linux-kernel - RE: how many memset(,0,) calls in kernel ?

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Message-ID: <88976a40175c491fb5e3349f6686ad67@AcuMS.aculab.com>
Date:   Mon, 13 Sep 2021 16:03:09 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>, Douglas Gilbert <dgilbert@...erlog.com>
CC:     LKML <linux-kernel@...r.kernel.org>
Subject: RE: how many memset(,0,) calls in kernel ?

>   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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