Update NRVO logic to support early return

Summary:
The previous implementation misses an opportunity to apply NRVO (Named Return Value
Optimization) below. That discourages user to write early return code.

```
struct Foo {};

Foo f(bool b) {
  if (b)
    return Foo();
  Foo oo;
  return oo;
}
```
That is, we can/should apply RVO for a local variable if:
 * It's directly returned by at least one return statement.
 * And, all reachable return statements in its scope returns the variable directly.
While, the previous implementation disables the RVO in a scope if there are multiple return
statements that refers different variables.

On the new algorithm, local variables are in NRVO_Candidate state at first, and a return
statement changes it to NRVO_Disabled for all visible variables but the return statement refers.
Then, at the end of the function AST traversal, NRVO is enabled for variables in NRVO_Candidate
state and refers from at least one return statement.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: xbolva00, Quuxplusone, arthur.j.odwyer, cfe-commits

Differential Revision: https://reviews.llvm.org/D47067

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@333500 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 44 insertions, 7 deletions

diff --git a/test/CodeGenCXX/nrvo.cpp b/test/CodeGenCXX/nrvo.cpp
index 0f359b9c90..1da4308e26 100644
--- a/test/CodeGenCXX/nrvo.cpp
+++ b/test/CodeGenCXX/nrvo.cpp
@@ -130,17 +130,13 @@ X test2(bool B) {
 }
 
 // CHECK-LABEL: define void @_Z5test3b
-X test3(bool B) {
+X test3(bool B, X x) {
   // CHECK: tail call {{.*}} @_ZN1XC1Ev
-  // CHECK-NOT: call {{.*}} @_ZN1XC1ERKS_
-  // CHECK: call {{.*}} @_ZN1XC1Ev
-  // CHECK: call {{.*}} @_ZN1XC1ERKS_
   if (B) {
     X y;
     return y;
   }
-  // FIXME: we should NRVO this variable too.
-  X x;
+  // CHECK: tail call {{.*}} @_ZN1XC1ERKS_
   return x;
 }
 
@@ -191,9 +187,13 @@ X test6() {
 }
 
 // CHECK-LABEL: define void @_Z5test7b
+// CHECK-EH-LABEL: define void @_Z5test7b
 X test7(bool b) {
   // CHECK: tail call {{.*}} @_ZN1XC1Ev
   // CHECK-NEXT: ret
+
+  // CHECK-EH: tail call {{.*}} @_ZN1XC1Ev
+  // CHECK-EH-NEXT: ret
   if (b) {
     X x;
     return x;
@@ -202,10 +202,14 @@ X test7(bool b) {
 }
 
 // CHECK-LABEL: define void @_Z5test8b
+// CHECK-EH-LABEL: define void @_Z5test8b
 X test8(bool b) {
   // CHECK: tail call {{.*}} @_ZN1XC1Ev
   // CHECK-NEXT: ret
-  if (b) {
+
+  // CHECK-EH: tail call {{.*}} @_ZN1XC1Ev
+  // CHECK-EH-NEXT: ret
+if (b) {
     X x;
     return x;
   } else {
@@ -221,4 +225,37 @@ Y<int> test9() {
 // CHECK-LABEL: define linkonce_odr void @_ZN1YIiE1fEv
 // CHECK: tail call {{.*}} @_ZN1YIiEC1Ev
 
+// CHECK-LABEL: define void @_Z6test10b
+X test10(bool B, X x) {
+  if (B) {
+    // CHECK: tail call {{.*}} @_ZN1XC1ERKS_
+    // CHECK-EH: tail call {{.*}} @_ZN1XC1ERKS_
+    return x;
+  }
+  // CHECK: tail call {{.*}} @_ZN1XC1Ev
+  // CHECK-NOT: call {{.*}} @_ZN1XC1ERKS_
+
+  // CHECK-EH: tail call {{.*}} @_ZN1XC1Ev
+  // CHECK-EH-NOT: call {{.*}} @_ZN1XC1ERKS_
+  X y;
+  return y;
+}
+
+// CHECK-LABEL: define {{.*}} void @_Z6test11I1XET_v
+// CHECK-EH-LABEL: define {{.*}} void @_Z6test11I1XET_v
+template <typename T>
+T test11() {
+  // CHECK:      tail call {{.*}} @_ZN1XC1Ev
+  // CHECK-NEXT: ret void
+
+  // CHECK-EH:      tail call {{.*}} @_ZN1XC1Ev
+  // CHECK-EH-NEXT: ret void
+  T t;
+  return t;
+}
+
+void test12() {
+  test11<X>();
+}
+
 // CHECK-EH-03: attributes [[NR_NUW]] = { noreturn nounwind }