https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66174
--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> --- The problem is in i386.c, function expand_vec_perm_blend, where: /* This matches five different patterns with the different modes. */ x = gen_rtx_VEC_MERGE (vmode, op1, op0, GEN_INT (mask)); x = gen_rtx_SET (target, x); emit_insn (x); AVX512F modes need to move mask to a mask register.