Horton

Re: Popcount optimization using AVX512

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Ants Aasma <ants.aasma@cybertec.at>, Alvaro Herrera <alvherre@alvh.no-ip.org>, "Amonson, Paul D" <paul.d.amonson@intel.com>, Andres Freund <andres@anarazel.de>, "Shankaran, Akash" <akash.shankaran@intel.com>, Noah Misch <noah@leadboat.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-04-08T00:42:12Z
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.

Nathan Bossart <nathandbossart@gmail.com> writes:
> Here is what I have staged for commit, which I intend to do shortly.

Today's Coverity run produced this warning, which seemingly was
triggered by one of these commits, but I can't make much sense
of it:

*** CID 1596255:  Uninitialized variables  (UNINIT)
/usr/lib/gcc/x86_64-linux-gnu/10/include/avxintrin.h: 1218 in _mm256_undefined_si256()
1214     extern __inline __m256i __attribute__((__gnu_inline__, __always_inline__, __artificial__))
1215     _mm256_undefined_si256 (void)
1216     {
1217       __m256i __Y = __Y;
>>>     CID 1596255:  Uninitialized variables  (UNINIT)
>>>     Using uninitialized value "__Y".
1218       return __Y;
1219     }

I see the same code in my local copy of avxintrin.h,
and I quite agree that it looks like either an undefined
value or something that properly ought to be an error.
If we are calling this, why (and from where)?

Anyway, we can certainly just dismiss this warning if it
doesn't correspond to any real problem in our code.
But I thought I'd raise the question.

			regards, tom lane