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Re: [PATCH] Do not copy std:call_once arguments (LWG 2442)
- From: Jonathan Wakely <jwakely at redhat dot com>
- To: libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Wed, 12 Oct 2016 13:16:15 +0100
- Subject: Re: [PATCH] Do not copy std:call_once arguments (LWG 2442)
- Authentication-results: sourceware.org; auth=none
- References: <20161012112225.GA14015@redhat.com>
On 12/10/16 12:22 +0100, Jonathan Wakely wrote:
This replaces the use of std::__bind_simple by direct calls to
std::__invoke so that the arguments are not decay-copied.
* doc/xml/manual/intro.xml: Document LWG 2442 status.
* include/std/mutex [_GLIBCXX_HAVE_TLS] (__once_call_impl): Remove.
[_GLIBCXX_HAVE_TLS] (_Once_call): Declare primary template and define
partial specialization to unpack args and forward to std::invoke.
(call_once) [_GLIBCXX_HAVE_TLS]: Use forward_as_tuple and _Once_call
instead of __bind_simple and __once_call_impl.
(call_once) [!_GLIBCXX_HAVE_TLS]: Use __invoke instead of
__bind_simple.
* testsuite/30_threads/call_once/dr2442.cc: New test.
Tested powerpc64le-linux, committed to trunk.
I don't know why I over-complicated it by packing and unpacking the
args in a tuple, this is much simpler, and makes the TLS and non-TLS
implementations more similar.
Tested powerpc64le-linux, committed to trunk.
commit f4f65251fbf84547db29d04bfa60ffcf0540a188
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Wed Oct 12 12:30:35 2016 +0100
Simplify std::call_once implementation
* include/std/mutex [_GLIBCXX_HAVE_TLS] (_Once_call): Remove.
(call_once) [_GLIBCXX_HAVE_TLS]: Simplify by removing _Once_call.
diff --git a/libstdc++-v3/include/std/mutex b/libstdc++-v3/include/std/mutex
index 4c6f036..e90006f 100644
--- a/libstdc++-v3/include/std/mutex
+++ b/libstdc++-v3/include/std/mutex
@@ -579,21 +579,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#ifdef _GLIBCXX_HAVE_TLS
extern __thread void* __once_callable;
extern __thread void (*__once_call)();
-
- template<typename _Tuple, typename _IndexSeq
- = typename _Build_index_tuple<tuple_size<_Tuple>::value>::__type>
- struct _Once_call;
-
- template<typename _Tuple, size_t... _Ind>
- struct _Once_call<_Tuple, _Index_tuple<_Ind...>>
- {
- static void
- _S_call()
- {
- auto& __f_args = *static_cast<_Tuple*>(__once_callable);
- std::__invoke(std::get<_Ind>(std::move(__f_args))...);
- }
- };
#else
extern function<void()> __once_functor;
@@ -613,17 +598,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2442. call_once() shouldn't DECAY_COPY()
-#ifdef _GLIBCXX_HAVE_TLS
- auto __f_args = std::forward_as_tuple(
- std::forward<_Callable>(__f), std::forward<_Args>(__args)...);
- __once_callable = std::__addressof(__f_args);
- __once_call = _Once_call<decltype(__f_args)>::_S_call;
-#else
- unique_lock<mutex> __functor_lock(__get_once_mutex());
- __once_functor = [&] {
+ auto __callable = [&] {
std::__invoke(std::forward<_Callable>(__f),
std::forward<_Args>(__args)...);
};
+#ifdef _GLIBCXX_HAVE_TLS
+ __once_callable = std::__addressof(__callable);
+ __once_call = []{ (*(decltype(__callable)*)__once_callable)(); };
+#else
+ unique_lock<mutex> __functor_lock(__get_once_mutex());
+ __once_functor = __callable;
__set_once_functor_lock_ptr(&__functor_lock);
#endif