[libc++] [FreeBSD] only use _umtx_op(2) on 64bit arches

[kib]

Summary

Only 64bit architectures can be supported this way, because libcxx
defines __cxx_contention_t to be int64_t for FreeBSD, and 32bit
arches do not have a kind of UMTX_OP_WAIT_INT64_PRIVATE operation.

Fixes: 83387dbc18e7998f87aa4a2d35320bcb2ed5c392

[arichardson]

Using __LP64__ means we won't use umtx for CHERI architectures. Can we change this to __SIZEOF_LONG__ == 8?

[kib]

use SIZEOF_LONG == 8 instead of LP64 for CHERI.

[arichardson]

In D142422#4076305, @kib wrote:

use SIZEOF_LONG == 8 instead of LP64 for CHERI.

Thanks!

[Mordante]

Thanks for working on this!

libcxx/src/atomic.cpp

80–85

Does this still work on FreeBSD platforms where __SIZEOF_LONG__ != 8 ?

[emaste]

In D142422#4080460, @Mordante wrote:

Does this still work on FreeBSD platforms where SIZEOF_LONG != 8 ?

This switches the __SIZEOF_LONG__ != 8 platforms back to the timed backoff fallback. This isn't great, but also relatively low priority, they're all Tier-2 platforms for us.

[Mordante]

In D142422#4080498, @emaste wrote:

In D142422#4080460, @Mordante wrote:

Does this still work on FreeBSD platforms where SIZEOF_LONG != 8 ?

This switches the __SIZEOF_LONG__ != 8 platforms back to the timed backoff fallback. This isn't great, but also relatively low priority, they're all Tier-2 platforms for us.

Can you this information as comment in the code?
Otherwise LGTM from libc++'s perspective.

I assume you want to backport this to the release branch, right?

[kib]

Add comment explaining why umtx is only used on 64bit arches.

[ldionne]

LGTM if this fixes FreeBSD.

[arichardson]

@emaste it looks like you landed an older diff?

libcxx/src/atomic.cpp

 
 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__)
+#elif defined(__FreeBSD__) && defined(__LP64__)
+/*
+ * Since __cxx_contention_t is int64_t even on 32bit FreeBSD
+ * platforms, we have to use umtx ops that work on the long type, and
+ * limit its use to architectures where long and int64_t are synonyms.
+ */
 
 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);
+             UMTX_OP_WAIT, __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);
+             UMTX_OP_WAKE, __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)