Horton

Re: add AVX2 support to simd.h

John Naylor <johncnaylorls@gmail.com>

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From: John Naylor <johncnaylorls@gmail.com>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Ants Aasma <ants@cybertec.at>, pgsql-hackers@postgresql.org
Date: 2024-03-19T03:03:36Z
Lists: pgsql-hackers

Commits

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  1. Improve style of pg_lfind32().

  2. Fix compiler warning for pg_lfind32().

  3. Micro-optimize pg_lfind32().

  4. Introduce helper SIMD functions for small byte arrays

  5. Optimize xid/subxid searches in XidInMVCCSnapshot().

On Tue, Mar 19, 2024 at 9:03 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
>
> On Sun, Mar 17, 2024 at 09:47:33AM +0700, John Naylor wrote:
> > I haven't looked at the patches, but the graphs look good.
>
> I spent some more time on these patches.  Specifically, I reordered them to
> demonstrate the effects on systems without AVX2 support.  I've also added a
> shortcut to jump to the one-by-one approach when there aren't many
> elements, as the overhead becomes quite noticeable otherwise.  Finally, I
> ran the same benchmarks again on x86 and Arm out to 128 elements.
>
> Overall, I think 0001 and 0002 are in decent shape, although I'm wondering
> if it's possible to improve the style a bit.

I took a brief look, and 0001 isn't quite what I had in mind. I can't
quite tell what it's doing with the additional branches and "goto
retry", but I meant something pretty simple:

- if short, do one element at a time and return
- if long, do one block unconditionally, then round the start pointer
up so that "end - start" is an exact multiple of blocks, and loop over
them