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From: Nathan Bossart <[email protected]>
To: Amonson, Paul D <[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 11:22:11 -0500
Message-ID: <20240329162211.GB1106154@nathanxps13> (raw)
In-Reply-To: <20240329155940.GC1046039@nathanxps13>
References: <[email protected]>
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	<20240325200551.GB2924132@nathanxps13>
	<20240327220010.GA231058@nathanxps13>
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	<20240329155940.GC1046039@nathanxps13>

On Fri, Mar 29, 2024 at 10:59:40AM -0500, Nathan Bossart wrote:
> It might be nice if we conditionally built pg_popcount_avx512.o in autoconf
> builds, too, but AFAICT we still need to wrap most of that code with
> macros, so I'm not sure it's worth the trouble.  I'll take another look at
> this...

If we assumed that TRY_POPCNT_FAST would be set and either
HAVE__GET_CPUID_COUNT or HAVE__CPUIDEX would be set whenever
USE_AVX512_POPCNT_WITH_RUNTIME_CHECK is set, we could probably remove the
surrounding macros and just compile pg_popcount_avx512.c conditionally
based on USE_AVX512_POPCNT_WITH_RUNTIME_CHECK.  However, the surrounding
code seems to be pretty cautious about these assumptions (e.g., the CPUID
macros are checked before setting TRY_POPCNT_FAST), so this would stray
from the nearby precedent a bit.

A counterexample is the CRC32C code.  AFAICT we assume the presence of
CPUID in that code (and #error otherwise).  I imagine its probably safe to
assume the compiler understands CPUID if it understands AVX512 intrinsics,
but that is still mostly a guess.

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

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