CodeGen: Have 'this'-returning constructors and destructors to take advantage of the new backend 'returned' attribute.

The backend will now use the generic 'returned' attribute to form tail calls where possible, as well as avoid save-restores of 'this' in some cases (specifically the cases that matter for the ARM C++ ABI).

This patch also reverts a prior front-end only partial implementation of these optimizations, since it's no longer required.

llvm-svn: 184205

1 files changed, 34 insertions, 29 deletions

diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 1bb2c55f559..e6eca28b29f 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -28,6 +28,8 @@ class MicrosoftCXXABI : public CGCXXABI {
 public:
   MicrosoftCXXABI(CodeGenModule &CGM) : CGCXXABI(CGM) {}
 
+  bool HasThisReturn(GlobalDecl GD) const;
+
   bool isReturnTypeIndirect(const CXXRecordDecl *RD) const {
     // Structures that are not C++03 PODs are always indirect.
     return !RD->isPOD();
@@ -71,13 +73,14 @@ public:
 
   void EmitInstanceFunctionProlog(CodeGenFunction &CGF);
 
-  llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
-                           const CXXConstructorDecl *D,
-                           CXXCtorType Type, bool ForVirtualBase,
-                           bool Delegating,
-                           llvm::Value *This,
-                           CallExpr::const_arg_iterator ArgBeg,
-                           CallExpr::const_arg_iterator ArgEnd);
+  RValue EmitConstructorCall(CodeGenFunction &CGF,
+                             const CXXConstructorDecl *D,
+                             CXXCtorType Type,
+                             bool ForVirtualBase, bool Delegating,
+                             ReturnValueSlot ReturnValue,
+                             llvm::Value *This,
+                             CallExpr::const_arg_iterator ArgBeg,
+                             CallExpr::const_arg_iterator ArgEnd);
 
   RValue EmitVirtualDestructorCall(CodeGenFunction &CGF,
                                    const CXXDestructorDecl *Dtor,
@@ -124,7 +127,6 @@ public:
   llvm::Value *readArrayCookieImpl(CodeGenFunction &CGF,
                                    llvm::Value *allocPtr,
                                    CharUnits cookieSize);
-  static bool needThisReturn(GlobalDecl GD);
 
 private:
   llvm::Constant *getZeroInt() {
@@ -299,19 +301,15 @@ MicrosoftCXXABI::GetVirtualBaseClassOffset(CodeGenFunction &CGF,
   return CGF.Builder.CreateNSWAdd(VBPtrOffset, VBPtrToNewBase);
 }
 
-bool MicrosoftCXXABI::needThisReturn(GlobalDecl GD) {
-  const CXXMethodDecl* MD = cast<CXXMethodDecl>(GD.getDecl());
-  return isa<CXXConstructorDecl>(MD);
+bool MicrosoftCXXABI::HasThisReturn(GlobalDecl GD) const {
+  return isa<CXXConstructorDecl>(GD.getDecl());
 }
 
 void MicrosoftCXXABI::BuildConstructorSignature(const CXXConstructorDecl *Ctor,
                                  CXXCtorType Type,
                                  CanQualType &ResTy,
                                  SmallVectorImpl<CanQualType> &ArgTys) {
-  // 'this' is already in place
-
-  // Ctor returns this ptr
-  ResTy = ArgTys[0];
+  // 'this' parameter and 'this' return are already in place
 
   const CXXRecordDecl *Class = Ctor->getParent();
   if (Class->getNumVBases()) {
@@ -346,6 +344,7 @@ void MicrosoftCXXABI::BuildDestructorSignature(const CXXDestructorDecl *Dtor,
                                                CanQualType &ResTy,
                                         SmallVectorImpl<CanQualType> &ArgTys) {
   // 'this' is already in place
+
   // TODO: 'for base' flag
 
   if (Type == Dtor_Deleting) {
@@ -366,9 +365,6 @@ void MicrosoftCXXABI::BuildInstanceFunctionParams(CodeGenFunction &CGF,
                                                   QualType &ResTy,
                                                   FunctionArgList &Params) {
   BuildThisParam(CGF, Params);
-  if (needThisReturn(CGF.CurGD)) {
-    ResTy = Params[0]->getType();
-  }
 
   ASTContext &Context = getContext();
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(CGF.CurGD.getDecl());
@@ -393,9 +389,17 @@ void MicrosoftCXXABI::BuildInstanceFunctionParams(CodeGenFunction &CGF,
 
 void MicrosoftCXXABI::EmitInstanceFunctionProlog(CodeGenFunction &CGF) {
   EmitThisParam(CGF);
-  if (needThisReturn(CGF.CurGD)) {
+
+  /// If this is a function that the ABI specifies returns 'this', initialize
+  /// the return slot to 'this' at the start of the function.
+  ///
+  /// Unlike the setting of return types, this is done within the ABI
+  /// implementation instead of by clients of CGCXXABI because:
+  /// 1) getThisValue is currently protected
+  /// 2) in theory, an ABI could implement 'this' returns some other way;
+  ///    HasThisReturn only specifies a contract, not the implementation    
+  if (HasThisReturn(CGF.CurGD))
     CGF.Builder.CreateStore(getThisValue(CGF), CGF.ReturnValue);
-  }
 
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(CGF.CurGD.getDecl());
   if (isa<CXXConstructorDecl>(MD) && MD->getParent()->getNumVBases()) {
@@ -417,11 +421,13 @@ void MicrosoftCXXABI::EmitInstanceFunctionProlog(CodeGenFunction &CGF) {
   }
 }
 
-llvm::Value *MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
-                                          const CXXConstructorDecl *D,
-                                          CXXCtorType Type, bool ForVirtualBase,
-                                          bool Delegating,
-                                          llvm::Value *This,
+RValue MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
+                                            const CXXConstructorDecl *D,
+                                            CXXCtorType Type, 
+                                            bool ForVirtualBase,
+                                            bool Delegating,
+                                            ReturnValueSlot ReturnValue,
+                                            llvm::Value *This,
                                           CallExpr::const_arg_iterator ArgBeg,
                                           CallExpr::const_arg_iterator ArgEnd) {
   assert(Type == Ctor_Complete || Type == Ctor_Base);
@@ -435,10 +441,9 @@ llvm::Value *MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
   }
 
   // FIXME: Provide a source location here.
-  CGF.EmitCXXMemberCall(D, SourceLocation(), Callee, ReturnValueSlot(), This,
-                        ImplicitParam, ImplicitParamTy,
-                        ArgBeg, ArgEnd);
-  return Callee;
+  return CGF.EmitCXXMemberCall(D, SourceLocation(), Callee, ReturnValue,
+                               This, ImplicitParam, ImplicitParamTy,
+                               ArgBeg, ArgEnd);
 }
 
 RValue MicrosoftCXXABI::EmitVirtualDestructorCall(CodeGenFunction &CGF,