[libcxx-commits] [libcxx] cc1dfb3 - [libc++] Fix bug in atomic_ref's calculation of lock_free-ness (#99570)
via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Jul 26 11:10:09 PDT 2024
Author: Louis Dionne
Date: 2024-07-26T14:10:06-04:00
New Revision: cc1dfb37aa84d1524243b83fadb8ff0f821e03e9
URL: https://github.com/llvm/llvm-project/commit/cc1dfb37aa84d1524243b83fadb8ff0f821e03e9
DIFF: https://github.com/llvm/llvm-project/commit/cc1dfb37aa84d1524243b83fadb8ff0f821e03e9.diff
LOG: [libc++] Fix bug in atomic_ref's calculation of lock_free-ness (#99570)
The builtin __atomic_always_lock_free takes into account the type of the
pointer provided as the second argument. Because we were passing void*,
rather than T*, the calculation failed. This meant that
atomic_ref<T>::is_always_lock_free was only true for char & bool.
This bug exists elsewhere in the atomic library (when using GCC, we fail
to pass a pointer at all, and we fail to correctly align the atomic like
_Atomic would).
This change also attempts to start sorting out testing difficulties with
this function that caused the bug to exist by using the
__GCC_ATOMIC_(CHAR|SHORT|INT|LONG|LLONG|POINTER)_IS_LOCK_FREE predefined
macros to establish an expected value for `is_always_lock_free` and
`is_lock_free` for the respective types, as well as types with matching
sizes and compatible alignment values.
Using these compiler pre-defines we can actually validate that certain
types, like char and int, are actually always lock free like they are on
every platform in the wild.
Note that this patch was actually authored by Eric Fiselier but I picked
up the patch and GitHub won't let me set Eric as the primary author.
Co-authored-by: Eric Fiselier <eric at efcs.ca>
Added:
libcxx/test/std/atomics/atomics.lockfree/is_always_lock_free.cpp
Modified:
libcxx/include/__atomic/atomic_ref.h
libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp
libcxx/test/support/atomic_helpers.h
Removed:
libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
################################################################################
diff --git a/libcxx/include/__atomic/atomic_ref.h b/libcxx/include/__atomic/atomic_ref.h
index 156f1961151c1..2849b82e1a3dd 100644
--- a/libcxx/include/__atomic/atomic_ref.h
+++ b/libcxx/include/__atomic/atomic_ref.h
@@ -42,6 +42,19 @@ _LIBCPP_BEGIN_NAMESPACE_STD
#if _LIBCPP_STD_VER >= 20
+// These types are required to make __atomic_is_always_lock_free work across GCC and Clang.
+// The purpose of this trick is to make sure that we provide an object with the correct alignment
+// to __atomic_is_always_lock_free, since that answer depends on the alignment.
+template <size_t _Alignment>
+struct __alignment_checker_type {
+ alignas(_Alignment) char __data;
+};
+
+template <size_t _Alignment>
+struct __get_aligner_instance {
+ static constexpr __alignment_checker_type<_Alignment> __instance{};
+};
+
template <class _Tp>
struct __atomic_ref_base {
protected:
@@ -105,7 +118,7 @@ struct __atomic_ref_base {
// that the pointer is going to be aligned properly at runtime because that is a (checked) precondition
// of atomic_ref's constructor.
static constexpr bool is_always_lock_free =
- __atomic_always_lock_free(sizeof(_Tp), reinterpret_cast<void*>(-required_alignment));
+ __atomic_always_lock_free(sizeof(_Tp), &__get_aligner_instance<required_alignment>::__instance);
_LIBCPP_HIDE_FROM_ABI bool is_lock_free() const noexcept { return __atomic_is_lock_free(sizeof(_Tp), __ptr_); }
diff --git a/libcxx/test/std/atomics/atomics.lockfree/is_always_lock_free.cpp b/libcxx/test/std/atomics/atomics.lockfree/is_always_lock_free.cpp
new file mode 100644
index 0000000000000..2dc7f5c765419
--- /dev/null
+++ b/libcxx/test/std/atomics/atomics.lockfree/is_always_lock_free.cpp
@@ -0,0 +1,165 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <atomic>
+//
+// template <class T>
+// class atomic;
+//
+// static constexpr bool is_always_lock_free;
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+
+#include "test_macros.h"
+#include "atomic_helpers.h"
+
+template <typename T>
+void check_always_lock_free(std::atomic<T> const& a) {
+ using InfoT = LockFreeStatusInfo<T>;
+
+ constexpr std::same_as<const bool> decltype(auto) is_always_lock_free = std::atomic<T>::is_always_lock_free;
+
+ // If we know the status of T for sure, validate the exact result of the function.
