http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57156
--- Comment #5 from Matthias Kretz <kretz at kde dot org> 2013-05-03 09:56:00 UTC --- (In reply to comment #4) > I wouldn't know how to counter this for the _mm_cmpeq_epi8 case Actually, I have yet to find something in the standard that says using an uninitialized __m128 is undefined behavior. ;) Thus the only standard that would define the rules here would be Intel's, no? And from reading their docs I understand that calling an intrinsics maps to the behavior you'd get when using the associated instruction. In this case comparing any XMM register with itself will result in what I expect.