[Bug target/82731] _mm256_set_epi8(array[offset[0]], array[offset[1]], ...) byte gather makes slow code, trying to zero-extend all the uint16_t offsets first and spilling them.

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82731

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
We do not BB vectorize gathers I think (ISTR some "loop" uses in the
infrastructure, not too difficult to fix I guess).

In the end the problem is RTL expansion of the CTOR and then lack of
combine?

Look at how we RTL expand

typedef char __v32qi __attribute__((vector_size(32)));

__v32qi
_mm256_set_epi8  (char __q31, char __q30, char __q29, char __q28,
                  char __q27, char __q26, char __q25, char __q24,
                  char __q23, char __q22, char __q21, char __q20,
                  char __q19, char __q18, char __q17, char __q16,
                  char __q15, char __q14, char __q13, char __q12,
                  char __q11, char __q10, char __q09, char __q08,
                  char __q07, char __q06, char __q05, char __q04,
                  char __q03, char __q02, char __q01, char __q00)
{
  return __extension__ (__v32qi){
    __q00, __q01, __q02, __q03, __q04, __q05, __q06, __q07,
    __q08, __q09, __q10, __q11, __q12, __q13, __q14, __q15,
    __q16, __q17, __q18, __q19, __q20, __q21, __q22, __q23,
    __q24, __q25, __q26, __q27, __q28, __q29, __q30, __q31
  };
}