+ if constexpr (InfoT::status_known) {
+ constexpr LockFreeStatus known_status = InfoT::value;
+ if constexpr (known_status == LockFreeStatus::always) {
+ static_assert(is_always_lock_free, "is_always_lock_free is inconsistent with known lock-free status");
+ assert(a.is_lock_free() && "is_lock_free() is inconsistent with known lock-free status");
+ } else if constexpr (known_status == LockFreeStatus::never) {
+ static_assert(!is_always_lock_free, "is_always_lock_free is inconsistent with known lock-free status");
+ assert(!a.is_lock_free() && "is_lock_free() is inconsistent with known lock-free status");
+ } else {
+ assert(a.is_lock_free() || !a.is_lock_free()); // This is kinda dumb, but we might as well call the function once.
+ }
+ }
+
+ // In all cases, also sanity-check it based on the implication always-lock-free => lock-free.
+ if (is_always_lock_free) {
+ std::same_as<bool> decltype(auto) is_lock_free = a.is_lock_free();
+ assert(is_lock_free);
+ }
+ ASSERT_NOEXCEPT(a.is_lock_free());
+}
+
+#define CHECK_ALWAYS_LOCK_FREE(T) \
+ do { \
+ typedef T type; \
+ type obj{}; \
+ std::atomic<type> a(obj); \
+ check_always_lock_free(a); \
+ } while (0)
+
+void test() {
+ char c = 'x';
+ check_always_lock_free(std::atomic<char>(c));
+
+ int i = 0;
+ check_always_lock_free(std::atomic<int>(i));
+
+ float f = 0.f;
+ check_always_lock_free(std::atomic<float>(f));
+
+ int* p = &i;
+ check_always_lock_free(std::atomic<int*>(p));
+
+ CHECK_ALWAYS_LOCK_FREE(bool);
+ CHECK_ALWAYS_LOCK_FREE(char);
+ CHECK_ALWAYS_LOCK_FREE(signed char);
+ CHECK_ALWAYS_LOCK_FREE(unsigned char);
+#if TEST_STD_VER > 17 && defined(__cpp_char8_t)
+ CHECK_ALWAYS_LOCK_FREE(char8_t);
+#endif
+ CHECK_ALWAYS_LOCK_FREE(char16_t);
+ CHECK_ALWAYS_LOCK_FREE(char32_t);
+ CHECK_ALWAYS_LOCK_FREE(wchar_t);
+ CHECK_ALWAYS_LOCK_FREE(short);
+ CHECK_ALWAYS_LOCK_FREE(unsigned short);
+ CHECK_ALWAYS_LOCK_FREE(int);
+ CHECK_ALWAYS_LOCK_FREE(unsigned int);
+ CHECK_ALWAYS_LOCK_FREE(long);
+ CHECK_ALWAYS_LOCK_FREE(unsigned long);
+ CHECK_ALWAYS_LOCK_FREE(long long);
+ CHECK_ALWAYS_LOCK_FREE(unsigned long long);
+ CHECK_ALWAYS_LOCK_FREE(std::nullptr_t);
+ CHECK_ALWAYS_LOCK_FREE(void*);
+ CHECK_ALWAYS_LOCK_FREE(float);
+ CHECK_ALWAYS_LOCK_FREE(double);
+ CHECK_ALWAYS_LOCK_FREE(long double);
+#if __has_attribute(vector_size) && defined(_LIBCPP_VERSION)
+ CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(1 * sizeof(int)))));
+ CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(2 * sizeof(int)))));
+ CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(4 * sizeof(int)))));
+ CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(16 * sizeof(int)))));
+ CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(32 * sizeof(int)))));
+ CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(1 * sizeof(float)))));
+ CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(2 * sizeof(float)))));
+ CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(4 * sizeof(float)))));
+ CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(16 * sizeof(float)))));
+ CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(32 * sizeof(float)))));
+ CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(1 * sizeof(double)))));
+ CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(2 * sizeof(double)))));
+ CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(4 * sizeof(double)))));
+ CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(16 * sizeof(double)))));
+ CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(32 * sizeof(double)))));
+#endif // __has_attribute(vector_size) && defined(_LIBCPP_VERSION)
+ CHECK_ALWAYS_LOCK_FREE(struct Empty{});
