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Fix warnings about pessimizing return moves for C++11 and higher

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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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245421 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dimitry Andric
2015-08-19 06:43:33 +00:00
parent 35a6c564bf
commit 8966350d61
7 changed files with 13 additions and 7 deletions
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4
include/__hash_table
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@@ -2108,7 +2108,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::__construct_node(const value_type& __v
__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>::__construct_node(const value_type& __v
__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>