[llvm-branch-commits] [libcxx] release/22.x: [libc++] Fix native wait alignment (#180928) (PR #181214)
Cullen Rhodes via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Mon Feb 16 00:53:50 PST 2026
https://github.com/c-rhodes updated https://github.com/llvm/llvm-project/pull/181214
>From ee57019e9ea6065fb26b63d8e962be4aa78abdb1 Mon Sep 17 00:00:00 2001
From: Hui <hui.xie1990 at gmail.com>
Date: Thu, 12 Feb 2026 19:14:18 +0000
Subject: [PATCH] [libc++] Fix native wait alignment (#180928)
This PR fixes two issues regarding the alignment of native wait:
- In the internal platform call, the local variable is copied from a
potentially non-aligned buffer
- Under the unstable ABI, the predicate to test eligibility of a type
being able to do native wait is purely on size. We should test also the
alignment of such type is qualified for platform call
---------
Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
(cherry picked from commit 155beb97492e14e29ab7af3a805bbfb97bee1e6b)
---
.../include/__atomic/atomic_waitable_traits.h | 9 ++--
libcxx/src/atomic.cpp | 15 ++++---
.../atomics.syn/wait.native.compile.pass.cpp | 45 +++++++++++++++++++
3 files changed, 60 insertions(+), 9 deletions(-)
create mode 100644 libcxx/test/libcxx/atomics/atomics.syn/wait.native.compile.pass.cpp
diff --git a/libcxx/include/__atomic/atomic_waitable_traits.h b/libcxx/include/__atomic/atomic_waitable_traits.h
index de06fe70b3e1a..849c33122bff4 100644
--- a/libcxx/include/__atomic/atomic_waitable_traits.h
+++ b/libcxx/include/__atomic/atomic_waitable_traits.h
@@ -69,8 +69,11 @@ concept __atomic_waitable = requires(const _Tp __t, memory_order __order) {
# if defined(_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE)
-_LIBCPP_HIDE_FROM_ABI constexpr bool __has_native_atomic_wait_impl(size_t __size) {
- switch (__size) {
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __has_native_atomic_wait_impl() {
+ if (alignof(_Tp) % sizeof(_Tp) != 0)
+ return false;
+ switch (sizeof(_Tp)) {
# define _LIBCPP_MAKE_CASE(n) \
case n: \
return true;
@@ -84,7 +87,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __has_native_atomic_wait_impl(size_t __size
template <class _Tp>
concept __has_native_atomic_wait =
has_unique_object_representations_v<_Tp> && is_trivially_copyable_v<_Tp> &&
- __has_native_atomic_wait_impl(sizeof(_Tp));
+ std::__has_native_atomic_wait_impl<_Tp>();
# else // _LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE
diff --git a/libcxx/src/atomic.cpp b/libcxx/src/atomic.cpp
index 637c971ed8896..1277b20caf641 100644
--- a/libcxx/src/atomic.cpp
+++ b/libcxx/src/atomic.cpp
@@ -66,7 +66,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
template <std::size_t _Size>
static void __platform_wait_on_address(void const* __ptr, void const* __val, uint64_t __timeout_ns) {
static_assert(_Size == 4, "Can only wait on 4 bytes value");
- char buffer[_Size];
+ alignas(__cxx_contention_t) char buffer[_Size];
std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
static constexpr timespec __default_timeout = {2, 0};
timespec __timeout;
@@ -99,15 +99,18 @@ extern "C" int __ulock_wake(uint32_t operation, void* addr, uint64_t wake_value)
template <std::size_t _Size>
static void __platform_wait_on_address(void const* __ptr, void const* __val, uint64_t __timeout_ns) {
static_assert(_Size == 8 || _Size == 4, "Can only wait on 8 bytes or 4 bytes value");
- char buffer[_Size];
- std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
auto __timeout_us = __timeout_ns == 0 ? 0 : static_cast<uint32_t>(__timeout_ns / 1000);
- if constexpr (_Size == 4)
+ if constexpr (_Size == 4) {
+ alignas(uint32_t) char buffer[_Size];
+ std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
__ulock_wait(
UL_COMPARE_AND_WAIT, const_cast<void*>(__ptr), *reinterpret_cast<uint32_t const*>(&buffer), __timeout_us);
- else
+ } else {
+ alignas(uint64_t) char buffer[_Size];
+ std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
__ulock_wait(
UL_COMPARE_AND_WAIT64, const_cast<void*>(__ptr), *reinterpret_cast<uint64_t const*>(&buffer), __timeout_us);
+ }
}
template <std::size_t _Size>
@@ -130,7 +133,7 @@ static void __platform_wake_by_address(void const* __ptr, bool __notify_one) {
template <std::size_t _Size>
static void __platform_wait_on_address(void const* __ptr, void const* __val, uint64_t __timeout_ns) {
static_assert(_Size == 8, "Can only wait on 8 bytes value");
- char buffer[_Size];
+ alignas(__cxx_contention_t) char buffer[_Size];
std::memcpy(&buffer, const_cast<const void*>(__val), _Size);
if (__timeout_ns == 0) {
_umtx_op(const_cast<void*>(__ptr), UMTX_OP_WAIT, *reinterpret_cast<__cxx_contention_t*>(&buffer), nullptr, nullptr);
diff --git a/libcxx/test/libcxx/atomics/atomics.syn/wait.native.compile.pass.cpp b/libcxx/test/libcxx/atomics/atomics.syn/wait.native.compile.pass.cpp
new file mode 100644
index 0000000000000..88dd2b58b8709
--- /dev/null
+++ b/libcxx/test/libcxx/atomics/atomics.syn/wait.native.compile.pass.cpp
@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-threads
+
+// When __has_native_atomic_wait<T> is true, the atomic object's address will be directly passed to the platform's wait.
+// This test ensures that types that do not satisfy the platform's wait requirement should have __has_native_atomic_wait<T> be false.
+
+#include <atomic>
+#include <cstddef>
+#include <type_traits>
+
+template <std::size_t Size, std::size_t Align>
+struct alignas(Align) Data {
+ char buffer[Size];
+};
+
+static_assert(std::__has_native_atomic_wait<std::__cxx_contention_t>);
+
+#if defined(_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE) && defined(__APPLE__)
+
+static_assert(std::__has_native_atomic_wait<Data<4, 4>>);
+static_assert(std::__has_native_atomic_wait<Data<8, 8>>);
+
+static_assert(!std::has_unique_object_representations_v<Data<4, 8>>);
+static_assert(!std::__has_native_atomic_wait<Data<4, 8>>,
+ "Object with !has_unique_object_representations_v should not have native wait");
+
+static_assert(!std::__has_native_atomic_wait<Data<1, 1>>, "Should only support native wait for 4 and 8 byte types");
+
+// `__ulock_wait` requires the address is aligned to the requested size (4 or 8)
+
+static_assert(!std::__has_native_atomic_wait<Data<4, 1>>,
+ "Should only support native wait for types with properly aligned types");
+
+static_assert(!std::__has_native_atomic_wait<Data<8, 1>>,
+ "Should only support native wait for types with properly aligned types");
+
+#endif
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