[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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@367048 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index 61ca544..682b990 100644
--- a/include/__config
+++ b/include/__config
@@ -1097,17 +1097,27 @@
_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)
