| From: | Noah Misch <noah(at)leadboat(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Nathan Bossart <nathandbossart(at)gmail(dot)com>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, "Amonson, Paul D" <paul(dot)d(dot)amonson(at)intel(dot)com>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, "Shankaran, Akash" <akash(dot)shankaran(at)intel(dot)com> |
| Subject: | Re: Popcount optimization using AVX512 |
| Date: | 2023-11-07 03:15:01 |
| Message-ID: | 20231107031501.61@rfd.leadboat.com |
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| Lists: | pgsql-hackers |
On Mon, Nov 06, 2023 at 09:52:58PM -0500, Tom Lane wrote:
> Nathan Bossart <nathandbossart(at)gmail(dot)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
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