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From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amonson, Paul D <paul.d.amonson@intel.com>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Shankaran, Akash <akash.shankaran@intel.com>
Cc: Noah Misch <noah@leadboat.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Date: Fri, 29 Mar 2024 11:41:53 -0500
Message-ID: <20240329164153.GC1106154@nathanxps13> (raw)
In-Reply-To: <6504.1711729814@sss.pgh.pa.us>
References: <BL1PR11MB53049F1796E834DBE0E05864DC362@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240325200551.GB2924132@nathanxps13>
	<20240327220010.GA231058@nathanxps13>
	<BL1PR11MB53047289F146DB43C9B2FC71DC342@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240328213854.GA916732@nathanxps13>
	<BL1PR11MB5304E51336123CE6F041A920DC3B2@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240329153515.GA1046039@nathanxps13>
	<BL1PR11MB5304D039E04286E9DF048473DC3A2@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240329161642.GA1106154@nathanxps13>
	<6504.1711729814@sss.pgh.pa.us>

On Fri, Mar 29, 2024 at 12:30:14PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>>> I see google web references to the xgetbv instruction as far back as 2009
>>> for Intel 64 bit HW and 2010 for AMD 64bit HW, maybe you could test for
>>> _xgetbv() MSVC built-in. How far back do you need to go?
> 
>> Hm.  It seems unlikely that a compiler would understand AVX512 intrinsics
>> and not XGETBV then.  I guess the other question is whether CPUID
>> indicating AVX512 is enabled implies the availability of XGETBV on the CPU.
>> If that's not safe, we might need to add another CPUID test.
> 
> Some quick googling says that (1) XGETBV predates AVX and (2) if you
> are worried about old CPUs, you should check CPUID to verify whether
> XGETBV exists before trying to use it.  I did not look for the
> bit-level details on how to do that.

That extra CPUID check should translate to exactly one additional line of
code, so I think I'm inclined to just add it.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com





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  Cc: nathandbossart@gmail.com, tgl@sss.pgh.pa.us, paul.d.amonson@intel.com, dgrowleyml@gmail.com, andres@anarazel.de, alvherre@alvh.no-ip.org, akash.shankaran@intel.com, noah@leadboat.com, boekewurm+postgres@gmail.com, pgsql-hackers@lists.postgresql.org
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