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From: Nathan Bossart <[email protected]>
To: Tom Lane <[email protected]>
Cc: Amonson, Paul D <[email protected]>
Cc: David Rowley <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Shankaran, Akash <[email protected]>
Cc: Noah Misch <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Popcount optimization using AVX512
Date: Fri, 29 Mar 2024 11:41:53 -0500
Message-ID: <20240329164153.GC1106154@nathanxps13> (raw)
In-Reply-To: <[email protected]>
References: <BL1PR11MB53049F1796E834DBE0E05864DC362@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240325200551.GB2924132@nathanxps13>
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	<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>
	<[email protected]>

On Fri, Mar 29, 2024 at 12:30:14PM -0400, Tom Lane wrote:
> Nathan Bossart <[email protected]> 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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