https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109741
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> (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.
That is even the comment:
// increase alignment to put each lock on a separate cache line