+ CHECK_ALWAYS_LOCK_FREE(struct OneInt { int i; });
+ CHECK_ALWAYS_LOCK_FREE(struct IntArr2 { int i[2]; });
+ CHECK_ALWAYS_LOCK_FREE(struct FloatArr3 { float i[3]; });
+ CHECK_ALWAYS_LOCK_FREE(struct LLIArr2 { long long int i[2]; });
+ CHECK_ALWAYS_LOCK_FREE(struct LLIArr4 { long long int i[4]; });
+ CHECK_ALWAYS_LOCK_FREE(struct LLIArr8 { long long int i[8]; });
+ CHECK_ALWAYS_LOCK_FREE(struct LLIArr16 { long long int i[16]; });
+ CHECK_ALWAYS_LOCK_FREE(struct Padding {
+ char c; /* padding */
+ long long int i;
+ });
+ CHECK_ALWAYS_LOCK_FREE(union IntFloat {
+ int i;
+ float f;
+ });
+ CHECK_ALWAYS_LOCK_FREE(enum class CharEnumClass : char{foo});
+
+ // C macro and static constexpr must be consistent.
+ enum class CharEnumClass : char { foo };
+ static_assert(std::atomic<bool>::is_always_lock_free == (2 == ATOMIC_BOOL_LOCK_FREE), "");
+ static_assert(std::atomic<char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
+ static_assert(std::atomic<CharEnumClass>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
+ static_assert(std::atomic<signed char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
+ static_assert(std::atomic<unsigned char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
+#if TEST_STD_VER > 17 && defined(__cpp_char8_t)
+ static_assert(std::atomic<char8_t>::is_always_lock_free == (2 == ATOMIC_CHAR8_T_LOCK_FREE), "");
+#endif
+ static_assert(std::atomic<char16_t>::is_always_lock_free == (2 == ATOMIC_CHAR16_T_LOCK_FREE), "");
+ static_assert(std::atomic<char32_t>::is_always_lock_free == (2 == ATOMIC_CHAR32_T_LOCK_FREE), "");
+ static_assert(std::atomic<wchar_t>::is_always_lock_free == (2 == ATOMIC_WCHAR_T_LOCK_FREE), "");
+ static_assert(std::atomic<short>::is_always_lock_free == (2 == ATOMIC_SHORT_LOCK_FREE), "");
+ static_assert(std::atomic<unsigned short>::is_always_lock_free == (2 == ATOMIC_SHORT_LOCK_FREE), "");
+ static_assert(std::atomic<int>::is_always_lock_free == (2 == ATOMIC_INT_LOCK_FREE), "");
+ static_assert(std::atomic<unsigned int>::is_always_lock_free == (2 == ATOMIC_INT_LOCK_FREE), "");
+ static_assert(std::atomic<long>::is_always_lock_free == (2 == ATOMIC_LONG_LOCK_FREE), "");
+ static_assert(std::atomic<unsigned long>::is_always_lock_free == (2 == ATOMIC_LONG_LOCK_FREE), "");
+ static_assert(std::atomic<long long>::is_always_lock_free == (2 == ATOMIC_LLONG_LOCK_FREE), "");
+ static_assert(std::atomic<unsigned long long>::is_always_lock_free == (2 == ATOMIC_LLONG_LOCK_FREE), "");
+ static_assert(std::atomic<void*>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE), "");
+ static_assert(std::atomic<std::nullptr_t>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE), "");
+
+#if TEST_STD_VER >= 20
+ static_assert(std::atomic_signed_lock_free::is_always_lock_free, "");
+ static_assert(std::atomic_unsigned_lock_free::is_always_lock_free, "");
+#endif
+}
+
+int main(int, char**) {
+ test();
+ return 0;
+}
diff --git a/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp b/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
deleted file mode 100644
index 6d6e6477bc251..0000000000000
--- a/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
+++ /dev/null
@@ -1,120 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-
-// <atomic>
-
-// static constexpr bool is_always_lock_free;
-
-#include <atomic>
-#include <cassert>
-#include <cstddef>
-
-#include "test_macros.h"
-
-template <typename T>
-void checkAlwaysLockFree() {
- if (std::atomic<T>::is_always_lock_free) {
- assert(std::atomic<T>().is_lock_free());
- }
-}
-
-void run()
-{
-// structs and unions can't be defined in the template invocation.
