Re: refactor architecture-specific popcount code

On Thu, Jan 22, 2026 at 11:50:38AM -0600, Nathan Bossart wrote:
> On Thu, Jan 22, 2026 at 04:50:26PM +0700, John Naylor wrote:
>> 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().
>
> Yeah, these seem like obvious places to use pg_popcount(). Note that
> bms_member_index() does a final popcount on a masked version of the last
> word. We could swap that with pg_popcount(), too, but it might be slower
> than just calling the word-sized function. However, it could be hard to
> tell the difference, as we'd be trading a function or function pointer call
> with an inlined loop over pg_number_of_ones. And even if it is slower, I'm
> not sure it matters all that much in the grand scheme of things.

I added a 0003 that swaps that final popcount with pg_popcount().

>> Then we could get rid of all the pointer indirection for the
>> word-sized functions.
>
> Do you mean that we'd just keep the portable ones around? I see some code
> in pgvector that might be negatively impacted by that, but if I understand
> correctly it would require an unusual setup.

I added a 0004 that removes the architecture-specific word-sized functions.

--
nathan