Re: optimize lookups in snapshot [sub]xip arrays

From: Andres Freund <andres(at)anarazel(dot)de>
To: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Cc: John Naylor <john(dot)naylor(at)enterprisedb(dot)com>, John Naylor <jcnaylor(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: optimize lookups in snapshot [sub]xip arrays
Date: 2022-08-05 22:04:34
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On 2022-08-05 13:25:10 -0700, Nathan Bossart wrote:
> I went ahead and renamed it to pg_lfind32() and switched it back to
> returning the pointer. That felt the cleanest from the naming perspective,
> but as Andres noted, it might not be as fast as just looking for the
> presence of the element. I modified my small testing program to perform
> many searches on small arrays, and I wasn't able to identify any impact, so
> perhaps thіs is good enough.

Why on small arrays? I'd expect a difference mainly if it there's at least a
few iterations.

But mainly I'd expect to find a difference if the SIMD code were optimized a
further on the basis of not needing to return the offset. E.g. by
replacing _mm_packs_epi32 with _mm_or_si128, that's cheaper.

- Andres

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