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.

Updated from r177211.
rdar://12818789


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177541 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 25 insertions, 0 deletions

diff --git a/lib/CodeGen/CGCall.cpp b/lib/CodeGen/CGCall.cpp
index 9e97bce6c3..9318973e64 100644
--- a/lib/CodeGen/CGCall.cpp
+++ b/lib/CodeGen/CGCall.cpp
@@ -1614,6 +1614,18 @@ static llvm::StoreInst *findDominatingStoreToReturnValue(CodeGenFunction &CGF) {
   return store;
 }
 
+/// Check whether 'this' argument of a callsite matches 'this' of the caller.
+static bool checkThisPointer(llvm::Value *ThisArg, llvm::Value *This) {
+  if (ThisArg == This)
+    return true;
+  // Check whether ThisArg is a bitcast of This.
+  llvm::BitCastInst *Bitcast;
+  if ((Bitcast = dyn_cast<llvm::BitCastInst>(ThisArg)) &&
+      Bitcast->getOperand(0) == This)
+    return true;
+  return false;
+}
+
 void CodeGenFunction::EmitFunctionEpilog(const CGFunctionInfo &FI) {
   // Functions with no result always return void.
   if (ReturnValue == 0) {
@@ -1705,6 +1717,19 @@ void CodeGenFunction::EmitFunctionEpilog(const CGFunctionInfo &FI) {
     llvm_unreachable("Invalid ABI kind for return argument");
   }
 
+  // If this function returns 'this', the last instruction is a CallInst
+  // that returns 'this', and 'this' argument of the CallInst points to
+  // the same object as CXXThisValue, 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 &&
+        checkThisPointer(Callsite->getOperand(0), CXXThisValue))
+      RV = Builder.CreateBitCast(Callsite, RetAI.getCoerceToType());
+  }
   llvm::Instruction *Ret = RV ? Builder.CreateRet(RV) : Builder.CreateRetVoid();
   if (!RetDbgLoc.isUnknown())
     Ret->setDebugLoc(RetDbgLoc);