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 18:13:26
Message-ID: 20220806181326.GA1216305@nathanxps13
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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:

0.19 │ packssdw %xmm1,%xmm0
0.62 │ packssdw %xmm1,%xmm0
7.14 │ packsswb %xmm1,%xmm0

1.52 │ por %xmm1,%xmm0
2.05 │ por %xmm1,%xmm0
5.66 │ por %xmm1,%xmm0

I also tried a combined approach where I replaced _mm_packs_epi16 with
1.16 │ packssdw %xmm1,%xmm0
1.47 │ packssdw %xmm1,%xmm0
8.17 │ por %xmm1,%xmm0

Of course, this simplistic analysis leaves out the impact of the
surrounding instructions, but it seems to support the idea that the
_mm_packs_epi* approach might have a slight edge.

Nathan Bossart
Amazon Web Services:

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