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: 2026-01-22T09:50:26Z
Lists: pgsql-hackers 
On Thu, Jan 22, 2026 at 3:23 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
>
> Committed, thanks for reviewing.

Sure. Now that that's in place, I wanted to brainstorm more
refactoring/rationalization ideas that seem on-topic for the thread
but have less clear payoff:

1) Nowadays, the only global call sites of the word-sized functions
are select_best_grantor() and in bitmapsets. The latter calls the
word-sized functions in a loop (could be just one word). It may be
more efficient to calculate the size in bytes and call pg_popcount().
Then we could get rid of all the pointer indirection for the
word-sized functions.

2) The x86 byte buffer variants expend a lot of effort to detect
whether the buffer is aligned on both 64- and 32-bit platforms, with
an optimized path for each. At least 64-bit doesn't care about
alignment, and 32-bit doesn't warrant anything fancier than pure C.
Simultaneously, the aarch64 equivalent doesn't seem to take care about
alignment. (I think Nathan mentioned he didn't see a difference during
testing, but I wonder how universal that is).

3) There is repeated code for the <8 bytes case, and the tail of the
"optimized" functions. I'm also not sure why the small case is inlined
everywhere.

--
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