Exploit this-return of a callsite in a this-return function.

For constructors/desctructors that return 'this', if there exists a callsite
that returns 'this' and is immediately before the return instruction, make
sure we are using the return value from the callsite.

We don't need to keep 'this' alive through the callsite. It also enables
optimizations in the backend, such as tail call optimization.

rdar://12818789

llvm-svn: 177211

1 files changed, 13 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index 287d164..2ececb0 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -1666,8 +1666,11 @@ CodeGenFunction::EmitCXXConstructorCall(const CXXConstructorDecl *D,
   }
 
   // Non-trivial constructors are handled in an ABI-specific manner.
-  CGM.getCXXABI().EmitConstructorCall(*this, D, Type, ForVirtualBase,
-                                      Delegating, This, ArgBeg, ArgEnd);
+  llvm::Value *Callee = CGM.getCXXABI().EmitConstructorCall(*this, D, Type,
+                            ForVirtualBase, Delegating, This, ArgBeg, ArgEnd);
+  if (CGM.getCXXABI().HasThisReturn(CurGD) &&
+      CGM.getCXXABI().HasThisReturn(GlobalDecl(D, Type)))
+     CalleeWithThisReturn = Callee;
 }
 
 void
@@ -1756,9 +1759,12 @@ CodeGenFunction::EmitDelegateCXXConstructorCall(const CXXConstructorDecl *Ctor,
     EmitDelegateCallArg(DelegateArgs, param);
   }
 
+  llvm::Value *Callee = CGM.GetAddrOfCXXConstructor(Ctor, CtorType);
   EmitCall(CGM.getTypes().arrangeCXXConstructorDeclaration(Ctor, CtorType),
-           CGM.GetAddrOfCXXConstructor(Ctor, CtorType), 
-           ReturnValueSlot(), DelegateArgs, Ctor);
+           Callee, ReturnValueSlot(), DelegateArgs, Ctor);
+  if (CGM.getCXXABI().HasThisReturn(CurGD) &&
+      CGM.getCXXABI().HasThisReturn(GlobalDecl(Ctor, CtorType)))
+     CalleeWithThisReturn = Callee;
 }
 
 namespace {
@@ -1825,6 +1831,9 @@ void CodeGenFunction::EmitCXXDestructorCall(const CXXDestructorDecl *DD,
   EmitCXXMemberCall(DD, SourceLocation(), Callee, ReturnValueSlot(), This,
                     VTT, getContext().getPointerType(getContext().VoidPtrTy),
                     0, 0);
+  if (CGM.getCXXABI().HasThisReturn(CurGD) &&
+      CGM.getCXXABI().HasThisReturn(GlobalDecl(DD, Type)))
+     CalleeWithThisReturn = Callee;
 }
 
 namespace {