build this example, use: + +.. code-block:: bash + + # Compile + clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy + # Run + ./toy + +On some platforms, you will need to specify -rdynamic or +-Wl,--export-dynamic when linking. This ensures that symbols defined in +the main executable are exported to the dynamic linker and so are +available for symbol resolution at run time. This is not needed if you +compile your support code into a shared library, although doing that +will cause problems on Windows. + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter6/toy.cpp + :language: c++ + +`Next: Extending the language: mutable variables / SSA +construction <LangImpl07.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl07.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl07.rst new file mode 100644 index 00000000000..582645f449b --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl07.rst @@ -0,0 +1,883 @@ +======================================================= +Kaleidoscope: Extending the Language: Mutable Variables +======================================================= + +.. contents:: + :local: + +Chapter 7 Introduction +====================== + +Welcome to Chapter 7 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. In chapters 1 through 6, we've built a +very respectable, albeit simple, `functional programming +language <http://en.wikipedia.org/wiki/Functional_programming>`_. In our +journey, we learned some parsing techniques, how to build and represent +an AST, how to build LLVM IR, and how to optimize the resultant code as +well as JIT compile it. + +While Kaleidoscope is interesting as a functional language, the fact +that it is functional makes it "too easy" to generate LLVM IR for it. In +particular, a functional language makes it very easy to build LLVM IR +directly in `SSA +form <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_. +Since LLVM requires that the input code be in SSA form, this is a very +nice property and it is often unclear to newcomers how to generate code +for an imperative language with mutable variables. + +The short (and happy) summary of this chapter is that there is no need +for your front-end to build SSA form: LLVM provides highly tuned and +well tested support for this, though the way it works is a bit +unexpected for some. + +Why is this a hard problem? +=========================== + +To understand why mutable variables cause complexities in SSA +construction, consider this extremely simple C example: + +.. code-block:: c + + int G, H; + int test(_Bool Condition) { + int X; + if (Condition) + X = G; + else + X = H; + return X; + } + +In this case, we have the variable "X", whose value depends on the path +executed in the program. Because there are two different possible values +for X before the return instruction, a PHI node is inserted to merge the +two values. The LLVM IR that we want for this example looks like this: + +.. code-block:: llvm + + @G = weak global i32 0 ; type of @G is i32* + @H = weak global i32 0 ; type of @H is i32* + + define i32 @test(i1 %Condition) { + entry: + br i1 %Condition, label %cond_true, label %cond_false + + cond_true: + %X.0 = load i32* @G + br label %cond_next + + cond_false: + %X.1 = load i32* @H + br label %cond_next + + cond_next: + %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] + ret i32 %X.2 + } + +In this example, the loads from the G and H global variables are +explicit in the LLVM IR, and they live in the then/else branches of the +if statement (cond\_true/cond\_false). In order to merge the incoming +values, the X.2 phi node in the cond\_next block selects the right value +to use based on where control flow is coming from: if control flow comes +from the cond\_false block, X.2 gets the value of X.1. Alternatively, if +control flow comes from cond\_true, it gets the value of X.0. The intent +of this chapter is not to explain the details of SSA form. For more +information, see one of the many `online +references <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_. + +The question for this article is "who places the phi nodes when lowering +assignments to mutable variables?". The issue here is that LLVM +*requires* that its IR be in SSA form: there is no "non-ssa" mode for +it. However, SSA construction requires non-trivial algorithms and data +structures, so it is inconvenient and wasteful for every front-end to +have to reproduce this logic. + +Memory in LLVM +============== + +The 'trick' here is that while LLVM does require all register values to +be in SSA form, it does not require (or permit) memory objects to be in +SSA form. In the example above, note that the loads from G and H are +direct accesses to G and H: they are not renamed or versioned. This +differs from some other compiler systems, which do try to version memory +objects. In LLVM, instead of encoding dataflow analysis of memory into +the LLVM IR, it is handled with `Analysis +Passes <../WritingAnLLVMPass.html>`_ which are computed on demand. + +With this in mind, the high-level idea is that we want to make a stack +variable (which lives in memory, because it is on the stack) for each +mutable object in a function. To take advantage of this trick, we need +to talk about how LLVM represents stack variables. + +In LLVM, all memory accesses are explicit with load/store instructions, +and it is carefully designed not to have (or need) an "address-of" +operator. Notice how the type of the @G/@H global variables is actually +"i32\*" even though the variable is defined as "i32". What this means is +that @G defines *space* for an i32 in the global data area, but its +*name* actually refers to the address for that space. Stack variables +work the same way, except that instead of being declared with global +variable definitions, they are declared with the `LLVM alloca +instruction <../LangRef.html#alloca-instruction>`_: + +.. code-block:: llvm + + define i32 @example() { + entry: + %X = alloca i32 ; type of %X is i32*. + ... + %tmp = load i32* %X ; load the stack value %X from the stack. + %tmp2 = add i32 %tmp, 1 ; increment it + store i32 %tmp2, i32* %X ; store it back + ... + +This code shows an example of how you can declare and manipulate a stack +variable in the LLVM IR. Stack memory allocated with the alloca +instruction is fully general: you can pass the address of the stack slot +to functions, you can store it in other variables, etc. In our example +above, we could rewrite the example to use the alloca technique to avoid +using a PHI node: + +.. code-block:: llvm + + @G = weak global i32 0 ; type of @G is i32* + @H = weak global i32 0 ; type of @H is i32* + + define i32 @test(i1 %Condition) { + entry: + %X = alloca i32 ; type of %X is i32*. + br i1 %Condition, label %cond_true, label %cond_false + + cond_true: + %X.0 = load i32* @G + store i32 %X.0, i32* %X ; Update X + br label %cond_next + + cond_false: + %X.1 = load i32* @H + store i32 %X.1, i32* %X ; Update X + br label %cond_next + + cond_next: + %X.2 = load i32* %X ; Read X + ret i32 %X.2 + } + +With this, we have discovered a way to handle arbitrary mutable +variables without the need to create Phi nodes at all: + +#. Each mutable variable becomes a stack allocation. +#. Each read of the variable becomes a load from the stack. +#. Each update of the variable becomes a store to the stack. +#. Taking the address of a variable just uses the stack address + directly. + +While this solution has solved our immediate problem, it introduced +another one: we have now apparently introduced a lot of stack traffic +for very simple and common operations, a major performance problem. +Fortunately for us, the LLVM optimizer has a highly-tuned optimization +pass named "mem2reg" that handles this case, promoting allocas like this +into SSA registers, inserting Phi nodes as appropriate. If you run this +example through the pass, for example, you'll get: + +.. code-block:: bash + + $ llvm-as < example.ll | opt -mem2reg | llvm-dis + @G = weak global i32 0 + @H = weak global i32 0 + + define i32 @test(i1 %Condition) { + entry: + br i1 %Condition, label %cond_true, label %cond_false + + cond_true: + %X.0 = load i32* @G + br label %cond_next + + cond_false: + %X.1 = load i32* @H + br label %cond_next + + cond_next: + %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] + ret i32 %X.01 + } + +The mem2reg pass implements the standard "iterated dominance frontier" +algorithm for constructing SSA form and has a number of optimizations +that speed up (very common) degenerate cases. The mem2reg optimization +pass is the answer to dealing with mutable variables, and we highly +recommend that you depend on it. Note that mem2reg only works on +variables in certain circumstances: + +#. mem2reg is alloca-driven: it looks for allocas and if it can handle + them, it promotes them. It does not apply to global variables or heap + allocations. +#. mem2reg only looks for alloca instructions in the entry block of the + function. Being in the entry block guarantees that the alloca is only + executed once, which makes analysis simpler. +#. mem2reg only promotes allocas whose uses are direct loads and stores. + If the address of the stack object is passed to a function, or if any + funny pointer arithmetic is involved, the alloca will not be + promoted. +#. mem2reg only works on allocas of `first + class <../LangRef.html#first-class-types>`_ values (such as pointers, + scalars and vectors), and only if the array size of the allocation is + 1 (or missing in the .ll file). mem2reg is not capable of promoting + structs or arrays to registers. Note that the "sroa" pass is + more powerful and can promote structs, "unions", and arrays in many + cases. + +All of these properties are easy to satisfy for most imperative +languages, and we'll illustrate it below with Kaleidoscope. The final +question you may be asking is: should I bother with this nonsense for my +front-end? Wouldn't it be better if I just did SSA construction +directly, avoiding use of the mem2reg optimization pass? In short, we +strongly recommend that you use this technique for building SSA form, +unless there is an extremely good reason not to. Using this technique +is: + +- Proven and well tested: clang uses this technique + for local mutable variables. As such, the most common clients of LLVM + are using this to handle a bulk of their variables. You can be sure + that bugs are found fast and fixed early. +- Extremely Fast: mem2reg has a number of special cases that make it + fast in common cases as well as fully general. For example, it has + fast-paths for variables that are only used in a single block, + variables that only have one assignment point, good heuristics to + avoid insertion of unneeded phi nodes, etc. +- Needed for debug info generation: `Debug information in + LLVM <../SourceLevelDebugging.html>`_ relies on having the address of + the variable exposed so that debug info can be attached to it. This + technique dovetails very naturally with this style of debug info. + +If nothing else, this makes it much easier to get your front-end up and +running, and is very simple to implement. Let's extend Kaleidoscope with +mutable variables now! + +Mutable Variables in Kaleidoscope +================================= + +Now that we know the sort of problem we want to tackle, let's see what +this looks like in the context of our little Kaleidoscope language. +We're going to add two features: + +#. The ability to mutate variables with the '=' operator. +#. The ability to define new variables. + +While the first item is really what this is about, we only have +variables for incoming arguments as well as for induction variables, and +redefining those only goes so far :). Also, the ability to define new +variables is a useful thing regardless of whether you will be mutating +them. Here's a motivating example that shows how we could use these: + +:: + + # Define ':' for sequencing: as a low-precedence operator that ignores operands + # and just returns the RHS. + def binary : 1 (x y) y; + + # Recursive fib, we could do this before. + def fib(x) + if (x < 3) then + 1 + else + fib(x-1)+fib(x-2); + + # Iterative fib. + def fibi(x) + var a = 1, b = 1, c in + (for i = 3, i < x in + c = a + b : + a = b : + b = c) : + b; + + # Call it. + fibi(10); + +In order to mutate variables, we have to change our existing variables +to use the "alloca trick". Once we have that, we'll add our new +operator, then extend Kaleidoscope to support new variable definitions. + +Adjusting Existing Variables for Mutation +========================================= + +The symbol table in Kaleidoscope is managed at code generation time by +the '``NamedValues``' map. This map currently keeps track of the LLVM +"Value\*" that holds the double value for the named variable. In order +to support mutation, we need to change this slightly, so that +``NamedValues`` holds the *memory location* of the variable in question. +Note that this change is a refactoring: it changes the structure of the +code, but does not (by itself) change the behavior of the compiler. All +of these changes are isolated in the Kaleidoscope code generator. + +At this point in Kaleidoscope's development, it only supports variables +for two things: incoming arguments to functions and the induction +variable of 'for' loops. For consistency, we'll allow mutation of these +variables in addition to other user-defined variables. This means that +these will both need memory locations. + +To start our transformation of Kaleidoscope, we'll change the +NamedValues map so that it maps to AllocaInst\* instead of Value\*. Once +we do this, the C++ compiler will tell us what parts of the code we need +to update: + +.. code-block:: c++ + + static std::map<std::string, AllocaInst*> NamedValues; + +Also, since we will need to create these allocas, we'll use a helper +function that ensures that the allocas are created in the entry block of +the function: + +.. code-block:: c++ + + /// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of + /// the function. This is used for mutable variables etc. + static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction, + const std::string &VarName) { + IRBuilder<> TmpB(&TheFunction->getEntryBlock(), + TheFunction->getEntryBlock().begin()); + return TmpB.CreateAlloca(Type::getDoubleTy(TheContext), 0, + VarName.c_str()); + } + +This funny looking code creates an IRBuilder object that is pointing at +the first instruction (.begin()) of the entry block. It then creates an +alloca with the expected name and returns it. Because all values in +Kaleidoscope are doubles, there is no need to pass in a type to use. + +With this in place, the first functionality change we want to make belongs to +variable references. In our new scheme, variables live on the stack, so +code generating a reference to them actually needs to produce a load +from the stack slot: + +.. code-block:: c++ + + Value *VariableExprAST::codegen() { + // Look this variable up in the function. + Value *V = NamedValues[Name]; + if (!V) + return LogErrorV("Unknown variable name"); + + // Load the value. + return Builder.CreateLoad(V, Name.c_str()); + } + +As you can see, this is pretty straightforward. Now we need to update +the things that define the variables to set up the alloca. We'll start +with ``ForExprAST::codegen()`` (see the `full code listing <#id1>`_ for +the unabridged code): + +.. code-block:: c++ + + Function *TheFunction = Builder.GetInsertBlock()->getParent(); + + // Create an alloca for the variable in the entry block. + AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); + + // Emit the start code first, without 'variable' in scope. + Value *StartVal = Start->codegen(); + if (!StartVal) + return nullptr; + + // Store the value into the alloca. + Builder.CreateStore(StartVal, Alloca); + ... + + // Compute the end condition. + Value *EndCond = End->codegen(); + if (!EndCond) + return nullptr; + + // Reload, increment, and restore the alloca. This handles the case where + // the body of the loop mutates the variable. + Value *CurVar = Builder.CreateLoad(Alloca); + Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar"); + Builder.CreateStore(NextVar, Alloca); + ... + +This code is virtually identical to the code `before we allowed mutable +variables <LangImpl5.html#code-generation-for-the-for-loop>`_. The big difference is that we +no longer have to construct a PHI node, and we use load/store to access +the variable as needed. + +To support mutable argument variables, we need to also make allocas