linux-kernel - Re: [PATCH] small update for strlen, strnlen, use less cpu instructions

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Message-ID: <CACVxJT-rvN-NX-Z+Eby9uYkubka9_a5ZnEPCBcRT6FRwdVZW3Q@mail.gmail.com>
Date:	Tue, 16 Jun 2015 14:41:31 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	richard.weinberger@...il.com
Cc:	lukes357@...il.com, Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] small update for strlen, strnlen, use less cpu instructions

> Now strlen() increments a variable for each character it faces,
> hence it will consume more cycles.

It doesn't matter if there is a dependency in a loop:

Before:
 520:   48 83 c0 01             add    $0x1,%rax
 524:   80 38 00                cmpb   $0x0,(%rax)
 527:   75 f7                   jne    520

After:
 500:   48 83 c0 01             add    $0x1,%rax
 504:   80 3c 07 00             cmpb   $0x0,(%rdi,%rax,1)
 508:   75 f6                   jne    500
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