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From: Ants Aasma <ants.aasma@cybertec.at>
Date: Thu, 4 Apr 2024 16:02:53 +0300
Message-ID: 
 <CANwKhkO9RS_O2H4oeaXzfmBanXFeETq9de7Fuw5kkTLGAQJEvw@mail.gmail.com>
Subject: Re: Popcount optimization using AVX512
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, "Amonson,
 Paul D" <paul.d.amonson@intel.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, David Rowley <dgrowleyml@gmail.com>,
	Andres Freund <andres@anarazel.de>, "Shankaran,
 Akash" <akash.shankaran@intel.com>,
	Noah Misch <noah@leadboat.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
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On Thu, 4 Apr 2024 at 01:50, Nathan Bossart <nathandbossart@gmail.com> wrote:
> If we can verify this approach won't cause segfaults and can stomach the
> regression between 8 and 16 bytes, I'd happily pivot to this approach so
> that we can avoid the function call dance that I have in v25.

The approach I posted does not rely on masking performing page fault
suppression. All loads are 64 byte aligned and always contain at least
one byte of the buffer and therefore are guaranteed to be within a
valid page.

I personally don't mind it being slower for the very small cases,
because when performance on those sizes really matters it makes much
more sense to shoot for an inlined version instead.

Speaking of which, what does bumping up the inlined version threshold
to 16 do with and without AVX-512 available? Linearly extrapolating
the 2 and 4 byte numbers it might just come ahead in both cases,
making the choice easy.

Regards,
Ants Aasma