for +them. The code for this is also pretty simple: + +.. code-block:: c++ + + Function *FunctionAST::codegen() { + ... + Builder.SetInsertPoint(BB); + + // Record the function arguments in the NamedValues map. + NamedValues.clear(); + for (auto &Arg : TheFunction->args()) { + // Create an alloca for this variable. + AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, Arg.getName()); + + // Store the initial value into the alloca. + Builder.CreateStore(&Arg, Alloca); + + // Add arguments to variable symbol table. + NamedValues[Arg.getName()] = Alloca; + } + + if (Value *RetVal = Body->codegen()) { + ... + +For each argument, we make an alloca, store the input value to the +function into the alloca, and register the alloca as the memory location +for the argument. This method gets invoked by ``FunctionAST::codegen()`` +right after it sets up the entry block for the function. + +The final missing piece is adding the mem2reg pass, which allows us to +get good codegen once again: + +.. code-block:: c++ + + // Promote allocas to registers. + TheFPM->add(createPromoteMemoryToRegisterPass()); + // Do simple "peephole" optimizations and bit-twiddling optzns. + TheFPM->add(createInstructionCombiningPass()); + // Reassociate expressions. + TheFPM->add(createReassociatePass()); + ... + +It is interesting to see what the code looks like before and after the +mem2reg optimization runs. For example, this is the before/after code +for our recursive fib function. Before the optimization: + +.. code-block:: llvm + + define double @fib(double %x) { + entry: + %x1 = alloca double + store double %x, double* %x1 + %x2 = load double, double* %x1 + %cmptmp = fcmp ult double %x2, 3.000000e+00 + %booltmp = uitofp i1 %cmptmp to double + %ifcond = fcmp one double %booltmp, 0.000000e+00 + br i1 %ifcond, label %then, label %else + + then: ; preds = %entry + br label %ifcont + + else: ; preds = %entry + %x3 = load double, double* %x1 + %subtmp = fsub double %x3, 1.000000e+00 + %calltmp = call double @fib(double %subtmp) + %x4 = load double, double* %x1 + %subtmp5 = fsub double %x4, 2.000000e+00 + %calltmp6 = call double @fib(double %subtmp5) + %addtmp = fadd double %calltmp, %calltmp6 + br label %ifcont + + ifcont: ; preds = %else, %then + %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] + ret double %iftmp + } + +Here there is only one variable (x, the input argument) but you can +still see the extremely simple-minded code generation strategy we are +using. In the entry block, an alloca is created, and the initial input +value is stored into it. Each reference to the variable does a reload +from the stack. Also, note that we didn't modify the if/then/else +expression, so it still inserts a PHI node. While we could make an +alloca for it, it is actually easier to create a PHI node for it, so we +still just make the PHI. + +Here is the code after the mem2reg pass runs: + +.. code-block:: llvm + + define double @fib(double %x) { + entry: + %cmptmp = fcmp ult double %x, 3.000000e+00 + %booltmp = uitofp i1 %cmptmp to double + %ifcond = fcmp one double %booltmp, 0.000000e+00 + br i1 %ifcond, label %then, label %else + + then: + br label %ifcont + + else: + %subtmp = fsub double %x, 1.000000e+00 + %calltmp = call double @fib(double %subtmp) + %subtmp5 = fsub double %x, 2.000000e+00 + %calltmp6 = call double @fib(double %subtmp5) + %addtmp = fadd double %calltmp, %calltmp6 + br label %ifcont + + ifcont: ; preds = %else, %then + %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] + ret double %iftmp + } + +This is a trivial case for mem2reg, since there are no redefinitions of +the variable. The point of showing this is to calm your tension about +inserting such blatent inefficiencies :). + +After the rest of the optimizers run, we get: + +.. code-block:: llvm + + define double @fib(double %x) { + entry: + %cmptmp = fcmp ult double %x, 3.000000e+00 + %booltmp = uitofp i1 %cmptmp to double + %ifcond = fcmp ueq double %booltmp, 0.000000e+00 + br i1 %ifcond, label %else, label %ifcont + + else: + %subtmp = fsub double %x, 1.000000e+00 + %calltmp = call double @fib(double %subtmp) + %subtmp5 = fsub double %x, 2.000000e+00 + %calltmp6 = call double @fib(double %subtmp5) + %addtmp = fadd double %calltmp, %calltmp6 + ret double %addtmp + + ifcont: + ret double 1.000000e+00 + } + +Here we see that the simplifycfg pass decided to clone the return +instruction into the end of the 'else' block. This allowed it to +eliminate some branches and the PHI node. + +Now that all symbol table references are updated to use stack variables, +we'll add the assignment operator. + +New Assignment Operator +======================= + +With our current framework, adding a new assignment operator is really +simple. We will parse it just like any other binary operator, but handle +it internally (instead of allowing the user to define it). The first +step is to set a precedence: + +.. code-block:: c++ + + int main() { + // Install standard binary operators. + // 1 is lowest precedence. + BinopPrecedence['='] = 2; + BinopPrecedence['<'] = 10; + BinopPrecedence['+'] = 20; + BinopPrecedence['-'] = 20; + +Now that the parser knows the precedence of the binary operator, it +takes care of all the parsing and AST generation. We just need to +implement codegen for the assignment operator. This looks like: + +.. code-block:: c++ + + Value *BinaryExprAST::codegen() { + // Special case '=' because we don't want to emit the LHS as an expression. + if (Op == '=') { + // Assignment requires the LHS to be an identifier. + VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS.get()); + if (!LHSE) + return LogErrorV("destination of '=' must be a variable"); + +Unlike the rest of the binary operators, our assignment operator doesn't +follow the "emit LHS, emit RHS, do computation" model. As such, it is +handled as a special case before the other binary operators are handled. +The other strange thing is that it requires the LHS to be a variable. It +is invalid to have "(x+1) = expr" - only things like "x = expr" are +allowed. + +.. code-block:: c++ + + // Codegen the RHS. + Value *Val = RHS->codegen(); + if (!Val) + return nullptr; + + // Look up the name. + Value *Variable = NamedValues[LHSE->getName()]; + if (!Variable) + return LogErrorV("Unknown variable name"); + + Builder.CreateStore(Val, Variable); + return Val; + } + ... + +Once we have the variable, codegen'ing the assignment is +straightforward: we emit the RHS of the assignment, create a store, and +return the computed value. Returning a value allows for chained +assignments like "X = (Y = Z)". + +Now that we have an assignment operator, we can mutate loop variables +and arguments. For example, we can now run code like this: + +:: + + # Function to print a double. + extern printd(x); + + # Define ':' for sequencing: as a low-precedence operator that ignores operands + # and just returns the RHS. + def binary : 1 (x y) y; + + def test(x) + printd(x) : + x = 4 : + printd(x); + + test(123); + +When run, this example prints "123" and then "4", showing that we did +actually mutate the value! Okay, we have now officially implemented our +goal: getting this to work requires SSA construction in the general +case. However, to be really useful, we want the ability to define our +own local variables, let's add this next! + +User-defined Local Variables +============================ + +Adding var/in is just like any other extension we made to +Kaleidoscope: we extend the lexer, the parser, the AST and the code +generator. The first step for adding our new 'var/in' construct is to +extend the lexer. As before, this is pretty trivial, the code looks like +this: + +.. code-block:: c++ + + enum Token { + ... + // var definition + tok_var = -13 + ... + } + ... + static int gettok() { + ... + if (IdentifierStr == "in") + return tok_in; + if (IdentifierStr == "binary") + return tok_binary; + if (IdentifierStr == "unary") + return tok_unary; + if (IdentifierStr == "var") + return tok_var; + return tok_identifier; + ... + +The next step is to define the AST node that we will construct. For +var/in, it looks like this: + +.. code-block:: c++ + + /// VarExprAST - Expression class for var/in + class VarExprAST : public ExprAST { + std::vector<std::pair<std::string, std::unique_ptr<ExprAST>>> VarNames; + std::unique_ptr<ExprAST> Body; + + public: + VarExprAST(std::vector<std::pair<std::string, std::unique_ptr<ExprAST>>> VarNames, + std::unique_ptr<ExprAST> Body) + : VarNames(std::move(VarNames)), Body(std::move(Body)) {} + + Value *codegen() override; + }; + +var/in allows a list of names to be defined all at once, and each name +can optionally have an initializer value. As such, we capture this +information in the VarNames vector. Also, var/in has a body, this body +is allowed to access the variables defined by the var/in. + +With this in place, we can define the parser pieces. The first thing we +do is add it as a primary expression: + +.. code-block:: c++ + + /// primary + /// ::= identifierexpr + /// ::= numberexpr + /// ::= parenexpr + /// ::= ifexpr + /// ::= forexpr + /// ::= varexpr + static std::unique_ptr<ExprAST> ParsePrimary() { + switch (CurTok) { + default: + return LogError("unknown token when expecting an expression"); + case tok_identifier: + return ParseIdentifierExpr(); + case tok_number: + return ParseNumberExpr(); + case '(': + return ParseParenExpr(); + case tok_if: + return ParseIfExpr(); + case tok_for: + return ParseForExpr(); + case tok_var: + return ParseVarExpr(); + } + } + +Next we define ParseVarExpr: + +.. code-block:: c++ + + /// varexpr ::= 'var' identifier ('=' expression)? + // (',' identifier ('=' expression)?)* 'in' expression + static std::unique_ptr<ExprAST> ParseVarExpr() { + getNextToken(); // eat the var. + + std::vector<std::pair<std::string, std::unique_ptr<ExprAST>>> VarNames; + + // At least one variable name is required. + if (CurTok != tok_identifier) + return LogError("expected identifier after var"); + +The first part of this code parses the list of identifier/expr pairs +into the local ``VarNames`` vector. + +.. code-block:: c++ + + while (1) { + std::string Name = IdentifierStr; + getNextToken(); // eat identifier. + + // Read the optional initializer. + std::unique_ptr<ExprAST> Init; + if (CurTok == '=') { + getNextToken(); // eat the '='. + + Init = ParseExpression(); + if (!Init) return nullptr; + } + + VarNames.push_back(std::make_pair(Name, std::move(Init))); + + // End of var list, exit loop. + if (CurTok != ',') break; + getNextToken(); // eat the ','. + + if (CurTok != tok_identifier) + return LogError("expected identifier list after var"); + } + +Once all the variables are parsed, we then parse the body and create the +AST node: + +.. code-block:: c++ + + // At this point, we have to have 'in'. + if (CurTok != tok_in) + return LogError("expected 'in' keyword after 'var'"); + getNextToken(); // eat 'in'. + + auto Body = ParseExpression(); + if (!Body) + return nullptr; + + return llvm::make_unique<VarExprAST>(std::move(VarNames), + std::move(Body)); + } + +Now that we can parse and represent the code, we need to support +emission of LLVM IR for it. This code starts out with: + +.. code-block:: c++ + + Value *VarExprAST::codegen() { + std::vector<AllocaInst *> OldBindings; + + Function *TheFunction = Builder.GetInsertBlock()->getParent(); + + // Register all variables and emit their initializer. + for (unsigned i = 0, e = VarNames.size(); i != e; ++i) { + const std::string &VarName = VarNames[i].first; + ExprAST *Init = VarNames[i].second.get(); + +Basically it loops over all the variables, installing them one at a +time. For each variable we put into the symbol table, we remember the +previous value that we replace in OldBindings. + +.. code-block:: c++ + + // Emit the initializer before adding the variable to scope, this prevents + // the initializer from referencing the variable itself, and permits stuff + // like this: + // var a = 1 in + // var a = a in ... # refers to outer 'a'. + Value *InitVal; + if (Init) { + InitVal = Init->codegen(); + if (!InitVal) + return nullptr; + } else { // If not specified, use 0.0. + InitVal = ConstantFP::get(TheContext, APFloat(0.0)); + } + + AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); + Builder.CreateStore(InitVal, Alloca); + + // Remember the old variable binding so that we can restore the binding when + // we unrecurse. + OldBindings.push_back(NamedValues[VarName]); + + // Remember this binding. + NamedValues[VarName] = Alloca; + } + +There are more comments here than code. The basic idea is that we emit +the initializer, create the alloca, then update the symbol table to +point to it. Once all the variables are installed in the symbol table, +we evaluate the body of the var/in expression: + +.. code-block:: c++ + + // Codegen the body, now that all vars are in scope. + Value *BodyVal = Body->codegen(); + if (!BodyVal) + return nullptr; + +Finally, before returning, we restore the previous variable bindings: + +.. code-block:: c++ + + // Pop all our variables from scope. + for (unsigned i = 0, e = VarNames.size(); i != e; ++i) + NamedValues[VarNames[i].first] = OldBindings[i]; + + // Return the body computation. + return BodyVal; + } + +The end result of all of this is that we get properly scoped variable +definitions, and we even (trivially) allow mutation of them :). + +With this, we completed what we set out to do. Our nice iterative fib +example from the intro compiles and runs just fine. The mem2reg pass +optimizes all of our stack variables into SSA registers, inserting PHI +nodes where needed, and our front-end remains simple: no "iterated +dominance frontier" computation anywhere in sight. + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +mutable variables and var/in support. To build this example, use: + +.. code-block:: bash + + # Compile + clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy + # Run + ./toy + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter7/toy.cpp + :language: c++ + +`Next: Compiling to Object Code <LangImpl08.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl08.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl08.rst new file mode 100644 index 00000000000..da4e60f84b8 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl08.rst @@ -0,0 +1,218 @@ +======================================== + Kaleidoscope: Compiling to Object Code +======================================== + +.. contents:: + :local: + +Chapter 8 Introduction +====================== + +Welcome to Chapter 8 of the "`Implementing a language with LLVM +<index.html>`_" tutorial. This chapter describes how to compile our +language down to object files. + +Choosing a target +================= + +LLVM has native support for cross-compilation. You can compile to the +architecture of your current machine, or just as easily compile for +other architectures. In this tutorial, we'll target the current +machine. + +To specify the architecture that you want to target, we use a string +called a "target triple". This takes the form +``<arch><sub>-<vendor>-<sys>-<abi>`` (see the `cross compilation docs +<http://clang.llvm.org/docs/CrossCompilation.html#target-triple>`_). + +As an example, we can see what clang thinks is our current target +triple: + +:: + + $ clang --version | grep Target + Target: x86_64-unknown-linux-gnu + +Running this command may show something different on your machine as +you might be using a different architecture or operating system to me. + +Fortunately, we don't need to hard-code a target triple to target the +current machine. LLVM provides ``sys::getDefaultTargetTriple``, which +returns the target triple of the current machine. + +.. code-block:: c++ + + auto TargetTriple = sys::getDefaultTargetTriple(); + +LLVM doesn't require us to link in all the target +functionality. For example, if we're just using the JIT, we don't need +the assembly printers. Similarly, if we're only targeting certain +architectures, we can only link in the functionality for those +architectures. + +For this example, we'll initialize all the targets for emitting object +code. + +.. code-block:: c++ + + InitializeAllTargetInfos(); + InitializeAllTargets(); + InitializeAllTargetMCs(); + InitializeAllAsmParsers(); + InitializeAllAsmPrinters(); + +We can now use our target triple to get a ``Target``: + +.. code-block:: c++ + + std::string Error; + auto Target = TargetRegistry::lookupTarget(TargetTriple, Error); + + // Print an error and exit if we couldn't find the requested target. + // This generally occurs if