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, 55 insertions, 0 deletions

diff --git a/clang/test/CodeGenCXX/nrvo.cpp b/clang/test/CodeGenCXX/nrvo.cpp
new file mode 100644
index 00000000000..7b4f20398d8
--- /dev/null
+++ b/clang/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;
+}