Re: add AVX2 support to simd.h

On Tue, Jan 9, 2024 at 12:37 AM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>
> > I suspect that there could be a regression lurking for some inputs
> > that the benchmark doesn't look at: pg_lfind32() currently needs to be
> > able to read 4 vector registers worth of elements before taking the
> > fast path. There is then a tail of up to 15 elements that are now
> > checked one-by-one, but AVX2 would increase that to 31. That's getting
> > big enough to be noticeable, I suspect. It would be good to understand
> > that case (n*32 + 31), because it may also be relevant now. It's also
> > easy to improve for SSE2/NEON for v17.
>
> Good idea. If it is indeed noticeable, we might be able to "fix" it by
> processing some of the tail with shorter vectors. But that probably means
> finding a way to support multiple vector sizes on the same build, which
> would require some work.

What I had in mind was an overlapping pattern I've seen in various
places: do one iteration at the beginning, then subtract the
aligned-down length from the end and do all those iterations. And
one-by-one is only used if the total length is small.