[RFC] libstdc++: optional: Set _M_engaged after the call to destructor [PR127005]

[Jonathan Wakely]

On Sun, 23 Aug 2026, 20:26 Andrea Pinski, <andrew.pinski@oss.qualcomm.com>
wrote:

> Sometimes the destructor will cause the optimizers not to see that
> _M_engaged has been to set to false (e.g. an external call). This
> means that we might get an uninitialized warning after the destructor
> in some cases.
> Setting it after the destructor will cause one extra store in some cases
> but that store allows the optimizers to get rid of other code.
>
> This is a RFC because I don't know if we want to remove the first store
> of _M_engaged or keep it.
> The patch has been bootstrapped and tested on x86_64-linux-gnu with no
> regressions.
> And tested provided in the bug report works and also the generated code
> looks much
> better too.
>
>         PR libstdc++/127005
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/optional (optional::_M_destroy): Set _M_engaged
>         to false after the destructor call.
>
> Signed-off-by: Andrea Pinski <andrew.pinski@oss.qualcomm.com>
> ---
>  libstdc++-v3/include/std/optional | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/libstdc++-v3/include/std/optional
> b/libstdc++-v3/include/std/optional
> index d6fd33e8c2e..8884c54f3e7 100644
> --- a/libstdc++-v3/include/std/optional
> +++ b/libstdc++-v3/include/std/optional
> @@ -303,6 +303,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>         _M_engaged = false;
>         _M_payload._M_value.~_Stored_type();
> +       _M_engaged = false;
>

Do you see the same benefits from

if (_M_engaged)
  __builtin_unreachable();

?

 #if defined(__clang__) && __cpp_lib_optional >= 202106L // full constexpr
> support
>         if (std::is_constant_evaluated())
>           // Work around PR124910 for Clang.
> --
> 2.43.0
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20260824/d847c398/attachment.htm>