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, 6 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index 772f640ed01..f51a899e7ff 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/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)