[PATCH] D47067: Update NRVO logic to support early return

[Taiju Tsuiki via Phabricator via cfe-commits]

tzik created this revision.
Herald added a subscriber: cfe-commits.

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 return statement if it refers a non-parameter local variable,
and the variable is referred by all return statements reachable from the variable declaration.
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.


Repository:
  rC Clang

https://reviews.llvm.org/D47067

Files:
  include/clang/AST/Decl.h
  include/clang/Sema/Scope.h
  lib/Sema/Scope.cpp
  lib/Sema/SemaDecl.cpp
  lib/Sema/SemaStmt.cpp
  lib/Serialization/ASTReaderDecl.cpp
  lib/Serialization/ASTWriterDecl.cpp
  test/CodeGenCXX/nrvo.cpp

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