we've forgotten to initialise the + // TargetRegistry or we have a bogus target triple. + if (!Target) { + errs() << Error; + return 1; + } + +Target Machine +============== + +We will also need a ``TargetMachine``. This class provides a complete +machine description of the machine we're targeting. If we want to +target a specific feature (such as SSE) or a specific CPU (such as +Intel's Sandylake), we do so now. + +To see which features and CPUs that LLVM knows about, we can use +``llc``. For example, let's look at x86: + +:: + + $ llvm-as < /dev/null | llc -march=x86 -mattr=help + Available CPUs for this target: + + amdfam10 - Select the amdfam10 processor. + athlon - Select the athlon processor. + athlon-4 - Select the athlon-4 processor. + ... + + Available features for this target: + + 16bit-mode - 16-bit mode (i8086). + 32bit-mode - 32-bit mode (80386). + 3dnow - Enable 3DNow! instructions. + 3dnowa - Enable 3DNow! Athlon instructions. + ... + +For our example, we'll use the generic CPU without any additional +features, options or relocation model. + +.. code-block:: c++ + + auto CPU = "generic"; + auto Features = ""; + + TargetOptions opt; + auto RM = Optional<Reloc::Model>(); + auto TargetMachine = Target->createTargetMachine(TargetTriple, CPU, Features, opt, RM); + + +Configuring the Module +====================== + +We're now ready to configure our module, to specify the target and +data layout. This isn't strictly necessary, but the `frontend +performance guide <../Frontend/PerformanceTips.html>`_ recommends +this. Optimizations benefit from knowing about the target and data +layout. + +.. code-block:: c++ + + TheModule->setDataLayout(TargetMachine->createDataLayout()); + TheModule->setTargetTriple(TargetTriple); + +Emit Object Code +================ + +We're ready to emit object code! Let's define where we want to write +our file to: + +.. code-block:: c++ + + auto Filename = "output.o"; + std::error_code EC; + raw_fd_ostream dest(Filename, EC, sys::fs::F_None); + + if (EC) { + errs() << "Could not open file: " << EC.message(); + return 1; + } + +Finally, we define a pass that emits object code, then we run that +pass: + +.. code-block:: c++ + + legacy::PassManager pass; + auto FileType = TargetMachine::CGFT_ObjectFile; + + if (TargetMachine->addPassesToEmitFile(pass, dest, FileType)) { + errs() << "TargetMachine can't emit a file of this type"; + return 1; + } + + pass.run(*TheModule); + dest.flush(); + +Putting It All Together +======================= + +Does it work? Let's give it a try. We need to compile our code, but +note that the arguments to ``llvm-config`` are different to the previous chapters. + +:: + + $ clang++ -g -O3 toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs all` -o toy + +Let's run it, and define a simple ``average`` function. Press Ctrl-D +when you're done. + +:: + + $ ./toy + ready> def average(x y) (x + y) * 0.5; + ^D + Wrote output.o + +We have an object file! To test it, let's write a simple program and +link it with our output. Here's the source code: + +.. code-block:: c++ + + #include <iostream> + + extern "C" { + double average(double, double); + } + + int main() { + std::cout << "average of 3.0 and 4.0: " << average(3.0, 4.0) << std::endl; + } + +We link our program to output.o and check the result is what we +expected: + +:: + + $ clang++ main.cpp output.o -o main + $ ./main + average of 3.0 and 4.0: 3.5 + +Full Code Listing +================= + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter8/toy.cpp + :language: c++ + +`Next: Adding Debug Information <LangImpl09.html>`_ diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl09.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl09.rst new file mode 100644 index 00000000000..d81f9fa0001 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl09.rst @@ -0,0 +1,465 @@ +====================================== +Kaleidoscope: Adding Debug Information +====================================== + +.. contents:: + :local: + +Chapter 9 Introduction +====================== + +Welcome to Chapter 9 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. In chapters 1 through 8, we've built a +decent little programming language with functions and variables. +What happens if something goes wrong though, how do you debug your +program? + +Source level debugging uses formatted data that helps a debugger +translate from binary and the state of the machine back to the +source that the programmer wrote. In LLVM we generally use a format +called `DWARF <http://dwarfstd.org>`_. DWARF is a compact encoding +that represents types, source locations, and variable locations. + +The short summary of this chapter is that we'll go through the +various things you have to add to a programming language to +support debug info, and how you translate that into DWARF. + +Caveat: For now we can't debug via the JIT, so we'll need to compile +our program down to something small and standalone. As part of this +we'll make a few modifications to the running of the language and +how programs are compiled. This means that we'll have a source file +with a simple program written in Kaleidoscope rather than the +interactive JIT. It does involve a limitation that we can only +have one "top level" command at a time to reduce the number of +changes necessary. + +Here's the sample program we'll be compiling: + +.. code-block:: python + + def fib(x) + if x < 3 then + 1 + else + fib(x-1)+fib(x-2); + + fib(10) + + +Why is this a hard problem? +=========================== + +Debug information is a hard problem for a few different reasons - mostly +centered around optimized code. First, optimization makes keeping source +locations more difficult. In LLVM IR we keep the original source location +for each IR level instruction on the instruction. Optimization passes +should keep the source locations for newly created instructions, but merged +instructions only get to keep a single location - this can cause jumping +around when stepping through optimized programs. Secondly, optimization +can move variables in ways that are either optimized out, shared in memory +with other variables, or difficult to track. For the purposes of this +tutorial we're going to avoid optimization (as you'll see with one of the +next sets of patches). + +Ahead-of-Time Compilation Mode +============================== + +To highlight only the aspects of adding debug information to a source +language without needing to worry about the complexities of JIT debugging +we're going to make a few changes to Kaleidoscope to support compiling +the IR emitted by the front end into a simple standalone program that +you can execute, debug, and see results. + +First we make our anonymous function that contains our top level +statement be our "main": + +.. code-block:: udiff + + - auto Proto = llvm::make_unique<PrototypeAST>("", std::vector<std::string>()); + + auto Proto = llvm::make_unique<PrototypeAST>("main", std::vector<std::string>()); + +just with the simple change of giving it a name. + +Then we're going to remove the command line code wherever it exists: + +.. code-block:: udiff + + @@ -1129,7 +1129,6 @@ static void HandleTopLevelExpression() { + /// top ::= definition | external | expression | ';' + static void MainLoop() { + while (1) { + - fprintf(stderr, "ready> "); + switch (CurTok) { + case tok_eof: + return; + @@ -1184,7 +1183,6 @@ int main() { + BinopPrecedence['*'] = 40; // highest. + + // Prime the first token. + - fprintf(stderr, "ready> "); + getNextToken(); + +Lastly we're going to disable all of the optimization passes and the JIT so +that the only thing that happens after we're done parsing and generating +code is that the LLVM IR goes to standard error: + +.. code-block:: udiff + + @@ -1108,17 +1108,8 @@ static void HandleExtern() { + static void HandleTopLevelExpression() { + // Evaluate a top-level expression into an anonymous function. + if (auto FnAST = ParseTopLevelExpr()) { + - if (auto *FnIR = FnAST->codegen()) { + - // We're just doing this to make sure it executes. + - TheExecutionEngine->finalizeObject(); + - // JIT the function, returning a function pointer. + - void *FPtr = TheExecutionEngine->getPointerToFunction(FnIR); + - + - // Cast it to the right type (takes no arguments, returns a double) so