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From: Tom Lane <[email protected]>
To: Nathan Bossart <[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 12:30:14 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <20240329161642.GA1106154@nathanxps13>
References: <[email protected]>
	<BL1PR11MB53043014C10F2D30B4231F9ADC362@BL1PR11MB5304.namprd11.prod.outlook.com>
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	<20240325200551.GB2924132@nathanxps13>
	<20240327220010.GA231058@nathanxps13>
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	<20240329153515.GA1046039@nathanxps13>
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	<20240329161642.GA1106154@nathanxps13>

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.

			regards, tom lane

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