https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99306
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- It's intended to be the cacheline size, so would use std::hardware_destructive_interference_size, but that's not implemented yet for the reasons given in PR 88466. And also because it's just a very verbose way to say 64 :-P