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From: Amonson, Paul D <[email protected]>
To: Nathan Bossart <[email protected]>
Cc: Tom Lane <[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 16:06:17 +0000
Message-ID: <BL1PR11MB5304D039E04286E9DF048473DC3A2@BL1PR11MB5304.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20240329153515.GA1046039@nathanxps13>
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	<20240329153515.GA1046039@nathanxps13>

> -----Original Message-----
>
> Cool.  I think we should run the benchmarks again to be safe, though.

Ok, sure go ahead. :)

> >> I forgot to mention that I also want to understand whether we can
> >> actually assume availability of XGETBV when CPUID says we support
> >> AVX512:
> >
> > You cannot assume as there are edge cases where AVX-512 was found on
> > system one during compile but it's not actually available in a kernel
> > on a second system at runtime despite the CPU actually having the
> > hardware feature.
> 
> Yeah, I understand that much, but I want to know how portable the XGETBV
> instruction is.  Unless I can assume that all x86_64 systems and compilers
> support that instruction, we might need an additional configure check and/or
> CPUID check.  It looks like MSVC has had support for the _xgetbv intrinsic for
> quite a while, but I'm still researching the other cases.

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?

Thanks,
Paul

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