https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98167
--- Comment #17 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #16)
> typedef int v4si __attribute__ ((vector_size(16)));
>
> v4si f(v4si a, v4si b) {
> v4si a1 = __builtin_shufflevector (a, a, 2, 3 ,1 ,0);
> v4si b1 = __builtin_shufflevector (b, a, 2, 3 ,1 ,0);
> return a1 * b1;
> }
>
> gcc generate
>
> f:
> vpshufd xmm1, xmm1, 30
> vpshufd xmm0, xmm0, 30
> vpmulld xmm0, xmm0, xmm1
> ret
>
> llvm generate
>
> f: # @f
> vpmulld xmm0, xmm1, xmm0
> vpshufd xmm0, xmm0, 30 # xmm0 = xmm0[2,3,1,0]
> ret
For the above, this is safe for -ftrapping-math as all elements are still used.
It is when elements that are not used it might not be safe ...
