Re: optimize lookups in snapshot [sub]xip arrays

On Tue, Aug 09, 2022 at 01:21:41PM +0700, John Naylor wrote:
> I decided I wasn't quite comfortable changing snapshot handling
> without further guarantees. To this end, 0002 in the attached v11 is
> an addendum that adds assert checking (also pgindent and some
> comment-smithing). As I suspected, make check-world passes even with
> purposefully screwed-up coding. 0003 uses pg_lfind32 in syscache.c and
> I verified that sticking in the wrong answer will lead to a crash in
> assert-enabled builds in short order. I'd kind of like to throw this
> (or something else suitable) at the build farm first for that reason.
> It's simpler than the qsort/qunique/binary search that was there
> before, so that's nice, but I've not tried to test performance.

Your adjustments in 0002 seem reasonable to me. I think it makes sense to
ensure there is test coverage for pg_lfind32(), but I don't know if that
syscache code is the right choice. For non-USE_SSE2 builds, it might make
these lookups more expensive. I'll look around to see if there are any
other suitable candidates.

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com