[cxx1z-constexpr-lambda] Implement captures - thus completing implementation of constexpr lambdas.

Enable evaluation of captures within constexpr lambdas by using a strategy similar to that used in CodeGen:
  - when starting evaluation of a lambda's call operator, create a map from VarDecl's to a closure's FieldDecls
  - every time a VarDecl (or '*this) that represents a capture is encountered while evaluating the expression via the expression evaluator (specifically the LValueEvaluator) in ExprConstant.cpp - it is replaced by the corresponding FieldDecl LValue (an Lvalue-to-Rvalue conversion on this LValue representation then determines the right rvalue when needed).

Thanks to Richard Smith and Hubert Tong for their review and feedback!

https://reviews.llvm.org/D29748

llvm-svn: 295279

1 files changed, 101 insertions, 7 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index dff126db0c2..337eaa40b95 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -425,6 +425,17 @@ namespace {
     /// Index - The call index of this call.
     unsigned Index;
 
+    // FIXME: Adding this to every 'CallStackFrame' may have a nontrivial impact
+    // on the overall stack usage of deeply-recursing constexpr evaluataions.
+    // (We should cache this map rather than recomputing it repeatedly.)
+    // But let's try this and see how it goes; we can look into caching the map
+    // as a later change.
+
+    /// LambdaCaptureFields - Mapping from captured variables/this to
+    /// corresponding data members in the closure class.
+    llvm::DenseMap<const VarDecl *, FieldDecl *> LambdaCaptureFields;
+    FieldDecl *LambdaThisCaptureField;
+
     CallStackFrame(EvalInfo &Info, SourceLocation CallLoc,
                    const FunctionDecl *Callee, const LValue *This,
                    APValue *Arguments);
@@ -2279,6 +2290,10 @@ static bool HandleLValueComplexElement(EvalInfo &Info, const Expr *E,
   return true;
 }
 
+static bool handleLValueToRValueConversion(EvalInfo &Info, const Expr *Conv,
+                                           QualType Type, const LValue &LVal,
+                                           APValue &RVal);
+
 /// Try to evaluate the initializer for a variable declaration.
 ///
 /// \param Info   Information about the ongoing evaluation.
@@ -2290,6 +2305,7 @@ static bool HandleLValueComplexElement(EvalInfo &Info, const Expr *E,
 static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
                                 const VarDecl *VD, CallStackFrame *Frame,
                                 APValue *&Result) {
+
   // If this is a parameter to an active constexpr function call, perform
   // argument substitution.
   if (const ParmVarDecl *PVD = dyn_cast<ParmVarDecl>(VD)) {
@@ -4180,6 +4196,10 @@ static bool HandleFunctionCall(SourceLocation CallLoc,
       return false;
     This->moveInto(Result);
     return true;
+  } else if (MD && isLambdaCallOperator(MD)) {
+    // We're in a lambda; determine the lambda capture field maps.
+    MD->getParent()->getCaptureFields(Frame.LambdaCaptureFields,
+                                      Frame.LambdaThisCaptureField);
   }
 
   StmtResult Ret = {Result, ResultSlot};
@@ -5041,6 +5061,33 @@ bool LValueExprEvaluator::VisitDeclRefExpr(const DeclRefExpr *E) {
 
