Horton

Re: refactor architecture-specific popcount code

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Greg Burd <greg@burd.me>
Cc: John Naylor <johncnaylorls@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2026-02-03T17:41:31Z
Lists: pgsql-hackers 
On Tue, Feb 03, 2026 at 12:19:48PM -0500, Greg Burd wrote:
> Thanks for the focus on this area of the code.  I've been looking into
> what to do with popcnt when building Win11/ARM64/MSVC.  I know that when
> _MSC_VER and _M_ARM64 are defined we can make use of the
> __popcnt(unsigned int) and __popcnt64(unsigned __int64) intrinsics which
> have been available since VS 2022 17.11+.  I thought I'd check that combo
> out and it turns out that it is identical to clang/gcc on that platform
> [0].
> 
> I'll wait for your work to land before proposing a patch to add these
> unless it is really easy to fit it and you feel like giving it a go. :)

We should probably just add something like

	#ifdef _MSC_VER
		return __popcnt(word);

for the new inlined versions of pg_popcount{32,64}.  We're already doing
that today for x86-64, and the AArch64-specific versions use intrinsics
that in theory compile to the same thing.  Plus, popcnt is required for
Windows these days.

I'm working on polishing/benchmarking these patches at the moment, so I
will work this change in.  Thanks!

-- 
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