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

llvm-svn: 333500

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index a1ce26d27ca..699a98ce4f5 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1326,7 +1326,7 @@ ASTDeclReader::RedeclarableResult ASTDeclReader::VisitVarDeclImpl(VarDecl *VD) {
     VD->NonParmVarDeclBits.IsThisDeclarationADemotedDefinition =
         Record.readInt();
     VD->NonParmVarDeclBits.ExceptionVar = Record.readInt();
-    VD->NonParmVarDeclBits.NRVOVariable = Record.readInt();
+    VD->NonParmVarDeclBits.NRVOMode = Record.readInt();
     VD->NonParmVarDeclBits.CXXForRangeDecl = Record.readInt();
     VD->NonParmVarDeclBits.ObjCForDecl = Record.readInt();
     VD->NonParmVarDeclBits.ARCPseudoStrong = Record.readInt();