[libcxx] r245421 - Fix warnings about pessimizing return moves for C++11 and higher

[Dimitry Andric via cfe-commits]

Author: dim
Date: Wed Aug 19 01:43:33 2015
New Revision: 245421

URL: http://llvm.org/viewvc/llvm-project?rev=245421&view=rev
Log:
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

Modified:
    libcxx/trunk/include/__config
    libcxx/trunk/include/__hash_table
    libcxx/trunk/include/__tree
    libcxx/trunk/include/algorithm
    libcxx/trunk/include/ext/hash_map
    libcxx/trunk/include/map
    libcxx/trunk/include/unordered_map

Modified: libcxx/trunk/include/__config
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__config?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/__config (original)
+++ libcxx/trunk/include/__config Wed Aug 19 01:43:33 2015
@@ -716,6 +716,12 @@ template <unsigned> struct __static_asse
 #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 *);

Modified: libcxx/trunk/include/__hash_table
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__hash_table?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/__hash_table (original)
+++ libcxx/trunk/include/__hash_table Wed Aug 19 01:43:33 2015
@@ -2108,7 +2108,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>
     __h.get_deleter().__value_constructed = true;
     __h->__hash_ = hash_function()(__h->__value_);
     __h->__next_ = nullptr;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
@@ -2124,7 +2124,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>
     __h.get_deleter().__value_constructed = true;
     __h->__hash_ = __hash;
     __h->__next_ = nullptr;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>

Modified: libcxx/trunk/include/__tree
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__tree?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/__tree (original)
+++ libcxx/trunk/include/__tree Wed Aug 19 01:43:33 2015
@@ -1843,7 +1843,7 @@ __tree<_Tp, _Compare, _Allocator>::__con
     __node_holder __h(__node_traits::allocate(__na, 1), _Dp(__na));
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_), __v);
     __h.get_deleter().__value_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES

Modified: libcxx/trunk/include/algorithm
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/algorithm?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/algorithm (original)
+++ libcxx/trunk/include/algorithm Wed Aug 19 01:43:33 2015
@@ -851,7 +851,7 @@ for_each(_InputIterator __first, _InputI
 {
     for (; __first != __last; ++__first)
         __f(*__first);
-    return _VSTD::move(__f);  // explicitly moved for (emulated) C++03
+    return _LIBCPP_EXPLICIT_MOVE(__f);  // explicitly moved for (emulated) C++03
 }
 
 // find

Modified: libcxx/trunk/include/ext/hash_map
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/ext/hash_map?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/ext/hash_map (original)
+++ libcxx/trunk/include/ext/hash_map Wed Aug 19 01:43:33 2015
@@ -681,7 +681,7 @@ hash_map<_Key, _Tp, _Hash, _Pred, _Alloc
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>

Modified: libcxx/trunk/include/map
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/map?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/map (original)
+++ libcxx/trunk/include/map Wed Aug 19 01:43:33 2015
@@ -1523,7 +1523,7 @@ map<_Key, _Tp, _Compare, _Allocator>::__
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Compare, class _Allocator>

Modified: libcxx/trunk/include/unordered_map
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/unordered_map?rev=245421&r1=245420&r2=245421&view=diff
==============================================================================
--- libcxx/trunk/include/unordered_map (original)
+++ libcxx/trunk/include/unordered_map Wed Aug 19 01:43:33 2015
@@ -1511,7 +1511,7 @@ unordered_map<_Key, _Tp, _Hash, _Pred, _
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>