-// Work around this with a typedef.
-#define CHECK_ALWAYS_LOCK_FREE(T) \
- do { \
- typedef T type; \
- checkAlwaysLockFree<type>(); \
- } while (0)
-
- CHECK_ALWAYS_LOCK_FREE(bool);
- CHECK_ALWAYS_LOCK_FREE(char);
- CHECK_ALWAYS_LOCK_FREE(signed char);
- CHECK_ALWAYS_LOCK_FREE(unsigned char);
-#if TEST_STD_VER > 17 && defined(__cpp_char8_t)
- CHECK_ALWAYS_LOCK_FREE(char8_t);
-#endif
- CHECK_ALWAYS_LOCK_FREE(char16_t);
- CHECK_ALWAYS_LOCK_FREE(char32_t);
- CHECK_ALWAYS_LOCK_FREE(wchar_t);
- CHECK_ALWAYS_LOCK_FREE(short);
- CHECK_ALWAYS_LOCK_FREE(unsigned short);
- CHECK_ALWAYS_LOCK_FREE(int);
- CHECK_ALWAYS_LOCK_FREE(unsigned int);
- CHECK_ALWAYS_LOCK_FREE(long);
- CHECK_ALWAYS_LOCK_FREE(unsigned long);
- CHECK_ALWAYS_LOCK_FREE(long long);
- CHECK_ALWAYS_LOCK_FREE(unsigned long long);
- CHECK_ALWAYS_LOCK_FREE(std::nullptr_t);
- CHECK_ALWAYS_LOCK_FREE(void*);
- CHECK_ALWAYS_LOCK_FREE(float);
- CHECK_ALWAYS_LOCK_FREE(double);
- CHECK_ALWAYS_LOCK_FREE(long double);
-#if __has_attribute(vector_size) && defined(_LIBCPP_VERSION)
- CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(1 * sizeof(int)))));
- CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(2 * sizeof(int)))));
- CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(4 * sizeof(int)))));
- CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(16 * sizeof(int)))));
- CHECK_ALWAYS_LOCK_FREE(int __attribute__((vector_size(32 * sizeof(int)))));
- CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(1 * sizeof(float)))));
- CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(2 * sizeof(float)))));
- CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(4 * sizeof(float)))));
- CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(16 * sizeof(float)))));
- CHECK_ALWAYS_LOCK_FREE(float __attribute__((vector_size(32 * sizeof(float)))));
- CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(1 * sizeof(double)))));
- CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(2 * sizeof(double)))));
- CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(4 * sizeof(double)))));
- CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(16 * sizeof(double)))));
- CHECK_ALWAYS_LOCK_FREE(double __attribute__((vector_size(32 * sizeof(double)))));
-#endif // __has_attribute(vector_size) && defined(_LIBCPP_VERSION)
- CHECK_ALWAYS_LOCK_FREE(struct Empty {});
- CHECK_ALWAYS_LOCK_FREE(struct OneInt { int i; });
- CHECK_ALWAYS_LOCK_FREE(struct IntArr2 { int i[2]; });
- CHECK_ALWAYS_LOCK_FREE(struct FloatArr3 { float i[3]; });
- CHECK_ALWAYS_LOCK_FREE(struct LLIArr2 { long long int i[2]; });
- CHECK_ALWAYS_LOCK_FREE(struct LLIArr4 { long long int i[4]; });
- CHECK_ALWAYS_LOCK_FREE(struct LLIArr8 { long long int i[8]; });
- CHECK_ALWAYS_LOCK_FREE(struct LLIArr16 { long long int i[16]; });
- CHECK_ALWAYS_LOCK_FREE(struct Padding { char c; /* padding */ long long int i; });
- CHECK_ALWAYS_LOCK_FREE(union IntFloat { int i; float f; });
- CHECK_ALWAYS_LOCK_FREE(enum class CharEnumClass : char { foo });
-
- // C macro and static constexpr must be consistent.
