[PATCH] libstdc++: Fix required aligment computation for floating points [PR122267]

[Tomasz Kaminski]

On Tue, Oct 14, 2025 at 11:25 AM Tomasz Kamiński <tkaminsk@redhat.com>
wrote:

> Before the r16-4349-g90dde804626f13 the required aligment of floating
> point,
> and pointer specialization used __alignof__(_Vt) as required_aligment, and
> mentioned commit changed them to alignof(_Vt). This values differs on i686
> for double, as alignof(double) is 4, and __alignof__(double) is 8.
>
> This patch restores the previous behavior.
>
>         PR libstdc++/122267
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/atomic_base.h
>         (__atomic_ref_base<const _Tp>::_S_required_aligment): Use
> __alignof__
>         instead of alignof.
> ---
> Tested 29_atomics/atomic_ref/* both with -m32 and without.
> Testing all on x86_64-linux locally. OK for trunk when tests passes?
>
All test passed.

>
>  libstdc++-v3/include/bits/atomic_base.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index 0f3f6b1925d..7e4ad2b1272 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1538,7 +1538,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        _S_required_aligment()
>        {
>         if constexpr (is_floating_point_v<_Vt> || is_pointer_v<_Vt>)
> -         return alignof(_Vt);
> +         return __alignof__(_Vt);
>         else if constexpr ((sizeof(_Vt) & (sizeof(_Vt) - 1)) ||
> sizeof(_Vt) > 16)
>           return alignof(_Vt);
>         else
> --
> 2.51.0
>
>
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