When none of mprefer-vector-width, avx256_optimal/avx128_optimal,
avx256_store_by_pieces/avx512_store_by_pieces is specified, GCC will
set ix86_{move_max,store_max} as max available vector length except
for AVX part.
So for -mavx2, vectorizer will choose 256-bit for vectorization, but
128-bit is used for struct copy, there could be a potential STLF issue
due to this "misalign".
The patch fixes that.
gcc/ChangeLog:
* config/i386/i386-options.cc (ix86_option_override_internal):
set ix86_{move_max,store_max} to PVW_AVX256 when TARGET_AVX
instead of PVW_AVX128.