On Wed, 29 Sept 2021 at 13:13, Jonathan Wakely wrote:
> We repeat this *a lot*. When I started work on this I defined a
> non-member function in the __atomic_impl namespace:
I've attached my incomplete patch from when I was working on this.
diff --cc libstdc++-v3/include/bits/atomic_base.h
index 71e1de078b5,35023ad30a8..00000000000
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@@ -944,6 -952,28 +944,27 @@@ _GLIBCXX_BEGIN_NAMESPACE_VERSIO
template<typename _Tp>
using _Val = remove_volatile_t<_Tp>;
+ template<typename _Tp>
+ constexpr bool
+ __maybe_has_padding()
+ {
-#if _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP
- return !__has_unique_object_representations(_Tp);
++#if __has_builtin(__has_unique_object_representations)
++ return !__has_unique_object_representations(_Tp)
++ && !is_floating_point<_Tp>::value;
+ #else
+ return true;
+ #endif
+ }
+
+ template<typename _Tp>
+ _GLIBCXX_ALWAYS_INLINE void
+ __clear_padding(_Tp& __val) noexcept
+ {
- auto* __ptr = std::__addressof(__val);
+ #if __has_builtin(__builtin_clear_padding)
- __builtin_clear_padding(__ptr);
++ __builtin_clear_padding(std::__addressof(__val));
+ #endif
- return __ptr;
+ }
+
// As above, but for difference_type arguments.
template<typename _Tp>
using _Diff = conditional_t<is_pointer_v<_Tp>, ptrdiff_t, _Val<_Tp>>;
@@@ -984,13 -1015,26 +1006,26 @@@
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE bool
compare_exchange_weak(_Tp* __ptr, _Val<_Tp>& __expected,
- _Val<_Tp> __desired, memory_order __success,
- memory_order __failure) noexcept
+ _Val<_Tp> __desired,
+ memory_order __success, memory_order __failure,
+ bool __weak = true) noexcept
{
+ __glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
-
+ #if __has_builtin(__builtin_clear_padding)
+ if _GLIBCXX_CONSTEXPR17 (__maybe_has_padding<_Tp>())
+ {
+ _Val<_Tp> __expected0 = __expected;
+ auto* __exp = __atomic_impl::__clear_padding(__expected0);
+ auto* __des = __atomic_impl::__clear_padding(__desired);
+ if (__atomic_compare_exchange(__ptr, __exp, __des, __weak,
+ int(__success), int(__failure)))
+ return true;
+ __builtin_memcpy(std::__addressof(__expected), __exp, sizeof(_Tp));
+ return false;
+ }
- else
+ #endif
return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
- std::__addressof(__desired), true,
+ std::__addressof(__desired), __weak,
int(__success), int(__failure));
}
@@@ -1000,11 -1044,8 +1035,9 @@@
_Val<_Tp> __desired, memory_order __success,
memory_order __failure) noexcept
{
+ __glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
-
- return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
- std::__addressof(__desired), false,
- int(__success), int(__failure));
+ return compare_exchange_weak(__ptr, __expected, __desired, __success,
+ __failure, false);
}
#if __cpp_lib_atomic_wait
