Fix warnings about pessimizing return moves for C++11 and higher
Summary:
Throughout the libc++ headers, there are a few instances where
_VSTD::move() is used to return a local variable. Howard commented in
r189039 that these were there "for non-obvious reasons such as to help
things limp along in C++03 language mode".
However, when compiling these headers with warnings on, and in C++11 or
higher mode (like we do in FreeBSD), they cause the following complaints
about pessimizing moves:
In file included from tests.cpp:26:
In file included from tests.hpp:29:
/usr/include/c++/v1/map:1368:12: error: moving a local object in a return statement prevents copy elision [-Werror,-Wpessimizing-move]
return _VSTD::move(__h); // explicitly moved for C++03
^
/usr/include/c++/v1/__config:368:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE
^
Attempt to fix this by adding a _LIBCPP_EXPLICIT_MOVE() macro to
__config, which gets defined to _VSTD::move for pre-C++11, and to
nothing for C++11 and later.
I am not completely satisfied with the macro name (I also considered
_LIBCPP_COMPAT_MOVE and some other variants), so suggestions are
welcome. :)
Reviewers: mclow.lists, howard.hinnant, EricWF
Subscribers: arthur.j.odwyer, cfe-commits
Differential Revision: http://reviews.llvm.org/D11394
llvm-svn: 245421
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 251c6291170cf6c999655626a0e246c9f7117dab
diff --git a/include/__config b/include/__config
index 600f0fb..5c15156 100644
--- a/include/__config
+++ b/include/__config
@@ -716,6 +716,12 @@
#define _LIBCPP_CONSTEXPR_AFTER_CXX11
#endif
+#ifdef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+# define _LIBCPP_EXPLICIT_MOVE(x) _VSTD::move(x)
+#else
+# define _LIBCPP_EXPLICIT_MOVE(x) (x)
+#endif
+
#ifndef _LIBCPP_HAS_NO_ASAN
extern "C" void __sanitizer_annotate_contiguous_container(
const void *, const void *, const void *, const void *);