Horton

Re: Popcount optimization using AVX512

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com>, "Shankaran, Akash" <akash.shankaran@intel.com>, Tom Lane <tgl@sss.pgh.pa.us>, 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>, Noah Misch <noah@leadboat.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-07-31T02:43:08Z
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.

Hi,

On 2024-07-30 21:01:31 -0500, Nathan Bossart wrote:
> On Tue, Jul 30, 2024 at 06:46:51PM -0700, Andres Freund wrote:
> > On 2024-07-30 20:20:34 -0500, Nathan Bossart wrote:
> >> On Tue, Jul 30, 2024 at 05:49:59PM -0700, Andres Freund wrote:
> >> > Why are we actually checking for xsave? We're not using xsave itself and I
> >> > couldn't find a comment in 792752af4eb5 explaining what we're using it as a
> >> > proxy for?  Is that just to know if _xgetbv() exists?  Is it actually possible
> >> > that xsave isn't available when avx512 is?
> >> 
> >> Yes, it's to verify we have XGETBV, which IIUC requires support from both
> >> the processor and the OS (see 598e011 and upthread discussion).  AFAIK the
> >> way we are detecting AVX-512 support is quite literally by-the-book unless
> >> I've gotten something wrong.
> > 
> > I'm basically wondering whether we need to check for compiler (not OS support)
> > support for xsave if we also check for -mavx512vpopcntdq -mavx512bw
> > support. Afaict the latter implies support for xsave.
> 
> The main purpose of the XSAVE compiler check is to determine whether we
> need to add -mxsave in order to use _xgetbv() [0].  If that wasn't a
> factor, we could probably skip it.  Earlier versions of the patch used
> inline assembly in the non-MSVC path to call XGETBV, which I was trying to
> avoid.

My point is that _xgetbv() is made available by -mavx512vpopcntdq -mavx512bw
alone, without needing -mxsave:

echo -e '#include <immintrin.h>\nint main() { return _xgetbv(0) & 0xe0; }'|time gcc -march=x86-64 -c -xc  - -o /dev/null
-> fails

echo -e '#include <immintrin.h>\nint main() { return _xgetbv(0) & 0xe0;}'|time gcc -march=x86-64 -mavx512vpopcntdq -mavx512bw -c -xc - -o /dev/null
-> succeeds

Greetings,

Andres Freund