[libcxx][libcxxabi] Fix build for OpenBSD (#92186)

- No indirect syscalls on OpenBSD. Instead there is a `futex` function
which issues a direct syscall.

- Monotonic clock is available despite the full POSIX suite of timers
not being available in its entirety.

  See https://lists.boost.org/boost-bugs/2015/07/41690.php and
  boostorg/log@c98b1f4
  for a description of an analogous problem and fix for Boost.

(cherry picked from commit af7467c)

libcxx/src/atomic.cpp

@@ -25,16 +25,28 @@
 #  if !defined(SYS_futex) && defined(SYS_futex_time64)
 #    define SYS_futex SYS_futex_time64
 #  endif
+#  define _LIBCPP_FUTEX(...) syscall(SYS_futex, __VA_ARGS__)
 
 #elif defined(__FreeBSD__)
 
 #  include <sys/types.h>
 #  include <sys/umtx.h>
 
+#  define _LIBCPP_FUTEX(...) syscall(SYS_futex, __VA_ARGS__)
+
+#elif defined(__OpenBSD__)
+
+#  include <sys/futex.h>
+
+// OpenBSD has no indirect syscalls
+#  define _LIBCPP_FUTEX(...) futex(__VA_ARGS__)
+
 #else // <- Add other operating systems here
 
 // Baseline needs no new headers
 
+#  define _LIBCPP_FUTEX(...) syscall(SYS_futex, __VA_ARGS__)
+
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -44,11 +56,11 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 static void
 __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) {
   static constexpr timespec __timeout = {2, 0};
-  syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
+  _LIBCPP_FUTEX(__ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
 }
 
 static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) {
-  syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
+  _LIBCPP_FUTEX(__ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
 }
 
 #elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK)

libcxx/src/chrono.cpp

@@ -31,7 +31,9 @@
 #  include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if defined(__APPLE__) || defined(__gnu_hurd__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+// OpenBSD does not have a fully conformant suite of POSIX timers, but
+// it does have clock_gettime and CLOCK_MONOTONIC which is all we need.
+#if defined(__APPLE__) || defined(__gnu_hurd__) || defined(__OpenBSD__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
 #  define _LIBCPP_HAS_CLOCK_GETTIME
 #endif
 

libcxxabi/src/cxa_guard_impl.h

@@ -47,6 +47,9 @@
 #include "__cxxabi_config.h"
 #include "include/atomic_support.h" // from libc++
 #if defined(__has_include)
+#  if __has_include(<sys/futex.h>)
+#    include <sys/futex.h>
+#  endif
 #  if __has_include(<sys/syscall.h>)
 #    include <sys/syscall.h>
 #  endif
@@ -411,7 +414,18 @@ struct InitByteGlobalMutex {
 //                         Futex Implementation
 //===----------------------------------------------------------------------===//
 
-#if defined(SYS_futex)
+#if defined(__OpenBSD__)
+void PlatformFutexWait(int* addr, int expect) {
+  constexpr int WAIT = 0;
+  futex(reinterpret_cast<volatile uint32_t*>(addr), WAIT, expect, NULL, NULL);
+  __tsan_acquire(addr);
+}
+void PlatformFutexWake(int* addr) {
+  constexpr int WAKE = 1;
+  __tsan_release(addr);
+  futex(reinterpret_cast<volatile uint32_t*>(addr), WAKE, INT_MAX, NULL, NULL);
+}
+#elif defined(SYS_futex)
 void PlatformFutexWait(int* addr, int expect) {
   constexpr int WAIT = 0;
   syscall(SYS_futex, addr, WAIT, expect, 0);