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[libc++] Use Fuchsia futex operations (#133571)

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The basic futex operations have always been available on Fuchsia.
Wire them up to properly support C++20 atomic notify_*/wait.
This commit is contained in:
Roland McGrath 2026-04-08 14:37:54 -07:00 committed by GitHub
parent be072baa33
commit fbdfff47c7
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GPG key ID: B5690EEEBB952194
3 changed files with 41 additions and 6 deletions
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8
libcxx/include/__atomic/contention_t.h
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@ -25,7 +25,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
// instead.
#if defined(_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE)
# ifdef __linux__
# if defined(__linux__) || defined(__Fuchsia__)
using __cxx_contention_t _LIBCPP_NODEBUG = int32_t;
# elif defined(__APPLE__)
using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
@ -37,15 +37,15 @@ using __cxx_contention_t _LIBCPP_NODEBUG = int32_t;
using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
# else
using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
# endif // __linux__
# endif // __linux__ || __Fuchsia__
#else // _LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE
# if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
# if defined(__linux__) || defined(__Fuchsia__) || (defined(_AIX) && !defined(__64BIT__))
using __cxx_contention_t _LIBCPP_NODEBUG = int32_t;
# else
using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
# endif // __linux__ || (_AIX && !__64BIT__)
# endif // __linux__ || __Fuchsia__ || (_AIX && !__64BIT__)
#endif // _LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE

4
libcxx/include/__cxx03/__atomic/contention_t.h
View file

@ -19,11 +19,11 @@
_LIBCPP_BEGIN_NAMESPACE_STD
#if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
#if defined(__linux__) || defined(__Fuchsia__) || (defined(_AIX) && !defined(__64BIT__))
using __cxx_contention_t = int32_t;
#else
using __cxx_contention_t = int64_t;
#endif // __linux__ || (_AIX && !__64BIT__)
#endif // __linux__ || __Fuchsia__ || (_AIX && !__64BIT__)
using __cxx_atomic_contention_t = __cxx_atomic_impl<__cxx_contention_t>;

35
libcxx/src/atomic.cpp
View file

@ -51,6 +51,10 @@
# include <memory>
# include <windows.h>
#elif defined(__Fuchsia__)
# include <zircon/syscalls.h>
#else // <- Add other operating systems here
// Baseline needs no new headers
@ -254,6 +258,37 @@ static void __platform_wake_by_address(void const* __ptr, bool __notify_one) {
}
}
#elif defined(__Fuchsia__)
template <std::size_t _Size>
static inline zx_futex_t const* __get_zx_futex(void const* __ptr) {
static_assert(_Size == sizeof(zx_futex_t), "Can only wait/wake on zx_futex_t-compatible value");
// Implicitly link against the vDSO system call ABI without requiring the
// final link to specify -lzircon explicitly when statically linking libc++.
# pragma comment(lib, "zircon")
return reinterpret_cast<zx_futex_t const*>(__ptr);
}
template <std::size_t _Size, class MaybeTimeout>
static void __platform_wait_on_address(void const* __ptr, void const* __val, MaybeTimeout maybe_timeout_ns) {
zx_futex_t val;
std::memcpy(&val, __val, _Size);
zx_instant_mono_t deadline;
if constexpr (is_same_v<MaybeTimeout, NoTimeout>) {
deadline = ZX_TIME_INFINITE;
} else {
deadline = _zx_deadline_after(maybe_timeout_ns);
}
_zx_futex_wait(__get_zx_futex<_Size>(__ptr), val, ZX_HANDLE_INVALID, deadline);
}
template <std::size_t _Size>
static void __platform_wake_by_address(void const* __ptr, bool __notify_one) {
_zx_futex_wake(__get_zx_futex<_Size>(__ptr), __notify_one ? 1 : UINT32_MAX);
}
#else // <- Add other operating systems here
// Baseline is just a timed backoff