Re: add AVX2 support to simd.h

Nathan Bossart <nathandbossart@gmail.com>

From: Nathan Bossart <nathandbossart@gmail.com>

To: John Naylor <johncnaylorls@gmail.com>

Cc: Ants Aasma <ants@cybertec.at>, pgsql-hackers@postgresql.org

Date: 2024-03-25T21:37:54Z

Lists: pgsql-hackers

Commits

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Improve style of pg_lfind32().
- 7188a7806d20 17.0 landed
Fix compiler warning for pg_lfind32().
- 1f42337be535 17.0 landed
Micro-optimize pg_lfind32().
- 7644a7340c8a 17.0 landed
Introduce helper SIMD functions for small byte arrays
- 9f225e992bed 17.0 cited
Optimize xid/subxid searches in XidInMVCCSnapshot().
- 37a6e5df3713 16.0 cited

Attachments

v9-0001-Micro-optimize-pg_lfind32.patch (text/x-diff)

Here is what I have staged for commit.  One notable difference in this
version of the patch is that I've changed

    +	if (nelem <= nelem_per_iteration)
    +		goto one_by_one;

to

    +	if (nelem < nelem_per_iteration)
    +		goto one_by_one;

I realized that there's no reason to jump to the one-by-one linear search
code when nelem == nelem_per_iteration, as the worst thing that will happen
is that we'll process all the elements twice if the value isn't present in
the array.  My benchmark that I've been using also shows a significant
speedup for this case with this change (on the order of 75%), which I
imagine might be due to a combination of branch prediction, caching, fewer
instructions, etc.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com