https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128009
--- Comment #2 from Kyuwon Cho <kyuwoncho18 at gmail dot com> --- Bytes 10-15 of union V are long double padding, but also value bits of the __int128 member. They get defined values from the v.b[] (unsigned char array) stores, and p->q (unsigned __int128) reads them.