we + - // can call it as a native function. + - double (*FP)() = (double (*)())(intptr_t)FPtr; + - // Ignore the return value for this. + - (void)FP; + + if (!F->codegen()) { + + fprintf(stderr, "Error generating code for top level expr"); + } + } else { + // Skip token for error recovery. + @@ -1439,11 +1459,11 @@ int main() { + // target lays out data structures. + TheModule->setDataLayout(TheExecutionEngine->getDataLayout()); + OurFPM.add(new DataLayoutPass()); + +#if 0 + OurFPM.add(createBasicAliasAnalysisPass()); + // Promote allocas to registers. + OurFPM.add(createPromoteMemoryToRegisterPass()); + @@ -1218,7 +1210,7 @@ int main() { + OurFPM.add(createGVNPass()); + // Simplify the control flow graph (deleting unreachable blocks, etc). + OurFPM.add(createCFGSimplificationPass()); + - + + #endif + OurFPM.doInitialization(); + + // Set the global so the code gen can use this. + +This relatively small set of changes get us to the point that we can compile +our piece of Kaleidoscope language down to an executable program via this +command line: + +.. code-block:: bash + + Kaleidoscope-Ch9 < fib.ks | & clang -x ir - + +which gives an a.out/a.exe in the current working directory. + +Compile Unit +============ + +The top level container for a section of code in DWARF is a compile unit. +This contains the type and function data for an individual translation unit +(read: one file of source code). So the first thing we need to do is +construct one for our fib.ks file. + +DWARF Emission Setup +==================== + +Similar to the ``IRBuilder`` class we have a +`DIBuilder <http://llvm.org/doxygen/classllvm_1_1DIBuilder.html>`_ class +that helps in constructing debug metadata for an LLVM IR file. It +corresponds 1:1 similarly to ``IRBuilder`` and LLVM IR, but with nicer names. +Using it does require that you be more familiar with DWARF terminology than +you needed to be with ``IRBuilder`` and ``Instruction`` names, but if you +read through the general documentation on the +`Metadata Format <http://llvm.org/docs/SourceLevelDebugging.html>`_ it +should be a little more clear. We'll be using this class to construct all +of our IR level descriptions. Construction for it takes a module so we +need to construct it shortly after we construct our module. We've left it +as a global static variable to make it a bit easier to use. + +Next we're going to create a small container to cache some of our frequent +data. The first will be our compile unit, but we'll also write a bit of +code for our one type since we won't have to worry about multiple typed +expressions: + +.. code-block:: c++ + + static DIBuilder *DBuilder; + + struct DebugInfo { + DICompileUnit *TheCU; + DIType *DblTy; + + DIType *getDoubleTy(); + } KSDbgInfo; + + DIType *DebugInfo::getDoubleTy() { + if (DblTy) + return DblTy; + + DblTy = DBuilder->createBasicType("double", 64, dwarf::DW_ATE_float); + return DblTy; + } + +And then later on in ``main`` when we're constructing our module: + +.. code-block:: c++ + + DBuilder = new DIBuilder(*TheModule); + + KSDbgInfo.TheCU = DBuilder->createCompileUnit( + dwarf::DW_LANG_C, DBuilder->createFile("fib.ks", "."), + "Kaleidoscope Compiler", 0, "", 0); + +There are a couple of things to note here. First, while we're producing a +compile unit for a language called Kaleidoscope we used the language +constant for C. This is because a debugger wouldn't necessarily understand +the calling conventions or default ABI for a language it doesn't recognize +and we follow the C ABI in our LLVM code generation so it's the closest +thing to accurate. This ensures we can actually call functions from the +debugger and have them execute. Secondly, you'll see the "fib.ks" in the +call to ``createCompileUnit``. This is a default hard coded value since +we're using shell redirection to put our source into the Kaleidoscope +compiler. In a usual front end you'd have an input file name and it would +go there. + +One last thing as part of emitting debug information via DIBuilder is that +we need to "finalize" the debug information. The reasons are part of the +underlying API for DIBuilder, but make sure you do this near the end of +main: + +.. code-block:: c++ + + DBuilder->finalize(); + +before you dump out the module. + +Functions +========= + +Now that we have our ``Compile Unit`` and our source locations, we can add +function definitions to the debug info. So in ``PrototypeAST::codegen()`` we +add a few lines of code to describe a context for our subprogram, in this +case the "File", and the actual definition of the function itself. + +So the context: + +.. code-block:: c++ + + DIFile *Unit = DBuilder->createFile(KSDbgInfo.TheCU.getFilename(), + KSDbgInfo.TheCU.getDirectory()); + +giving us an DIFile and asking the ``Compile Unit`` we created above for the +directory and filename where we are currently. Then, for now, we use some +source locations of 0 (since our AST doesn't currently have source location +information) and construct our function definition: + +.. code-block:: c++ + + DIScope *FContext = Unit; + unsigned LineNo = 0; + unsigned ScopeLine = 0; + DISubprogram *SP = DBuilder->createFunction( + FContext, P.getName(), StringRef(), Unit, LineNo, + CreateFunctionType(TheFunction->arg_size(), Unit), + false /* internal linkage */, true /* definition */, ScopeLine, + DINode::FlagPrototyped, false); + TheFunction->setSubprogram(SP); + +and we now have an DISubprogram that contains a reference to all of our +metadata for the function. + +Source Locations +================ + +The most important thing for debug information is accurate source location - +this makes it possible to map your source code back. We have a problem though, +Kaleidoscope really doesn't have any source location information in the lexer +or parser so we'll need to add it. + +.. code-block:: c++ + + struct SourceLocation { + int Line; + int Col; + }; + static SourceLocation CurLoc; + static SourceLocation LexLoc = {1, 0}; + + static int advance() { + int LastChar = getchar(); + + if (LastChar == '\n' || LastChar == '\r') { + LexLoc.Line++; + LexLoc.Col = 0; + } else + LexLoc.Col++; + return LastChar; + } + +In this set of code we've added some functionality on how to keep track of the +line and column of the "source file". As we lex every token we set our current +current "lexical location" to the assorted line and column for the beginning +of the token. We do this by overriding all of the previous calls to +``getchar()`` with our new ``advance()`` that keeps track of the information +and then we have added to all of our AST classes a source location: + +.. code-block:: c++ + + class ExprAST { + SourceLocation Loc; + + public: + ExprAST(SourceLocation Loc = CurLoc) : Loc(Loc) {} + virtual ~ExprAST() {} + virtual Value* codegen() = 0; + int getLine() const { return Loc.Line; } + int getCol() const { return Loc.Col; } + virtual raw_ostream &dump(raw_ostream &out, int ind) { + return out << ':' << getLine() << ':' << getCol() << '\n'; + } + +that we pass down through when we create a new expression: + +.. code-block:: c++ + + LHS = llvm::make_unique<BinaryExprAST>(BinLoc, BinOp, std::move(LHS), + std::move(RHS)); + +giving us locations for each of our expressions and variables. + +To make sure that every instruction gets proper source location information, +we have to tell ``Builder`` whenever we're at a new source location. +We use a small helper function for this: + +.. code-block:: c++ + + void DebugInfo::emitLocation(ExprAST *AST) { + DIScope *Scope; + if (LexicalBlocks.empty()) + Scope = TheCU; + else + Scope = LexicalBlocks.back(); + Builder.SetCurrentDebugLocation( + DebugLoc::get(AST->getLine(), AST->getCol(), Scope)); + } + +This both tells the main ``IRBuilder`` where we are, but also what scope +we're in. The scope can either be on compile-unit level or be the nearest +enclosing lexical block like the current function. +To represent this we create a stack of scopes: + +.. code-block:: c++ + + std::vector<DIScope *> LexicalBlocks; + +and push the scope (function) to the top of the stack when we start +generating the code for each function: + +.. code-block:: c++ + + KSDbgInfo.LexicalBlocks.push_back(SP); + +Also, we may not forget to pop the scope back off of the scope stack at the +end of the code