Horton

Re: Popcount optimization using AVX512

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, "Shankaran, Akash" <akash.shankaran@intel.com>, Nathan Bossart <nathandbossart@gmail.com>, "Amonson, Paul D" <paul.d.amonson@intel.com>, Tom Lane <tgl@sss.pgh.pa.us>, Matthias van de Meent <boekewurm+postgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-02-09T23:27:57Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix __attribute__((target(...))) usage.

  2. Use __attribute__((target(...))) for AVX-512 support.

  3. Fix code for probing availability of AVX-512.

  4. Optimize visibilitymap_count() with AVX-512 instructions.

  5. Optimize pg_popcount() with AVX-512 instructions.

  6. Inline pg_popcount() for small buffers.

  7. Avoid function call overhead of pg_popcount() in syslogger.c.

  8. Refactor code for setting pg_popcount* function pointers.

  9. Inline pg_popcount{32,64} into pg_popcount().

  10. Remove MSVC scripts

  11. Use ARMv8 CRC instructions where available.

  12. Use Intel SSE 4.2 CRC instructions where available.

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.