- enum class CharEnumClass : char { foo };
- static_assert(std::atomic<bool>::is_always_lock_free == (2 == ATOMIC_BOOL_LOCK_FREE), "");
- static_assert(std::atomic<char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
- static_assert(std::atomic<CharEnumClass>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
- static_assert(std::atomic<signed char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
- static_assert(std::atomic<unsigned char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
-#if TEST_STD_VER > 17 && defined(__cpp_char8_t)
- static_assert(std::atomic<char8_t>::is_always_lock_free == (2 == ATOMIC_CHAR8_T_LOCK_FREE), "");
-#endif
- static_assert(std::atomic<char16_t>::is_always_lock_free == (2 == ATOMIC_CHAR16_T_LOCK_FREE), "");
- static_assert(std::atomic<char32_t>::is_always_lock_free == (2 == ATOMIC_CHAR32_T_LOCK_FREE), "");
- static_assert(std::atomic<wchar_t>::is_always_lock_free == (2 == ATOMIC_WCHAR_T_LOCK_FREE), "");
- static_assert(std::atomic<short>::is_always_lock_free == (2 == ATOMIC_SHORT_LOCK_FREE), "");
- static_assert(std::atomic<unsigned short>::is_always_lock_free == (2 == ATOMIC_SHORT_LOCK_FREE), "");
- static_assert(std::atomic<int>::is_always_lock_free == (2 == ATOMIC_INT_LOCK_FREE), "");
- static_assert(std::atomic<unsigned int>::is_always_lock_free == (2 == ATOMIC_INT_LOCK_FREE), "");
- static_assert(std::atomic<long>::is_always_lock_free == (2 == ATOMIC_LONG_LOCK_FREE), "");
- static_assert(std::atomic<unsigned long>::is_always_lock_free == (2 == ATOMIC_LONG_LOCK_FREE), "");
- static_assert(std::atomic<long long>::is_always_lock_free == (2 == ATOMIC_LLONG_LOCK_FREE), "");
- static_assert(std::atomic<unsigned long long>::is_always_lock_free == (2 == ATOMIC_LLONG_LOCK_FREE), "");
- static_assert(std::atomic<void*>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE), "");
- static_assert(std::atomic<std::nullptr_t>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE), "");
-
-#if TEST_STD_VER >= 20
- static_assert(std::atomic_signed_lock_free::is_always_lock_free, "");
- static_assert(std::atomic_unsigned_lock_free::is_always_lock_free, "");
-#endif
-}
-
-int main(int, char**) { run(); return 0; }
diff --git a/libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp b/libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp
index 94f65e3b4b669..acdbf63a24d85 100644
--- a/libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp
@@ -9,7 +9,10 @@
// UNSUPPORTED: c++03, c++11, c++14, c++17
// <atomic>
-
+//
+// template <class T>
+// class atomic_ref;
+//
// static constexpr bool is_always_lock_free;
// bool is_lock_free() const noexcept;
@@ -18,10 +21,29 @@
#include <concepts>
#include "test_macros.h"
+#include "atomic_helpers.h"
template <typename T>
-void check_always_lock_free(std::atomic_ref<T> const a) {
- std::same_as<const bool> decltype(auto) is_always_lock_free = std::atomic_ref<T>::is_always_lock_free;
+void check_always_lock_free(std::atomic_ref<T> const& a) {
+ using InfoT = LockFreeStatusInfo<T>;
+
+ constexpr std::same_as<const bool> decltype(auto) is_always_lock_free = std::atomic_ref<T>::is_always_lock_free;
+
+ // If we know the status of T for sure, validate the exact result of the function.
+ if constexpr (InfoT::status_known) {
+ constexpr LockFreeStatus known_status = InfoT::value;
+ if constexpr (known_status == LockFreeStatus::always) {
+ static_assert(is_always_lock_free, "is_always_lock_free is inconsistent with known lock-free status");
+ assert(a.is_lock_free() && "is_lock_free() is inconsistent with known lock-free status");
+ } else if constexpr (known_status == LockFreeStatus::never) {
+ static_assert(!is_always_lock_free, "is_always_lock_free is inconsistent with known lock-free status");
+ assert(!a.is_lock_free() && "is_lock_free() is inconsistent with known lock-free status");
+ } else {
+ assert(a.is_lock_free() || !a.is_lock_free()); // This is kinda dumb, but we might as well call the function once.
