Re: Popcount optimization using AVX512 - Mailing list pgsql-hackers

From Nathan Bossart
Subject Re: Popcount optimization using AVX512
Date
Msg-id 20240329164153.GC1106154@nathanxps13
Whole thread Raw
In response to Re: Popcount optimization using AVX512  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-hackers
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



pgsql-hackers by date:

Previous
From: Andrew Dunstan
Date:
Subject: Re: WIP Incremental JSON Parser
Next
From: Andrew Dunstan
Date:
Subject: Re: WIP Incremental JSON Parser