[Bug tree-optimization/114908] fails to optimize avx2 in-register permute written with std::experimental::simd

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114908

--- Comment #4 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 6 May 2024, mkretz at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114908
> 
> --- Comment #3 from Matthias Kretz (Vir) <mkretz at gcc dot gnu.org> ---
> The stdx::simd implementation in this area is old and mainly tuned to be
> correct. I can rewrite the split and concat implementation to use
> __builtin_shufflevector (which wasn't available in GCC at the time when I
> originally implemented it). Doing so I can resolve this issue.
> 
> How do you want to handle this? Because it would certainly be nice if the
> compiler can optimize this in the same way as Clang can. Should I try to come
> up with a testcase that doesn't need stdx::simd and then improve stdx::simd
> independently?

Yes, that would be nice (best the testcase w/o stdx::simd being a C
testcase even, no C++ abstraction).

I do think stdx::simd should be improved to use __builtin_shufflevector
though.