https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109741
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Janez Zemva from comment #1)
> alternatively, the line could be changed into:
>
> struct alignas(void*) M : __gnu_cxx::__mutex { };
>
> since this was probably meant with the magic number 64.
The magic number 64 is the cache line size to make sure there are less sharing.
