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From: David Laight <David.Laight@ACULAB.COM>
To: 'Willy Tarreau' <w@1wt.eu>, Douglas Gilbert <dgilbert@interlog.com>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: RE: how many memset(,0,) calls in kernel ?
Date: Mon, 13 Sep 2021 16:03:09 +0000	[thread overview]
Message-ID: <88976a40175c491fb5e3349f6686ad67@AcuMS.aculab.com> (raw)
In-Reply-To: <20210912045608.GB16216@1wt.eu>

>   36:   b9 06 00 00 00          mov    $0x6,%ecx
>   3b:   4c 89 e7                mov    %r12,%rdi
>   3e:   f3 ab                   rep stos %eax,%es:(%rdi)
> 
> The last line does exactly "memset(%rdi, %eax, %ecx)". Just two bytes
> for some code that modern processors are even able to optimize.

Hmmm I'd bet that 6 stores will be faster on ~everything.
'modern' processors do better than some older ones [1], but 6
writes isn't enough to get into the really fast paths.
So you'll still take a few cycles of setup.

[1] P4 netburst had a ~40 clock setup for any 'rep' operation.

	David

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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  reply	other threads:[~2021-09-13 16:03 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-12  3:36 how many memset(,0,) calls in kernel ? Douglas Gilbert
2021-09-12  4:56 ` Willy Tarreau
2021-09-13 16:03   ` David Laight [this message]
2021-09-13 16:09     ` Willy Tarreau
2021-09-14  8:23       ` David Laight
2021-09-14 16:46         ` Willy Tarreau

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