Horton

Re: Popcount optimization using AVX512

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Noah Misch <noah@leadboat.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, 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>
Date: 2023-11-07T03:59:26Z
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 Mon, Nov 06, 2023 at 07:15:01PM -0800, Noah Misch wrote:
> 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

Thanks, that seems promising for the function pointer cases.  I'll plan on
trying to convert one of the existing ones to use it.  BTW it looks like
LLVM has something similar [0].

IIUC this unfortunately wouldn't help for cases where we wanted to keep
stuff inlined, such as is_valid_ascii() and the functions in pg_lfind.h,
unless we applied it to the calling functions, but that doesn't ѕound
particularly maintainable.

[0] https://llvm.org/docs/LangRef.html#ifuncs

-- 
Nathan Bossart
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