Date: Tue, 30 Jul 2024 16:54:54 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com>,
	"Shankaran, Akash" <akash.shankaran@intel.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, David Rowley <dgrowleyml@gmail.com>,
	Ants Aasma <ants.aasma@cybertec.at>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
	"Amonson, Paul D" <paul.d.amonson@intel.com>,
	Noah Misch <noah@leadboat.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Message-ID: <ZqlhLn9PwBTpdy-n@nathan>
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On Tue, Jul 30, 2024 at 04:32:07PM -0500, Nathan Bossart wrote:
> On Tue, Jul 30, 2024 at 02:07:01PM -0700, Andres Freund wrote:
>> Afaict we could just check for predefined preprocessor macros:
>> 
>> echo|time gcc -c -mxsave -mavx512vpopcntdq -mavx512bw -xc -dM -E  - -o -|grep -E '__XSAVE__|__AVX512BW__|__AVX512VPOPCNTDQ__'
>> #define __AVX512BW__ 1
>> #define __AVX512VPOPCNTDQ__ 1
>> #define __XSAVE__ 1
>> 0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 13292maxresident)k
>> 
>> echo|time gcc -c -march=nehalem -xc -dM -E  - -o -|grep -E '__XSAVE__|__AVX512BW__|__AVX512VPOPCNTDQ__'
>> 0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 10972maxresident)k
> 
> Seems promising.  I can't think of a reason that wouldn't work.
> 
>> Now, a reasonable counter-argument would be that only some of these macros are
>> defined for msvc ([1]).  However, as it turns out, the test is broken
>> today, as msvc doesn't error out when using an intrinsic that's not
>> "available" by the target architecture, it seems to assume that the caller did
>> a cpuid check ahead of time.

Hm.  Upon further inspection, I see that MSVC appears to be missing
__XSAVE__ and __AVX512VPOPCNTDQ__, which is unfortunate.  Still, I think
the worst case scenario is that the CPUID check fails and we don't use
AVX-512 instructions.  AFAICT we aren't adding new function pointers in any
builds that don't already have them, just compiling some extra unused code.

-- 
nathan