+ }
+ }
+
+ // In all cases, also sanity-check it based on the implication always-lock-free => lock-free.
if (is_always_lock_free) {
std::same_as<bool> decltype(auto) is_lock_free = a.is_lock_free();
assert(is_lock_free);
@@ -33,10 +55,14 @@ void check_always_lock_free(std::atomic_ref<T> const a) {
do { \
typedef T type; \
type obj{}; \
- check_always_lock_free(std::atomic_ref<type>(obj)); \
+ std::atomic_ref<type> a(obj); \
+ check_always_lock_free(a); \
} while (0)
void test() {
+ char c = 'x';
+ check_always_lock_free(std::atomic_ref<char>(c));
+
int i = 0;
check_always_lock_free(std::atomic_ref<int>(i));
diff --git a/libcxx/test/support/atomic_helpers.h b/libcxx/test/support/atomic_helpers.h
index 0266a0961067b..d2f2b751cb47d 100644
--- a/libcxx/test/support/atomic_helpers.h
+++ b/libcxx/test/support/atomic_helpers.h
@@ -11,9 +11,112 @@
#include <cassert>
#include <cstdint>
+#include <cstddef>
+#include <type_traits>
#include "test_macros.h"
+#if defined(TEST_COMPILER_CLANG)
+# define TEST_ATOMIC_CHAR_LOCK_FREE __CLANG_ATOMIC_CHAR_LOCK_FREE
+# define TEST_ATOMIC_SHORT_LOCK_FREE __CLANG_ATOMIC_SHORT_LOCK_FREE
+# define TEST_ATOMIC_INT_LOCK_FREE __CLANG_ATOMIC_INT_LOCK_FREE
+# define TEST_ATOMIC_LONG_LOCK_FREE __CLANG_ATOMIC_LONG_LOCK_FREE
+# define TEST_ATOMIC_LLONG_LOCK_FREE __CLANG_ATOMIC_LLONG_LOCK_FREE
+# define TEST_ATOMIC_POINTER_LOCK_FREE __CLANG_ATOMIC_POINTER_LOCK_FREE
+#elif defined(TEST_COMPILER_GCC)
+# define TEST_ATOMIC_CHAR_LOCK_FREE __GCC_ATOMIC_CHAR_LOCK_FREE
+# define TEST_ATOMIC_SHORT_LOCK_FREE __GCC_ATOMIC_SHORT_LOCK_FREE
+# define TEST_ATOMIC_INT_LOCK_FREE __GCC_ATOMIC_INT_LOCK_FREE
+# define TEST_ATOMIC_LONG_LOCK_FREE __GCC_ATOMIC_LONG_LOCK_FREE
+# define TEST_ATOMIC_LLONG_LOCK_FREE __GCC_ATOMIC_LLONG_LOCK_FREE
+# define TEST_ATOMIC_POINTER_LOCK_FREE __GCC_ATOMIC_POINTER_LOCK_FREE
+#elif TEST_COMPILER_MSVC
+// This is lifted from STL/stl/inc/atomic on github for the purposes of
+// keeping the tests compiling for MSVC's STL. It's not a perfect solution
+// but at least the tests will keep running.
+//
+// Note MSVC's STL never produces a type that is sometimes lock free, but not always lock free.
