Date: Mon, 6 Nov 2023 19:15:01 -0800
From: Noah Misch <noah@leadboat.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Nathan Bossart <nathandbossart@gmail.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	"Amonson, Paul D" <paul.d.amonson@intel.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
	"Shankaran, Akash" <akash.shankaran@intel.com>
Subject: Re: Popcount optimization using AVX512
Message-ID: <20231107031501.61@rfd.leadboat.com>
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On Mon, Nov 06, 2023 at 09:52:58PM -0500, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
> > Like I said, I don't have any proposals yet, but assuming we do want to
> > support newer intrinsics, either open-coded or via auto-vectorization, I
> > suspect we'll need to gather consensus for a new policy/strategy.
> 
> Yeah.  The function-pointer solution kind of sucks, because for the
> sort of operation we're considering here, adding a call and return
> is probably order-of-100% overhead.  Worse, it adds similar overhead
> for everyone who doesn't get the benefit of the optimization.

The glibc/gcc "ifunc" mechanism was designed to solve this problem of choosing
a function implementation based on the runtime CPU, without incurring function
pointer overhead.  I would not attempt to use AVX512 on non-glibc systems, and
I would use ifunc to select the desired popcount implementation on glibc:
https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Function-Attributes.html