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[NFC][libcxx] Add comments about making mutex/condition_variable trivial on Apple platforms 

Leaving some comments behind so that we avoid re-having that discussion
in the future.

llvm-svn: 367048
This commit is contained in:
Louis Dionne 2019-07-25 20:29:20 +00:00
parent e1c159e86a
commit 568bb7eeb6
1 changed files with 13 additions and 3 deletions
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16
libcxx/include/__config
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@ -1097,17 +1097,27 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
_LIBCPP_HAS_NO_THREADS is defined.
#endif
// The Apple, glibc, and Bionic implementation of pthreads implements
// The glibc and Bionic implementation of pthreads implements
// pthread_mutex_destroy as nop for regular mutexes. Additionally, Win32
// mutexes have no destroy mechanism.
// TODO(EricWF): Enable this optimization on Apple and Bionic platforms after
// speaking to their respective stakeholders.
//
// This optimization can't be performed on Apple platforms, where
// pthread_mutex_destroy can allow the kernel to release resources.
// See https://llvm.org/D64298 for details.
//
// TODO(EricWF): Enable this optimization on Bionic after speaking to their
// respective stakeholders.
#if (defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && defined(__GLIBC__)) \
|| defined(_LIBCPP_HAS_THREAD_API_WIN32)
# define _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION
#endif
// Destroying a condvar is a nop on Windows.
//
// This optimization can't be performed on Apple platforms, where
// pthread_cond_destroy can allow the kernel to release resources.
// See https://llvm.org/D64298 for details.
//
// TODO(EricWF): This is potentially true for some pthread implementations
// as well.
#if defined(_LIBCPP_HAS_THREAD_API_WIN32)