https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120527
Bug ID: 120527
Summary: Native platform wait on darwin
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
CC: iains at gcc dot gnu.org
Target Milestone: ---
Target: *-*-darwin*
Darwin provides __ulock_wait and __ulock_wake which should be sufficient to
implement the internals needed for efficient std::atomic::wait and
std::atomic::notify.
Unfortunately these aren't documented and are not really public.
Libc++ uses them based on this version check:
#if defined(__APPLE__)
# if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
# if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500
# define _LIBCPP_USE_ULOCK
# endif
With these manual declarations (because there's no header that declares them):
extern "C" int __ulock_wait(
uint32_t operation, void* addr, uint64_t value, uint32_t timeout); /*
timeout is specified in microseconds */
extern "C" int __ulock_wake(uint32_t operation, void* addr, uint64_t
wake_value);
//
https://github.com/apple/darwin-xnu/blob/2ff845c2e033bd0ff64b5b6aa6063a1f8f65aa32/bsd/sys/ulock.h#L82
# define UL_COMPARE_AND_WAIT64 5
# define ULF_WAKE_ALL 0x00000100
So we would want something like:
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -51,6 +51,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#define _GLIBCXX_HAVE_PLATFORM_WAIT 1
using __platform_wait_t = int;
inline constexpr size_t __platform_wait_alignment = 4;
+#elif defined _GLIBCXX_HAVE_DARWIN_ULOCK
+ using __platform_wait_t = uint64_t;
+ inline constexpr size_t __platform_wait_alignment = 8;
#else
// define _GLIBCX_HAVE_PLATFORM_WAIT and implement __platform_wait()
// and __platform_notify() if there is a more efficient primitive supported
and:
--- a/libstdc++-v3/src/c++20/atomic.cc
+++ b/libstdc++-v3/src/c++20/atomic.cc
@@ -95,6 +95,8 @@ namespace
static_cast<int>(__futex_wait_flags::__wake_private),
__all ? INT_MAX : 1);
}
+#elif defined _GLIBCXX_HAVE_DARWIN_ULOCK
+ // TODO define __platform_wait and __platform_notify
#endif
// The state used by atomic waiting and notifying functions.
@@ -371,6 +373,8 @@ __platform_wait_until(const __platform_wait_t* __addr,
}
return true;
}
+#elif defined _GLIBCXX_HAVE_DARWIN_ULOCK
+ // TODO define __platform_wait_until
#endif // HAVE_LINUX_FUTEX
#ifndef _GLIBCXX_HAVE_PLATFORM_TIMED_WAIT
