[libc++] add FreeBSD atomic wait support

[emaste]

Summary

From Konstantin Belousov <[e-mail]>

[Mordante]

Thanks for working on this! Does this fix existing tests?

[emaste]

libcxx/src/atomic.cpp

29–33

The CI format job complained about the indentation on the changes in this file, but they are consistent with what is in the file now. Should we run clang-format on it first, and then update this to match?

[emaste]

In D142134#4066210, @Mordante wrote:

Thanks for working on this!

In fact Kostik found this independent of my work on bringing up CI. It's fortuitous timing.

Does this fix existing tests?

There were a number of atomic tests that timed out on occasion or were inconsistent before (and I temporarily removed them in my work to get the CI going).
I applied this change and restored those tests and (in the one run so far) they passed.

[emaste]

I applied this change and restored those tests and (in the one run so far) they passed.

After a couple more runs it looks like the tests now pass consistently.

[philnik]

LGTM % nit.

libcxx/src/atomic.cpp

29–33

Just leave it as-is for now. Most of the code-base isn't formatted and we don't enforce it currently in any way.

82

You don't need the underscores in the argument names, since this is dylib code.

[kib]

libcxx/src/atomic.cpp

82

It is just consistent with the rest of the file (in other words, functions were copy/pasted from the Linux block and edited for FreeBSD).

[emaste]

libcxx/src/atomic.cpp

82

OK, I would rather keep this consistent with the other implementations, similar to the clang-format discussion above.

libcxx/src/atomic.cpp

 #include <sys/syscall.h>
 
 // libc++ uses SYS_futex as a universal syscall name. However, on 32 bit architectures
 // with a 64 bit time_t, we need to specify SYS_futex_time64.
 #if !defined(SYS_futex) && defined(SYS_futex_time64)
 # define SYS_futex SYS_futex_time64
 #endif
 
+#elif defined(__FreeBSD__)
+
+#include <sys/types.h>
+#include <sys/umtx.h>
+
 #else // <- Add other operating systems here
 
 // Baseline needs no new headers
 
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 

 
 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
                                               bool __notify_one)
 {
     __ulock_wake(UL_COMPARE_AND_WAIT | (__notify_one ? 0 : ULF_WAKE_ALL),
                  const_cast<__cxx_atomic_contention_t*>(__ptr), 0);
 }
 
+#elif defined(__FreeBSD__)
+
+static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              __cxx_contention_t __val)
+{
+    _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr),
+             UMTX_OP_WAIT_UINT_PRIVATE, __val, NULL, NULL);
+}
+
+static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
+                                              bool __notify_one)
+{
+    _umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr),
+             UMTX_OP_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, NULL, NULL);
+}
+
 #else // <- Add other operating systems here
 
 // Baseline is just a timed backoff
 
 static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
                                               __cxx_contention_t __val)
 {
     __libcpp_thread_poll_with_backoff([=]() -> bool {