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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [TRIVIAL] kstrdup
Date: 18 Apr 2003 12:24:09 -0700
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Followup to:  <3EA0469D.7090602@pobox.com>
By author:    Jeff Garzik <jgarzik@pobox.com>
In newsgroup: linux.dev.kernel
> 
> That's kinda cute.  Why not submit a patch to the strcpy implementation 
> in include/asm-i386/string.h?  :)  Ours is shorter, but does have a jump:
>          "1:\tlodsb\n\t"
>          "stosb\n\t"
>          "testb %%al,%%al\n\t"
>          "jne 1b"
> 
> Which is better?  I don't know; I'm still learning the performance 
> eccentricities of x86 insns on various processors.
> 

It varies from porocessor to processor, and also depends on usage.

> 
> Related x86 question:  if the memory buffer is not dword-aligned, is 
> 'rep movsl' the best idea?  On RISC it's usually smarter to unroll the 
> head of the loop to avoid unaligned accesses; but from reading x86 asm 
> code in the kernel, nobody seems to care about that.  Is the 
> unaligned-access penalty so small that the increased code size of the 
> head-unroll is never worth it?
> 

A lot of the newer x86 processors will perform this unrolling in
microcode.

	-hpa
-- 
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