Implement a simple form of the C++ named return value optimization for

return statements. We perform NRVO only when all of the return
statements in the function return the same variable. Fixes some link
failures in Boost.Interprocess (which is relying on NRVO), and
probably improves performance for some C++ applications.

llvm-svn: 103867

1 files changed, 36 insertions, 22 deletions

diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 48198ff5761..c93d9ba33df 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -398,16 +398,19 @@ void CodeGenFunction::EmitLocalBlockVarDecl(const VarDecl &D) {
   CharUnits Align = CharUnits::Zero();
   bool IsSimpleConstantInitializer = false;
 
+  bool NRVO = false;
   llvm::Value *DeclPtr;
   if (Ty->isConstantSizeType()) {
     if (!Target.useGlobalsForAutomaticVariables()) {
-      
+      NRVO = getContext().getLangOptions().ElideConstructors && 
+             D.isNRVOVariable();
       // If this value is an array or struct, is POD, and if the initializer is
-      // a staticly determinable constant, try to optimize it.
+      // a staticly determinable constant, try to optimize it (unless the NRVO
+      // is already optimizing this).
       if (D.getInit() && !isByRef &&
           (Ty->isArrayType() || Ty->isRecordType()) &&
           Ty->isPODType() &&
-          D.getInit()->isConstantInitializer(getContext())) {
+          D.getInit()->isConstantInitializer(getContext()) && !NRVO) {
         // If this variable is marked 'const', emit the value as a global.
         if (CGM.getCodeGenOpts().MergeAllConstants &&
             Ty.isConstant(getContext())) {
@@ -418,19 +421,29 @@ void CodeGenFunction::EmitLocalBlockVarDecl(const VarDecl &D) {
         IsSimpleConstantInitializer = true;
       }
       
-      // A normal fixed sized variable becomes an alloca in the entry block.
+      // A normal fixed sized variable becomes an alloca in the entry block,
+      // unless it's an NRVO variable.
       const llvm::Type *LTy = ConvertTypeForMem(Ty);
-      if (isByRef)
-        LTy = BuildByRefType(&D);
-      llvm::AllocaInst *Alloc = CreateTempAlloca(LTy);
-      Alloc->setName(D.getNameAsString());
-
-      Align = getContext().getDeclAlign(&D);
-      if (isByRef)
-        Align = std::max(Align, 
-            CharUnits::fromQuantity(Target.getPointerAlign(0) / 8));
-      Alloc->setAlignment(Align.getQuantity());
-      DeclPtr = Alloc;
+      
+      if (NRVO) {
+        // The named return value optimization: allocate this variable in the
+        // return slot, so that we can elide the copy when returning this
+        // variable (C++0x [class.copy]p34).
+        DeclPtr = ReturnValue;
+      } else {
+        if (isByRef)
+          LTy = BuildByRefType(&D);
+        
+        llvm::AllocaInst *Alloc = CreateTempAlloca(LTy);
+        Alloc->setName(D.getNameAsString());
+
+        Align = getContext().getDeclAlign(&D);
+        if (isByRef)
+          Align = std::max(Align, 
+              CharUnits::fromQuantity(Target.getPointerAlign(0) / 8));
+        Alloc->setAlignment(Align.getQuantity());
+        DeclPtr = Alloc;
+      }
     } else {
       // Targets that don't support recursion emit locals as globals.
       const char *Class =
@@ -645,13 +658,14 @@ void CodeGenFunction::EmitLocalBlockVarDecl(const VarDecl &D) {
   while (const ArrayType *Array = getContext().getAsArrayType(DtorTy))
     DtorTy = getContext().getBaseElementType(Array);
   if (const RecordType *RT = DtorTy->getAs<RecordType>())
-    if (CXXRecordDecl *ClassDecl = dyn_cast<CXXRecordDecl>(RT->getDecl())) {
-      llvm::Value *Loc = DeclPtr;
-      if (isByRef)
-        Loc = Builder.CreateStructGEP(DeclPtr, getByRefValueLLVMField(&D), 
-                                      D.getNameAsString());
-      
-      if (!ClassDecl->hasTrivialDestructor()) {
+    if (CXXRecordDecl *ClassDecl = dyn_cast<CXXRecordDecl>(RT->getDecl())) {      
+      if (!ClassDecl->hasTrivialDestructor() && !NRVO) {
+        // Note: We suppress the destructor call when this is an NRVO variable.
+        llvm::Value *Loc = DeclPtr;
+        if (isByRef)
+          Loc = Builder.CreateStructGEP(DeclPtr, getByRefValueLLVMField(&D), 
+                                        D.getNameAsString());
+        
         const CXXDestructorDecl *D = ClassDecl->getDestructor(getContext());
         assert(D && "EmitLocalBlockVarDecl - destructor is nul");