https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107304
--- Comment #17 from H.J. Lu <hjl.tools at gmail dot com> --- Since this type typedef struct { unsigned char v __attribute__((aligned(256))) __attribute__ ((vector_size(64 * sizeof(unsigned char)))); } stress_vec_u8_64_t; is processed outside of the function and AVX512 is enabled by default, it has V64QImode instead of BLKmode. If the target attribute disables AVX512, V64QImode becomes unusable.