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