https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85538
--- Comment #1 from Matthias Kretz <kretz at kde dot org> --- Sorry, I was trying to force GCC to use the k1 register and playing with register asm (which didn't have any effect at all). f8 should actually be (cf. https://godbolt.org/g/hSkoJV): bool f8(__m512i x, __m512i y) { __mmask64 k = _mm512_cmp_epi8_mask(x, y, _MM_CMPINT_EQ); return _kortestc_mask64_u8(k, k); }