https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68484
--- Comment #6 from Marc Glisse <glisse at gcc dot gnu.org> --- clang has: static __inline__ void __DEFAULT_FN_ATTRS _mm_storel_epi64(__m128i *__p, __m128i __a) { struct __mm_storel_epi64_struct { long long __u; } __attribute__((__packed__, __may_alias__)); ((struct __mm_storel_epi64_struct*)__p)->__u = __a[0]; } It doesn't prove anything, this is just a point of comparison. (I haven't checked, but I suspect that they have an implicit may_alias on all vector types)