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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177211 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGCXXABI.h

@@ -91,6 +91,10 @@ public:
     return *MangleCtx;
   }
 
+  /// Returns true if the given instance method is one of the
+  /// kinds that the ABI says returns 'this'.
+  virtual bool HasThisReturn(GlobalDecl GD) const { return false; }
+
   /// Find the LLVM type used to represent the given member pointer
   /// type.
   virtual llvm::Type *
@@ -209,7 +213,8 @@ public:
   /// Emit the ABI-specific prolog for the function.
   virtual void EmitInstanceFunctionProlog(CodeGenFunction &CGF) = 0;
 
-  virtual void EmitConstructorCall(CodeGenFunction &CGF,
+  /// Emit the constructor call. Return the function that is called.
+  virtual llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
                                    const CXXConstructorDecl *D,
                                    CXXCtorType Type, bool ForVirtualBase,
                                    bool Delegating,

lib/CodeGen/CGCall.cpp

@@ -1705,6 +1705,18 @@ void CodeGenFunction::EmitFunctionEpilog(const CGFunctionInfo &FI) {
     llvm_unreachable("Invalid ABI kind for return argument");
   }
 
+  // If this function returns 'this' and the last instruction is a CallInst
+  // that returns 'this', use the return value from the CallInst. We will not
+  // need to keep 'this' alive through the callsite. It also enables
+  // optimizations in the backend, such as tail call optimization.
+  if (CalleeWithThisReturn && CGM.getCXXABI().HasThisReturn(CurGD)) {
+    llvm::BasicBlock *IP = Builder.GetInsertBlock();
+    llvm::CallInst *Callsite;
+    if (!IP->empty() && (Callsite = dyn_cast<llvm::CallInst>(&IP->back())) &&
+        Callsite->getCalledFunction() == CalleeWithThisReturn)
+      // Create a bitcast of Callsite.
+      RV = Builder.CreateBitCast(Callsite, RetAI.getCoerceToType());
+  }
   llvm::Instruction *Ret = RV ? Builder.CreateRet(RV) : Builder.CreateRetVoid();
   if (!RetDbgLoc.isUnknown())
     Ret->setDebugLoc(RetDbgLoc);

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 {

lib/CodeGen/CodeGenFunction.cpp

@@ -564,6 +564,9 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
   SourceRange BodyRange;
   if (Stmt *Body = FD->getBody()) BodyRange = Body->getSourceRange();
 
+  // Reset CalleeWithThisReturn.
+  CalleeWithThisReturn = 0;
+
   // Emit the standard function prologue.
   StartFunction(GD, ResTy, Fn, FnInfo, Args, BodyRange.getBegin());
 
@@ -615,6 +618,8 @@ void CodeGenFunction::GenerateCode(GlobalDecl GD, llvm::Function *Fn,
 
   // Emit the standard function epilogue.
   FinishFunction(BodyRange.getEnd());
+  // Reset CalleeWithThisReturn.
+  CalleeWithThisReturn = 0;
 
   // If we haven't marked the function nothrow through other means, do
   // a quick pass now to see if we can.

lib/CodeGen/CodeGenFunction.h

@@ -1131,6 +1131,10 @@ private:
   CGDebugInfo *DebugInfo;
   bool DisableDebugInfo;
 
+  /// If the current function returns 'this', use the field to keep track of
+  /// the callee that returns 'this'.
+  llvm::Value *CalleeWithThisReturn;
+
   /// DidCallStackSave - Whether llvm.stacksave has been called. Used to avoid
   /// calling llvm.stacksave for multiple VLAs in the same scope.
   bool DidCallStackSave;

lib/CodeGen/ItaniumCXXABI.cpp

@@ -112,7 +112,7 @@ public:
 
   void EmitInstanceFunctionProlog(CodeGenFunction &CGF);
 
-  void EmitConstructorCall(CodeGenFunction &CGF,
+  llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
                            const CXXConstructorDecl *D,
                            CXXCtorType Type, bool ForVirtualBase,
                            bool Delegating,
@@ -177,11 +177,11 @@ public:
   llvm::Value *readArrayCookieImpl(CodeGenFunction &CGF, llvm::Value *allocPtr,
                                    CharUnits cookieSize);
 
