https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116331
--- Comment #4 from Carl Dehlin <carl at dehlin dot com> --- Found answer or 3) here: https://sourceware.org/glibc/manual/latest/html_mono/libc.html#Known-Maximum-Errors-in-Math-Functions . For x86_64 cos is accurate up to 1 ulp from the correctly rounded value. As I understand the builtin version is always correctly rounded, hence maximum 1 ulp difference.
