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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103867 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/CGDecl.cpp b/lib/CodeGen/CGDecl.cpp
index 48198ff5761bcadfb9903a802b11b9aaa983b53c..c93d9ba33df671212b15ef108e54fad3fff62e64 100644 (file)
--- a/lib/CodeGen/CGDecl.cpp
+++ b/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");
         

diff --git a/lib/CodeGen/CGStmt.cpp b/lib/CodeGen/CGStmt.cpp
index 772f640ed01f3828969cbfdd25aaee16471a63f1..f51a899e7ffacfe1a1d69ce6962657bd36e86245 100644 (file)
--- a/lib/CodeGen/CGStmt.cpp
+++ b/lib/CodeGen/CGStmt.cpp
@@ -607,7 +607,12 @@ void CodeGenFunction::EmitReturnStmt(const ReturnStmt &S) {
 
   // FIXME: Clean this up by using an LValue for ReturnTemp,
   // EmitStoreThroughLValue, and EmitAnyExpr.
-  if (!ReturnValue) {
+  if (S.getNRVOCandidate() && S.getNRVOCandidate()->isNRVOVariable() &&
+      !Target.useGlobalsForAutomaticVariables()) {
+    // Apply the named return value optimization for this return statement,
+    // which means doing nothing: the appropriate result has already been
+    // constructed into the NRVO variable.
+  } else if (!ReturnValue) {
     // Make sure not to return anything, but evaluate the expression
     // for side effects.
     if (RV)

diff --git a/lib/Sema/SemaStmt.cpp b/lib/Sema/SemaStmt.cpp
index f8fe4fe21c04bdd73898dade54f908a2daba46ce..d904907e97d97c4288cb2c3012722d47635a29e5 100644 (file)
--- a/lib/Sema/SemaStmt.cpp
+++ b/lib/Sema/SemaStmt.cpp
@@ -1090,7 +1090,8 @@ static const VarDecl *getNRVOCandidate(ASTContext &Ctx, QualType RetType,
     return 0;
   
   if (VD->getKind() == Decl::Var && VD->hasLocalStorage() && 
-      !VD->getType()->isReferenceType() && !VD->getType().isVolatileQualified())
+      !VD->getType()->isReferenceType() && !VD->hasAttr<BlocksAttr>() &&
+      !VD->getType().isVolatileQualified())
     return VD;
   
   return 0;

diff --git a/test/CodeGenCXX/nrvo.cpp b/test/CodeGenCXX/nrvo.cpp
new file mode 100644 (file)
index 0000000..7b4f203
--- /dev/null
+++ b/test/CodeGenCXX/nrvo.cpp
@@ -0,0 +1,55 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+
+// Test code generation for the named return value optimization.
+class X {
+public:
+  X();
+  X(const X&);
+  ~X();
+};
+
+// CHECK: define void @_Z5test0v
+X test0() {
+  X x;
+  // CHECK-NOT: call void @_ZN1XD1Ev
+  // CHECK: ret void
+  return x;
+}
+
+// CHECK: define void @_Z5test1b(
+X test1(bool B) {
+  // CHECK: call void @_ZN1XC1Ev
+  X x;
+  // CHECK-NOT: call void @_ZN1XD1Ev
+  // CHECK: ret void
+  if (B)
+    return (x);
+  return x;
+}
+
+// CHECK: define void @_Z5test2b
+X test2(bool B) {
+  // No NRVO
+  // CHECK: call void @_ZN1XC1Ev
+  X x;
+  // CHECK: call void @_ZN1XC1Ev
+  X y;
+  // CHECK: call void @_ZN1XC1ERKS_
+  if (B)
+    return y;
+  // CHECK: call void @_ZN1XC1ERKS_
+  return x;
+  // CHECK: call void @_ZN1XD1Ev
+  // CHECK: call void @_ZN1XD1Ev
+  // CHECK: ret void
+}
+
+X test3(bool B) {
+  // FIXME: We don't manage to apply NRVO here, although we could.
+  {
+    X y;
+    return y;
+  }
+  X x;
+  return x;
+}