generation for the function: + +.. code-block:: c++ + + // Pop off the lexical block for the function since we added it + // unconditionally. + KSDbgInfo.LexicalBlocks.pop_back(); + +Then we make sure to emit the location every time we start to generate code +for a new AST object: + +.. code-block:: c++ + + KSDbgInfo.emitLocation(this); + +Variables +========= + +Now that we have functions, we need to be able to print out the variables +we have in scope. Let's get our function arguments set up so we can get +decent backtraces and see how our functions are being called. It isn't +a lot of code, and we generally handle it when we're creating the +argument allocas in ``FunctionAST::codegen``. + +.. code-block:: c++ + + // Record the function arguments in the NamedValues map. + NamedValues.clear(); + unsigned ArgIdx = 0; + for (auto &Arg : TheFunction->args()) { + // Create an alloca for this variable. + AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, Arg.getName()); + + // Create a debug descriptor for the variable. + DILocalVariable *D = DBuilder->createParameterVariable( + SP, Arg.getName(), ++ArgIdx, Unit, LineNo, KSDbgInfo.getDoubleTy(), + true); + + DBuilder->insertDeclare(Alloca, D, DBuilder->createExpression(), + DebugLoc::get(LineNo, 0, SP), + Builder.GetInsertBlock()); + + // Store the initial value into the alloca. + Builder.CreateStore(&Arg, Alloca); + + // Add arguments to variable symbol table. + NamedValues[Arg.getName()] = Alloca; + } + + +Here we're first creating the variable, giving it the scope (``SP``), +the name, source location, type, and since it's an argument, the argument +index. Next, we create an ``lvm.dbg.declare`` call to indicate at the IR +level that we've got a variable in an alloca (and it gives a starting +location for the variable), and setting a source location for the +beginning of the scope on the declare. + +One interesting thing to note at this point is that various debuggers have +assumptions based on how code and debug information was generated for them +in the past. In this case we need to do a little bit of a hack to avoid +generating line information for the function prologue so that the debugger +knows to skip over those instructions when setting a breakpoint. So in +``FunctionAST::CodeGen`` we add some more lines: + +.. code-block:: c++ + + // Unset the location for the prologue emission (leading instructions with no + // location in a function are considered part of the prologue and the debugger + // will run past them when breaking on a function) + KSDbgInfo.emitLocation(nullptr); + +and then emit a new location when we actually start generating code for the +body of the function: + +.. code-block:: c++ + + KSDbgInfo.emitLocation(Body.get()); + +With this we have enough debug information to set breakpoints in functions, +print out argument variables, and call functions. Not too bad for just a +few simple lines of code! + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +debug information. To build this example, use: + +.. code-block:: bash + + # Compile + clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy + # Run + ./toy + +Here is the code: + +.. literalinclude:: ../../examples/Kaleidoscope/Chapter9/toy.cpp + :language: c++ + +`Next: Conclusion and other useful LLVM tidbits <LangImpl10.html>`_ + diff --git a/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl10.rst b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl10.rst new file mode 100644 index 00000000000..b1d19c2cdd8 --- /dev/null +++ b/llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl10.rst @@ -0,0 +1,254 @@ +====================================================== +Kaleidoscope: Conclusion and other useful LLVM tidbits +====================================================== + +.. contents:: + :local: + +Tutorial Conclusion +=================== + +Welcome to the final chapter of the "`Implementing a language with +LLVM <index.html>`_" tutorial. In the course of this tutorial, we have +grown our little Kaleidoscope language from being a useless toy, to +being a semi-interesting (but probably still useless) toy. :) + +It is interesting to see how far we've come, and how little code it has +taken. We built the entire lexer, parser, AST, code generator, an +interactive run-loop (with a JIT!), and emitted debug information in +standalone executables - all in under 1000 lines of (non-comment/non-blank) +code. + +Our little language supports a couple of interesting features: it +supports user defined binary and unary operators, it uses JIT +compilation for immediate evaluation, and it supports a few control flow +constructs with SSA construction. + +Part of the idea of this tutorial was to show you how easy and fun it +can be to define, build, and play with languages. Building a compiler +need not be a scary or mystical process! Now that you've seen some of +the basics, I strongly encourage you to take the code and hack on it. +For example, try adding: + +- **global variables** - While global variables have questional value + in modern software engineering, they are often useful when putting + together quick little hacks like the Kaleidoscope compiler itself. + Fortunately, our current setup makes it very easy to add global + variables: just have value lookup check to see if an unresolved + variable is in the global variable symbol table before rejecting it. + To create a new global variable, make an instance of the LLVM + ``GlobalVariable`` class. +- **typed variables** - Kaleidoscope currently only supports variables + of type double. This gives the language a very nice elegance, because + only supporting one type means that you never have to specify types. + Different languages have different ways of handling this. The easiest + way is to require the user to specify types for every variable + definition, and record the type of the variable in the symbol table + along with its Value\*. +- **arrays, structs, vectors, etc** - Once you add types, you can start + extending the type system in all sorts of interesting ways. Simple + arrays are very easy and are quite useful for many different + applications. Adding them is mostly an exercise in learning how the + LLVM `getelementptr <../LangRef.html#getelementptr-instruction>`_ instruction + works: it is so nifty/unconventional, it `has its own + FAQ <../GetElementPtr.html>`_! +- **standard runtime** - Our current language allows the user to access + arbitrary external functions, and we use it for things like "printd" + and "putchard". As you extend the language to add higher-level + constructs, often these constructs make the most sense if they are + lowered to calls into a language-supplied runtime. For example, if + you add hash tables to the language, it would probably make sense to + add the routines to a runtime, instead of inlining them all the way. +- **memory management** - Currently we can only access the stack in + Kaleidoscope. It would also be useful to be able to allocate heap + memory, either with calls to the standard libc malloc/free interface + or with a garbage collector. If you would like to use garbage + collection, note that LLVM fully supports `Accurate Garbage + Collection <../GarbageCollection.html>`_ including algorithms that + move objects and need to scan/update the stack. +- **exception handling support** - LLVM supports generation of `zero + cost exceptions <../ExceptionHandling.html>`_ which interoperate with + code compiled in other languages. You could also generate code by + implicitly making every function return an error value and checking + it. You could also make explicit use of setjmp/longjmp. There are + many different ways to go here. +- **object orientation, generics, database access, complex numbers, + geometric programming, ...