-private:
   /// \brief Returns true if the given instance method is one of the
   /// kinds that the ARM ABI says returns 'this'.
-  static bool HasThisReturn(GlobalDecl GD) {
-    const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
+  bool HasThisReturn(GlobalDecl GD) const {
+    const CXXMethodDecl *MD = dyn_cast_or_null<CXXMethodDecl>(GD.getDecl());
+    if (!MD) return false;
     return ((isa<CXXDestructorDecl>(MD) && GD.getDtorType() != Dtor_Deleting) ||
             (isa<CXXConstructorDecl>(MD)));
   }
@@ -834,7 +834,7 @@ void ARMCXXABI::EmitInstanceFunctionProlog(CodeGenFunction &CGF) {
     CGF.Builder.CreateStore(getThisValue(CGF), CGF.ReturnValue);
 }
 
-void ItaniumCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
+llvm::Value *ItaniumCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
                                         const CXXConstructorDecl *D,
                                         CXXCtorType Type, bool ForVirtualBase,
                                         bool Delegating,
@@ -849,6 +849,7 @@ void ItaniumCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
   // FIXME: Provide a source location here.
   CGF.EmitCXXMemberCall(D, SourceLocation(), Callee, ReturnValueSlot(), This,
                         VTT, VTTTy, ArgBeg, ArgEnd);
+  return Callee;
 }
 
 RValue ItaniumCXXABI::EmitVirtualDestructorCall(CodeGenFunction &CGF,

lib/CodeGen/MicrosoftCXXABI.cpp

@@ -55,7 +55,7 @@ public:
 
   void EmitInstanceFunctionProlog(CodeGenFunction &CGF);
 
-  void EmitConstructorCall(CodeGenFunction &CGF,
+  llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
                            const CXXConstructorDecl *D,
                            CXXCtorType Type, bool ForVirtualBase,
                            bool Delegating,
@@ -238,7 +238,7 @@ void MicrosoftCXXABI::EmitInstanceFunctionProlog(CodeGenFunction &CGF) {
   }
 }
 
-void MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
+llvm::Value *MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
                                           const CXXConstructorDecl *D,
                                           CXXCtorType Type, bool ForVirtualBase,
                                           bool Delegating,
@@ -259,6 +259,7 @@ void MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
   CGF.EmitCXXMemberCall(D, SourceLocation(), Callee, ReturnValueSlot(), This,
                         ImplicitParam, ImplicitParamTy,
                         ArgBeg, ArgEnd);
+  return Callee;
 }
 
 RValue MicrosoftCXXABI::EmitVirtualDestructorCall(CodeGenFunction &CGF,

test/CodeGenCXX/arm.cpp

@@ -56,15 +56,15 @@ namespace test1 {
   // CHECK:   [[THIS:%.*]] = alloca [[A]]*, align 4
   // CHECK:   store [[A]]* {{.*}}, [[A]]** [[THIS]]
   // CHECK:   [[THIS1:%.*]] = load [[A]]** [[THIS]]
-  // CHECK:   call [[A]]* @_ZN5test11AC2Ei(
-  // CHECK:   ret [[A]]* [[THIS1]]
+  // CHECK:   [[THIS2:%.*]] = call [[A]]* @_ZN5test11AC2Ei(
+  // CHECK:   ret [[A]]* [[THIS2]]
 
   // CHECK: define linkonce_odr [[A]]* @_ZN5test11AD1Ev([[A]]* %this) unnamed_addr
   // CHECK:   [[THIS:%.*]] = alloca [[A]]*, align 4
   // CHECK:   store [[A]]* {{.*}}, [[A]]** [[THIS]]
   // CHECK:   [[THIS1:%.*]] = load [[A]]** [[THIS]]
-  // CHECK:   call [[A]]* @_ZN5test11AD2Ev(
-  // CHECK:   ret [[A]]* [[THIS1]]
+  // CHECK:   [[THIS2:%.*]] = call [[A]]* @_ZN5test11AD2Ev(
+  // CHECK:   ret [[A]]* [[THIS2]]
 }
 
 // Awkward virtual cases.