From: Nathan Bossart <nathandbossart@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: Greg Burd <greg@burd.me>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: refactor architecture-specific popcount code
Date: Fri, 20 Feb 2026 09:39:38 -0600
Message-ID: <aZiAOo25VBa6PoQi@nathan> (raw)
In-Reply-To: <CANWCAZaCaWR0SKkrBcm9WWRHMRs-Mbj8Wr+on2m8TPk0BiMt4A@mail.gmail.com>
References: <aYIzS4-IJOOgpIql@nathan>
<E22D691E-A655-44B7-9C1B-C7A86735FE15@greg.burd.me>
<aYJeGs-xz3NEEdSe@nathan>
<CANWCAZa_d9yZ-V7eU57M9UzB=BY+BUFDtdiF79bY5oyWYYSYLA@mail.gmail.com>
<aYO9llwttJEtl7er@nathan>
<CANWCAZaCaWR0SKkrBcm9WWRHMRs-Mbj8Wr+on2m8TPk0BiMt4A@mail.gmail.com>
On Fri, Feb 20, 2026 at 03:21:05PM +0700, John Naylor wrote:
> On Thu, Feb 5, 2026 at 4:43 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
>> Yeah, the plain C version might be marginally slower than the built-in
>> version for that test, but it still seems quite a bit faster than HEAD.
>>
>> HEAD v8 v10
>> 40 25 29
>
> (for the following, numbers are nanoseconds per call from
> drive_bms_num_members())
>
> Seems similar on S390X / gcc 13.3 (last week I only tested a single
> bitmapword and feel don't like repeating):
>
> master (older): 4.1083 (call builtin)
> v8: 2.8889 (inline builtin)
> v10: 2.7961 (inline pure C)
Thanks for testing it.
> On ppc64le / gcc 8.5, without native popcount it suffers:
>
> words master v14
> 1 4.5 6.5
> 2 5.8 9.7
> 64 67.9 101
> 128 143 190
>
> So one up, one down among obscure platforms. There seems to be a
> fairly thin case for the builtin anymore, although it's not zero.
I spent some time looking at how clang/gcc compiled the plain-C version on
various architectures [0], and I was pleasantly surprised to discover that
at some point in recent history they started automatically converting it to
special popcount instructions. I suspect that you'd see better results on
ppc64le if you upgraded the compiler...
[0] https://godbolt.org/z/v9vvx7E89
--
nathan
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To: pgsql-hackers@postgresql.org
Cc: nathandbossart@gmail.com, johncnaylorls@gmail.com, greg@burd.me, hlinnaka@iki.fi
Subject: Re: refactor architecture-specific popcount code
In-Reply-To: <aZiAOo25VBa6PoQi@nathan>
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