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[libc++] Use Fuchsia futex operations #133571

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[libc++] Use Fuchsia futex operations #133571

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[libc++] Use Fuchsia futex operations
frobtech Mar 29, 2025
ba63615
Merge branch 'main' into p/libcxx-fuchsia-futex
frobtech Jun 27, 2025
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2 changes: 1 addition & 1 deletion libcxx/include/__atomic/contention_t.h
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,7 @@

_LIBCPP_BEGIN_NAMESPACE_STD

#if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
#if defined(__linux__) || defined(__Fuchsia__) || (defined(_AIX) && !defined(__64BIT__))
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@huixie90 huixie90 Jul 12, 2025

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Might be bit unfortunate, but isn’t this an ABI break for your platform ?

using __cxx_contention_t _LIBCPP_NODEBUG = int32_t;
#else
using __cxx_contention_t _LIBCPP_NODEBUG = int64_t;
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2 changes: 1 addition & 1 deletion libcxx/include/__cxx03/__atomic/contention_t.h
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Expand Up @@ -19,7 +19,7 @@

_LIBCPP_BEGIN_NAMESPACE_STD

#if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
#if defined(__linux__) || defined(__Fuchsia__) || (defined(_AIX) && !defined(__64BIT__))
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@philnik777 philnik777 Mar 29, 2025

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Please don't modify the C++03 headers for now.

using __cxx_contention_t = int32_t;
#else
using __cxx_contention_t = int64_t;
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22 changes: 22 additions & 0 deletions libcxx/src/atomic.cpp
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Expand Up @@ -41,6 +41,10 @@
// OpenBSD has no indirect syscalls
# define _LIBCPP_FUTEX(...) futex(__VA_ARGS__)

#elif defined(__Fuchsia__)

# include <zircon/syscalls.h>

#else // <- Add other operating systems here

// Baseline needs no new headers
Expand Down Expand Up @@ -101,6 +105,24 @@ static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const vo
_umtx_op(const_cast<__cxx_atomic_contention_t*>(__ptr), UMTX_OP_WAKE, __notify_one ? 1 : INT_MAX, nullptr, nullptr);
}

#elif defined(__Fuchsia__)

static inline zx_futex_t const* __libcpp_zx_futex(__cxx_atomic_contention_t const volatile* ptr) {
// 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 const_cast<zx_futex_t const*>(reinterpret_cast<zx_futex_t const volatile*>(&ptr->__a_value));
}

static void
__libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr, __cxx_contention_t __val) {
_zx_futex_wait(__libcpp_zx_futex(__ptr), __val, ZX_HANDLE_INVALID, ZX_TIME_INFINITE);
}

static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr, bool __notify_one) {
_zx_futex_wake(__libcpp_zx_futex(__ptr), __notify_one ? 1 : UINT32_MAX);
}

#else // <- Add other operating systems here

// Baseline is just a timed backoff
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