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From: Nathan Bossart <[email protected]>
To: Ants Aasma <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Amonson, Paul D <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: David Rowley <[email protected]>
Cc: Andres Freund <[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: Mon, 1 Apr 2024 16:31:40 -0500
Message-ID: <20240401213140.GA2362108@nathanxps13> (raw)
In-Reply-To: <CANwKhkM-YZGE527y00LPU1660GW6zicvX7a=ZOsxnYYU0hng3g@mail.gmail.com>
References: <20240401011708.GA2128739@nathanxps13>
	<[email protected]>
	<20240401155338.GA2296072@nathanxps13>
	<CANwKhkM-YZGE527y00LPU1660GW6zicvX7a=ZOsxnYYU0hng3g@mail.gmail.com>

On Tue, Apr 02, 2024 at 12:11:59AM +0300, Ants Aasma wrote:
> What about using the masking capabilities of AVX-512 to handle the
> tail in the same code path? Masked out portions of a load instruction
> will not generate an exception. To allow byte level granularity
> masking, -mavx512bw is needed. Based on wikipedia this will only
> disable this fast path on Knights Mill (Xeon Phi), in all other cases
> VPOPCNTQ implies availability of BW.

Sounds promising.  IMHO we should really be sure that these kinds of loads
won't generate segfaults and the like due to the masked-out portions.  I
searched around a little bit but haven't found anything that seemed
definitive.

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

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