 
 bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
+
+  // If we are within a lambda's call operator, check whether the 'VD' referred
+  // to within 'E' actually represents a lambda-capture that maps to a
+  // data-member/field within the closure object, and if so, evaluate to the
+  // field or what the field refers to.
+  if (Info.CurrentCall && isLambdaCallOperator(Info.CurrentCall->Callee)) {
+    if (auto *FD = Info.CurrentCall->LambdaCaptureFields.lookup(VD)) {
+      if (Info.checkingPotentialConstantExpression())
+        return false;
+      // Start with 'Result' referring to the complete closure object...
+      Result = *Info.CurrentCall->This;
+      // ... then update it to refer to the field of the closure object
+      // that represents the capture.
+      if (!HandleLValueMember(Info, E, Result, FD))
+        return false;
+      // And if the field is of reference type, update 'Result' to refer to what
+      // the field refers to.
+      if (FD->getType()->isReferenceType()) {
+        APValue RVal;
+        if (!handleLValueToRValueConversion(Info, E, FD->getType(), Result,
+                                            RVal))
+          return false;
+        Result.setFrom(Info.Ctx, RVal);
+      }
+      return true;
+    }
+  }
   CallStackFrame *Frame = nullptr;
   if (VD->hasLocalStorage() && Info.CurrentCall->Index > 1) {
     // Only if a local variable was declared in the function currently being
@@ -5440,6 +5487,27 @@ public:
       return false;
     }
     Result = *Info.CurrentCall->This;
+    // If we are inside a lambda's call operator, the 'this' expression refers
+    // to the enclosing '*this' object (either by value or reference) which is
+    // either copied into the closure object's field that represents the '*this'
+    // or refers to '*this'.
+    if (isLambdaCallOperator(Info.CurrentCall->Callee)) {
+      // Update 'Result' to refer to the data member/field of the closure object
+      // that represents the '*this' capture.
+      if (!HandleLValueMember(Info, E, Result,
+                             Info.CurrentCall->LambdaThisCaptureField)) 
+        return false;
+      // If we captured '*this' by reference, replace the field with its referent.
+      if (Info.CurrentCall->LambdaThisCaptureField->getType()
+              ->isPointerType()) {
+        APValue RVal;
+        if (!handleLValueToRValueConversion(Info, E, E->getType(), Result,
+                                            RVal))
+          return false;
+
+        Result.setFrom(Info.Ctx, RVal);
+      }
+    }
     return true;
   }
 
@@ -6269,14 +6337,40 @@ bool RecordExprEvaluator::VisitLambdaExpr(const LambdaExpr *E) {
   if (ClosureClass->isInvalidDecl()) return false;
 
   if (Info.checkingPotentialConstantExpression()) return true;
-  if (E->capture_size()) {
-    Info.FFDiag(E, diag::note_unimplemented_constexpr_lambda_feature_ast)
-        << "can not evaluate lambda expressions with captures";
-    return false;
+  
+  const size_t NumFields =
+      std::distance(ClosureClass->field_begin(), ClosureClass->field_end());
+  
+  assert(NumFields ==
+    std::distance(E->capture_init_begin(), E->capture_init_end()) &&
+    "The number of lambda capture initializers should equal the number of "
+    "fields within the closure type");
+  
+  Result = APValue(APValue::UninitStruct(), /*NumBases*/0, NumFields);
+  // Iterate through all the lambda's closure object's fields and initialize
+  // them.
+  auto *CaptureInitIt = E->capture_init_begin();
+  const LambdaCapture *CaptureIt = ClosureClass->captures_begin();
+  bool Success = true;
+  for (const auto *Field : ClosureClass->fields()) {
+    assert(CaptureInitIt != E->capture_init_end());
+    // Get the initializer for this field
+    Expr *const CurFieldInit = *CaptureInitIt++;
+    
+    // If there is no initializer, either this is a VLA or an error has
+    // occurred.
+    if (!CurFieldInit)
+      return Error(E);
+
+    APValue &FieldVal = Result.getStructField(Field->getFieldIndex());
+    if (!EvaluateInPlace(FieldVal, Info, This, CurFieldInit)) {
+      if (!Info.keepEvaluatingAfterFailure())
+        return false;
+      Success = false;
+    }
+    ++CaptureIt;
   }
-  // FIXME: Implement captures.
-  Result = APValue(APValue::UninitStruct(), /*NumBases*/0, /*NumFields*/0);
-  return true;
+  return Success;
 }
 
 static bool EvaluateRecord(const Expr *E, const LValue &This,