Horton

Re: refactor architecture-specific popcount code

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: Greg Burd <greg@burd.me>, Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2026-02-20T21:33:34Z
Lists: pgsql-hackers

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On Fri, Feb 20, 2026 at 09:39:38AM -0600, Nathan Bossart wrote:
> I spent some time looking at how clang/gcc compiled the plain-C version on
> various architectures [0], and I was pleasantly surprised to discover that
> at some point in recent history they started automatically converting it to
> special popcount instructions.  I suspect that you'd see better results on
> ppc64le if you upgraded the compiler...

If we're willing to rely on this behavior, we could even remove
pg_popcount64_neon() and pg_popcount64_sse42().  We still need to add
"pg_attribute_target("popcnt")" and a corresponding configure check for the
x86 stuff, so it's not as impressive from a code-removal standpoint, but it
at least allows us to remove some uses of intrinsics and inline assembly.

-- 
nathan

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