Horton

Re: refactor architecture-specific popcount code

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: John Naylor <johncnaylorls@gmail.com>, Nathan Bossart <nathandbossart@gmail.com>
Cc: pgsql-hackers@postgresql.org
Date: 2026-01-15T09:42:14Z
Lists: pgsql-hackers 
On 15/01/2026 11:07, John Naylor wrote:
> 0003
> 
> s/fast/sse42/:
> 
> Seems okay in this file, but this isn't the best name, either. Maybe a
> comment to head off future "corrections", something like:
> "Technically, POPCNT is not part of SSE 4.2, and is not even a vector
> operation, but many compilers emit the popcnt instruction with
> -msse4.2 anyway."
> 
> s/slow/generic/:
> 
> I'm ambivalent about this. The "slow" designation is flat-out wrong
> since at least Power and aarch64 can emit a single instruction here
> without prodding the compiler. On the other hand, "generic" seems
> wrong too, since e.g. pg_popcount64_slow() has three configure symbols
> and two compiler builtins. :-D

"fallback", or "portable" ?

> A possible future project would be to have a truly generic simple
> fallback in pure C and put all the fancy stuff in the header for
> architectures that have unconditional hardware support. It would make
> more sense to revisit the name then.
Yeah, I noticed that on x86_64, pg_popcount_optimized is always a 
function pointer with runtime check, even if you use compiler flags to 
target a CPU where the special instructions are available unconditionally.

- Heikki

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