+template <class T, size_t Size = sizeof(T)>
+constexpr bool msvc_is_lock_free_macro_value() {
+ return (Size <= 8 && (Size & Size - 1) == 0) ? 2 : 0;
+}
+# define TEST_ATOMIC_CHAR_LOCK_FREE ::msvc_is_lock_free_macro_value<char>()
+# define TEST_ATOMIC_SHORT_LOCK_FREE ::msvc_is_lock_free_macro_value<short>()
+# define TEST_ATOMIC_INT_LOCK_FREE ::msvc_is_lock_free_macro_value<int>()
+# define TEST_ATOMIC_LONG_LOCK_FREE ::msvc_is_lock_free_macro_value<long>()
+# define TEST_ATOMIC_LLONG_LOCK_FREE ::msvc_is_lock_free_macro_value<long long>()
+# define TEST_ATOMIC_POINTER_LOCK_FREE ::msvc_is_lock_free_macro_value<void*>()
+#else
+# error "Unknown compiler"
+#endif
+
+#ifdef TEST_COMPILER_CLANG
+# pragma clang diagnostic push
+# pragma clang diagnostic ignored "-Wc++11-extensions"
+#endif
+
+enum class LockFreeStatus : int { unknown = -1, never = 0, sometimes = 1, always = 2 };
+
+// We should really be checking whether the alignment of T is greater-than-or-equal-to the alignment required
+// for T to be atomic, but this is basically impossible to implement portably. Instead, we assume that any type
+// aligned to at least its size is going to be atomic if there exists atomic operations for that size at all,
+// which is true on most platforms. This technically reduces our test coverage in the sense that if a type has
+// an alignment requirement less than its size but could still be made lockfree, LockFreeStatusInfo will report
+// that we don't know whether it is lockfree or not.
+#define COMPARE_TYPES(T, FundamentalT) (sizeof(T) == sizeof(FundamentalT) && TEST_ALIGNOF(T) >= sizeof(T))
+
+template <class T>
+struct LockFreeStatusInfo {
+ static const LockFreeStatus value = LockFreeStatus(
+ COMPARE_TYPES(T, char)
+ ? TEST_ATOMIC_CHAR_LOCK_FREE
+ : (COMPARE_TYPES(T, short)
+ ? TEST_ATOMIC_SHORT_LOCK_FREE
+ : (COMPARE_TYPES(T, int)
+ ? TEST_ATOMIC_INT_LOCK_FREE
+ : (COMPARE_TYPES(T, long)
+ ? TEST_ATOMIC_LONG_LOCK_FREE
+ : (COMPARE_TYPES(T, long long)
+ ? TEST_ATOMIC_LLONG_LOCK_FREE
+ : (COMPARE_TYPES(T, void*) ? TEST_ATOMIC_POINTER_LOCK_FREE : -1))))));
+
+ static const bool status_known = LockFreeStatusInfo::value != LockFreeStatus::unknown;
+};
+
+#undef COMPARE_TYPES
+
+// This doesn't work in C++03 due to issues with scoped enumerations. Just disable the test.
+#if TEST_STD_VER >= 11
+static_assert(LockFreeStatusInfo<char>::status_known, "");
+static_assert(LockFreeStatusInfo<short>::status_known, "");
+static_assert(LockFreeStatusInfo<int>::status_known, "");
+static_assert(LockFreeStatusInfo<long>::status_known, "");
+static_assert(LockFreeStatusInfo<void*>::status_known, "");
+
+// long long is a bit funky: on some platforms, its alignment is 4 bytes but its size is
+// 8 bytes. In that case, atomics may or may not be lockfree based on their address.
+static_assert(alignof(long long) == sizeof(long long) ? LockFreeStatusInfo<long long>::status_known : true, "");
+
+// Those should always be lock free: hardcode some expected values to make sure our tests are actually
+// testing something meaningful.
+static_assert(LockFreeStatusInfo<char>::value == LockFreeStatus::always, "");
+static_assert(LockFreeStatusInfo<short>::value == LockFreeStatus::always, "");
+static_assert(LockFreeStatusInfo<int>::value == LockFreeStatus::always, "");
+#endif
+
+// These macros are somewhat suprising to use, since they take the values 0, 1, or 2.
+// To make the tests clearer, get rid of them in preference of LockFreeStatusInfo.
+#undef TEST_ATOMIC_CHAR_LOCK_FREE
+#undef TEST_ATOMIC_SHORT_LOCK_FREE
+#undef TEST_ATOMIC_INT_LOCK_FREE
+#undef TEST_ATOMIC_LONG_LOCK_FREE
+#undef TEST_ATOMIC_LLONG_LOCK_FREE
+#undef TEST_ATOMIC_POINTER_LOCK_FREE
+
+#ifdef TEST_COMPILER_CLANG
+# pragma clang diagnostic pop
+#endif
+
struct UserAtomicType {
int i;
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