Re: optimize lookups in snapshot [sub]xip arrays

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
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: 2022-08-06 21:25:22
Message-ID: 20220806212522.GA1221353@nathanxps13
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On Sat, Aug 06, 2022 at 11:13:26AM -0700, Nathan Bossart wrote:
> On Fri, Aug 05, 2022 at 03:04:34PM -0700, Andres Freund wrote:
>> 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.
> I haven't been able to find a significant difference between the two. If
> anything, the _mm_packs_epi* approach actually seems to be slightly faster
> in some cases. For something marginally more concrete, I compared the two
> in perf-top and saw the following for the relevant instructions:

Nevermind, I'm wrong. When compiled with -O2, it uses more than just the
xmm0 and xmm1 registers, and the _mm_or_si128 approach consistently shows a
speedup of slightly more than 5%. Patches attached.

Nathan Bossart
Amazon Web Services:

Attachment Content-Type Size
v8-0001-Introduce-optimized-routine-for-linear-searches-t.patch text/x-diff 3.0 KB
v8-0002-Optimize-linear-searches-in-XidInMVCCSnapshot.patch text/x-diff 2.8 KB

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