Horton

Re: refactor architecture-specific popcount code

John Naylor <johncnaylorls@gmail.com>

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From: John Naylor <johncnaylorls@gmail.com>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers@postgresql.org
Date: 2026-02-02T14:16:42Z
Lists: pgsql-hackers

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On Sat, Jan 31, 2026 at 4:33 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
>
> On Fri, Jan 30, 2026 at 03:22:45PM +0700, John Naylor wrote:
> > 0001 - I'm pretty sure this is comparable to HEAD if the optimized
> > function is pg_popcount_sse42(). Has the AVX512 version been tested
> > with 8-byte inputs? That seems to have a lot of pre- and
> > post-processing involved. The inline wrapper only bypasses for 7 or
> > less bytes.
>
> Here [0] is the latest perf data I see for the AVX-512 popcount patch,
> although that's comparing to v16, which IIRC lacks a few other inlining
> tricks.  There's a chance the SSE4.2 version is faster at that particular
> length.  I'm not sure we need to worry about that, but I can do a bit of
> testing if you'd like.

It might be a good idea to do a little new testing, and I see a use
for a special 8-byte path independent of AVX512: v6 seems to regress a
little for single-words. But, it turns out that when gcc turns
__builtin_popcountl into a single instruction, it's inline, but if it
emits portable bitwise ops, it does so in a function called
__popcountdi2(). That can be avoided by hand-coding in C for normal
builds (and for 32-bit looks cleaner anyway), as in the attached 0005.

My laptop here is really too old to make decisions that are
micro-architecture dependent, but with that caveat, I dusted off the
popcount benchmark and added a test for counting bitmapsets (v7-0004,
applies on top of v6):

select drive_bms_num_members(10000000, 1);

master:    13.2 ticks per call
v6:        15.3
v6+v7-0005 10.8

Again, take this with a grain of salt, but 0005 seems worth looking at.

--
John Naylor
Amazon Web Services

Commits

  1. Make use of pg_popcount() in more places.

  2. Remove uses of popcount builtins.

  3. Remove some unnecessary optimizations in popcount code.

  4. Remove specialized word-length popcount implementations.

  5. Move x86-64-specific popcount code to pg_popcount_x86.c.

  6. Refactor some SIMD and popcount macros.

  7. Rename "fast" and "slow" popcount functions.

  8. Rename pg_popcount_avx512.c to pg_popcount_x86.c.

  9. Remove trailing zero words from Bitmapsets

  10. Don't use _BitScanForward64/_BitScanReverse64 on 32-bit MSVC builds