| From: | Noah Misch <noah(at)leadboat(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, "Shankaran, Akash" <akash(dot)shankaran(at)intel(dot)com>, Nathan Bossart <nathandbossart(at)gmail(dot)com>, "Amonson, Paul D" <paul(dot)d(dot)amonson(at)intel(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Popcount optimization using AVX512 |
| Date: | 2024-02-09 23:27:57 |
| Message-ID: | 20240209232757.5d@rfd.leadboat.com |
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| Lists: | pgsql-hackers |
On Fri, Feb 09, 2024 at 10:24:32AM -0800, Andres Freund wrote:
> On 2024-01-26 07:42:33 +0100, Alvaro Herrera wrote:
> > This suggests that finding a way to make the ifunc stuff work (with good
> > performance) is critical to this work.
>
> Ifuncs are effectively implemented as a function call via a pointer, they're
> not magic, unfortunately. The sole trick they provide is that you don't
> manually have to use the function pointer.
The IFUNC creators introduced it so glibc could use arch-specific memcpy with
the instruction sequence of a non-pointer, extern function call, not the
instruction sequence of a function pointer call. I don't know why the
upthread ifunc_test.patch benchmark found ifunc performing worse than function
pointers. However, it would be odd if toolchains have replaced the original
IFUNC with something equivalent to or slower than function pointers.
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