https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726
--- Comment #11 from Chip Salzenberg <chip at pobox dot com> --- Indeed, 16 is required by the ABI; see http://www.x86-64.org/documentation/abi.pdf page 12. Only the SIMD __m256 is bigger than 16, and there seems no end to Intel's extensions to SIMD registers, so holding at 16 seems like the Right Thing.
