[TblGen] Extend !if semantics through new feature !cond

This patch extends TableGen language with !cond operator.
Instead of embedding !if inside !if which can get cumbersome,
one can now use !cond.
Below is an example to convert an integer 'x' into a string:

    !cond(!lt(x,0) : "Negative",
          !eq(x,0) : "Zero",
          !eq(x,1) : "One,
          1        : "MoreThanOne")

Reviewed By: hfinkel, simon_tatham, greened
Differential Revision: https://reviews.llvm.org/D55758

llvm-svn: 352185

2 files changed, 23 insertions, 1 deletions

diff --git a/llvm/docs/TableGen/LangIntro.rst b/llvm/docs/TableGen/LangIntro.rst
index ea46550ffc0..390f941f0ca 100644
--- a/llvm/docs/TableGen/LangIntro.rst
+++ b/llvm/docs/TableGen/LangIntro.rst
@@ -258,6 +258,20 @@ supported include:
 ``!if(a,b,c)``
   'b' if the result of 'int' or 'bit' operator 'a' is nonzero, 'c' otherwise.
 
+``!cond(condition_1 : val1, condition_2 : val2, ..., condition_n : valn)``
+    Instead of embedding !if inside !if which can get cumbersome,
+    one can use !cond. !cond returns 'val1' if the result of 'int' or 'bit'
+    operator 'condition1' is nonzero. Otherwise, it checks 'condition2'.
+    If 'condition2' is nonzero, returns 'val2', and so on.
+    If all conditions are zero, it reports an error.
+
+    Below is an example to convert an integer 'x' into a string:
+
+    !cond(!lt(x,0) : "Negative",
+          !eq(x,0) : "Zero",
+          !eq(x,1) : "One,
+          1        : "MoreThanOne")
+
 ``!eq(a,b)``
     'bit 1' if string a is equal to string b, 0 otherwise.  This only operates
     on string, int and bit objects.  Use !cast<string> to compare other types of
diff --git a/llvm/docs/TableGen/LangRef.rst b/llvm/docs/TableGen/LangRef.rst
index 2efee12ec9d..a3dbf363151 100644
--- a/llvm/docs/TableGen/LangRef.rst
+++ b/llvm/docs/TableGen/LangRef.rst
@@ -102,6 +102,12 @@ wide variety of meanings:
                :!isa    !dag     !le      !lt        !ge
                :!gt     !ne
 
+TableGen also has !cond operator that needs a slightly different
+syntax compared to other "bang operators":
+
+.. productionlist::
+   CondOperator: !cond
+
 
 Syntax
 ======
@@ -140,7 +146,7 @@ considered to define the class if any of the following is true:
 #. The :token:`Body` in the :token:`ObjectBody` is present and is not empty.
 #. The :token:`BaseClassList` in the :token:`ObjectBody` is present.
 
-You can declare an empty class by giving and empty :token:`TemplateArgList`
+You can declare an empty class by giving an empty :token:`TemplateArgList`
 and an empty :token:`ObjectBody`. This can serve as a restricted form of
 forward declaration: note that records deriving from the forward-declared
 class will inherit no fields from it since the record expansion is done
@@ -315,6 +321,8 @@ The initial :token:`DagArg` is called the "operator" of the dag.
 
 .. productionlist::
    SimpleValue: `BangOperator` ["<" `Type` ">"] "(" `ValueListNE` ")"
+              :| `CondOperator` "(" `CondVal` ("," `CondVal`)* ")"
+   CondVal: `Value` ":" `Value`
 
 Bodies
 ------