Re: Popcount optimization using AVX512

Nathan Bossart <nathandbossart@gmail.com>

From: Nathan Bossart <nathandbossart@gmail.com>

To: "Amonson, Paul D" <paul.d.amonson@intel.com>

Cc: Andres Freund <andres@anarazel.de>, Alvaro Herrera <alvherre@alvh.no-ip.org>, "Shankaran, Akash" <akash.shankaran@intel.com>, Noah Misch <noah@leadboat.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: 2024-03-13T16:39:04Z

Lists: pgsql-hackers

Commits

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Fix __attribute__((target(...))) usage.
- 41b98ddb77bf 18.0 landed
Use __attribute__((target(...))) for AVX-512 support.
- f78667bd910e 18.0 landed
Fix code for probing availability of AVX-512.
- 598e0114a3b1 17.0 landed
Optimize visibilitymap_count() with AVX-512 instructions.
- 41c51f0c68b2 17.0 landed
Optimize pg_popcount() with AVX-512 instructions.
- 792752af4eb5 17.0 landed
Inline pg_popcount() for small buffers.
- deb1486c7d36 17.0 landed
Avoid function call overhead of pg_popcount() in syslogger.c.
- 4133c1f45c54 17.0 landed
Refactor code for setting pg_popcount* function pointers.
- 6687430c98f3 17.0 landed
Inline pg_popcount{32,64} into pg_popcount().
- cc4826dd5e52 17.0 landed
Remove MSVC scripts
- 1301c80b2167 17.0 cited
Use ARMv8 CRC instructions where available.
- f044d71e331d 11.0 cited
Use Intel SSE 4.2 CRC instructions where available.
- 3dc2d62d0486 9.5.0 cited

A couple of thoughts on v7-0001:

+extern int  pg_popcount32_slow(uint32 word);
+extern int  pg_popcount64_slow(uint64 word);

+/* In pg_popcnt_*_accel source file. */
+extern int  pg_popcount32_fast(uint32 word);
+extern int  pg_popcount64_fast(uint64 word);

Can these prototypes be moved to a header file (maybe pg_bitutils.h)?  It
looks like these are defined twice in the patch, and while I'm not positive
that it's against project policy to declare extern function prototypes in
.c files, it appears to be pretty rare.

+  'pg_popcnt_choose.c',
+  'pg_popcnt_x86_64_accel.c',

I think we want these to be architecture-specific, i.e., only built for
x86_64 if the compiler knows how to use the relevant instructions.  There
is a good chance that we'll want to add similar support for other systems.
The CRC32C files are probably a good reference point for how to do this.

+#ifdef TRY_POPCNT_FAST

IIUC this macro can be set if either 1) the popcntq test in the
autoconf/meson scripts passes or 2) we're building with MSVC on x86_64.  I
wonder if it would be better to move the MSVC/x86_64 check to the
autoconf/meson scripts so that we could avoid surrounding large portions of
the popcount code with this macro.  This might even be a necessary step
towards building these files in an architecture-specific fashion.

-- 
Nathan Bossart
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