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, 1 insertions, 21 deletions

diff --git a/include/clang/Sema/Scope.h b/include/clang/Sema/Scope.h
index a3379ff34f..298c5461ad 100644
--- a/include/clang/Sema/Scope.h
+++ b/include/clang/Sema/Scope.h
@@ -201,10 +201,6 @@ private:
   /// Used to determine if errors occurred in this scope.
   DiagnosticErrorTrap ErrorTrap;
 
-  /// A lattice consisting of undefined, a single NRVO candidate variable in
-  /// this scope, or over-defined. The bit is true when over-defined.
-  llvm::PointerIntPair<VarDecl *, 1, bool> NRVO;
-
   void setFlags(Scope *Parent, unsigned F);
 
 public:
@@ -466,23 +462,7 @@ public:
                                   UsingDirectives.end());
   }
 
-  void addNRVOCandidate(VarDecl *VD) {
-    if (NRVO.getInt())
-      return;
-    if (NRVO.getPointer() == nullptr) {
-      NRVO.setPointer(VD);
-      return;
-    }
-    if (NRVO.getPointer() != VD)
-      setNoNRVO();
-  }
-
-  void setNoNRVO() {
-    NRVO.setInt(true);
-    NRVO.setPointer(nullptr);
-  }
-
-  void mergeNRVOIntoParent();
+  void setNRVOCandidate(VarDecl *Candidate);
 
   /// Init - This is used by the parser to implement scope caching.
   void Init(Scope *parent, unsigned flags);