** - Really, there is no end of crazy + features that you can add to the language. +- **unusual domains** - We've been talking about applying LLVM to a + domain that many people are interested in: building a compiler for a + specific language. However, there are many other domains that can use + compiler technology that are not typically considered. For example, + LLVM has been used to implement OpenGL graphics acceleration, + translate C++ code to ActionScript, and many other cute and clever + things. Maybe you will be the first to JIT compile a regular + expression interpreter into native code with LLVM? + +Have fun - try doing something crazy and unusual. Building a language +like everyone else always has, is much less fun than trying something a +little crazy or off the wall and seeing how it turns out. If you get +stuck or want to talk about it, feel free to email the `llvm-dev mailing +list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_: it has lots +of people who are interested in languages and are often willing to help +out. + +Before we end this tutorial, I want to talk about some "tips and tricks" +for generating LLVM IR. These are some of the more subtle things that +may not be obvious, but are very useful if you want to take advantage of +LLVM's capabilities. + +Properties of the LLVM IR +========================= + +We have a couple of common questions about code in the LLVM IR form - +let's just get these out of the way right now, shall we? + +Target Independence +------------------- + +Kaleidoscope is an example of a "portable language": any program written +in Kaleidoscope will work the same way on any target that it runs on. +Many other languages have this property, e.g. lisp, java, haskell, +javascript, python, etc (note that while these languages are portable, +not all their libraries are). + +One nice aspect of LLVM is that it is often capable of preserving target +independence in the IR: you can take the LLVM IR for a +Kaleidoscope-compiled program and run it on any target that LLVM +supports, even emitting C code and compiling that on targets that LLVM +doesn't support natively. You can trivially tell that the Kaleidoscope +compiler generates target-independent code because it never queries for +any target-specific information when generating code. + +The fact that LLVM provides a compact, target-independent, +representation for code gets a lot of people excited. Unfortunately, +these people are usually thinking about C or a language from the C +family when they are asking questions about language portability. I say +"unfortunately", because there is really no way to make (fully general) +C code portable, other than shipping the source code around (and of +course, C source code is not actually portable in general either - ever +port a really old application from 32- to 64-bits?). + +The problem with C (again, in its full generality) is that it is heavily +laden with target specific assumptions. As one simple example, the +preprocessor often destructively removes target-independence from the +code when it processes the input text: + +.. code-block:: c + + #ifdef __i386__ + int X = 1; + #else + int X = 42; + #endif + +While it is possible to engineer more and more complex solutions to +problems like this, it cannot be solved in full generality in a way that +is better than shipping the actual source code. + +That said, there are interesting subsets of C that can be made portable. +If you are willing to fix primitive types to a fixed size (say int = +32-bits, and long = 64-bits), don't care about ABI compatibility with +existing binaries, and are willing to give up some other minor features, +you can have portable code. This can make sense for specialized domains +such as an in-kernel language. + +Safety Guarantees +----------------- + +Many of the languages above are also "safe" languages: it is impossible +for a program written in Java to corrupt its address space and crash the +process (assuming the JVM has no bugs). Safety is an interesting +property that requires a combination of language design, runtime +support, and often operating system support. + +It is certainly possible to implement a safe language in LLVM, but LLVM +IR does not itself guarantee safety. The LLVM IR allows unsafe pointer +casts, use after free bugs, buffer over-runs, and a variety of other +problems. Safety needs to be implemented as a layer on top of LLVM and, +conveniently, several groups have investigated this. Ask on the `llvm-dev +mailing list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ if +you are interested in more details. + +Language-Specific Optimizations +------------------------------- + +One thing about LLVM that turns off many people is that it does not +solve all the world's problems in one system. One specific +complaint is that people perceive LLVM as being incapable of performing +high-level language-specific optimization: LLVM "loses too much +information". Here are a few observations about this: + +First, you're right that LLVM does lose information. For example, as of +this writing, there is no way to distinguish in the LLVM IR whether an +SSA-value came from a C "int" or a C "long" on an ILP32 machine (other +than debug info). Both get compiled down to an 'i32' value and the +information about what it came from is lost. The more general issue +here, is that the LLVM type system uses "structural equivalence" instead +of "name equivalence". Another place this surprises people is if you +have two types in a high-level language that have the same structure +(e.g. two different structs that have a single int field): these types +will compile down into a single LLVM type and it will be impossible to +tell what it came from. + +Second, while LLVM does lose information, LLVM is not a fixed target: we +continue to enhance and improve it in many different ways. In addition +to adding new features (LLVM did not always support exceptions or debug +info), we also extend the IR to capture important information for +optimization (e.g. whether an argument is sign or zero extended, +information about pointers aliasing, etc). Many of the enhancements are +user-driven: people want LLVM to include some specific feature, so they +go ahead and extend it. + +Third, it is *possible and easy* to add language-specific optimizations, +and you have a number of choices in how to do it. As one trivial +example, it is easy to add language-specific optimization passes that +"know" things about code compiled for a language. In the case of the C +family, there is an optimization pass that "knows" about the standard C +library functions. If you call "exit(0)" in main(), it knows that it is +safe to optimize that into "return 0;" because C specifies what the +'exit' function does. + +In addition to simple library knowledge, it is possible to embed a +variety of other language-specific information into the LLVM IR. If you +have a specific need and run into a wall, please bring the topic up on +the llvm-dev list. At the very worst, you can always treat LLVM as if it +were a "dumb code generator" and implement the high-level optimizations +you desire in your front-end, on the language-specific AST. + +Tips and Tricks +=============== + +There is a variety of useful tips and tricks that you come to know after +working on/with LLVM that aren't obvious at first glance. Instead of +letting everyone rediscover them, this section talks about some of these +issues. + +Implementing portable offsetof/sizeof +------------------------------------- + +One interesting thing that comes up, if you are trying to keep the code +generated by your compiler "target independent", is that you often need +to know the size of some LLVM type or the offset of some field in an +llvm structure. For example, you might need to pass the size of a type +into a function that allocates memory. + +Unfortunately, this can vary widely across targets: for example the +width of a pointer is trivially target-specific. However, there is a +`clever way to use the getelementptr +instruction <http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt>`_ +that allows you to compute this in a portable way. + +Garbage Collected Stack Frames +------------------------------ + +Some languages want to explicitly manage their stack frames, often so +that they are garbage collected or to allow easy implementation of +closures. There are often better ways to implement these features than +explicit stack frames, but `LLVM does support +them, <http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt>`_ +if you want. It requires your front-end to convert the code into +`Continuation Passing +Style <http://en.wikipedia.org/wiki/Continuation-passing_style>`_ and +the use of tail calls (which LLVM also supports). + |
