linux-kernel - RE: [PATCH V2 -tip] lib,x86_64: improve the performance of memcpy() for unaligned copy

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Date:	Mon, 18 Oct 2010 16:01:13 +0800
From:	"Ma, Ling" <ling.ma@...el.com>
To:	"miaox@...fujitsu.com" <miaox@...fujitsu.com>
CC:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Zhao, Yakui" <yakui.zhao@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V2 -tip] lib,x86_64: improve the performance of memcpy()
 for unaligned copy




>> rep_good will cause memcpy jump to memcpy_c, so not run this patch,
>> we may continue to do further optimization on it later.

>Yes, but in fact, the performance of memcpy_c is not better on some micro-architecture(such as:
>Wolfdale-3M, ), especially in the unaligned cases, so we need do optimization for it, and I think
>the first step of optimization is optimizing the original code of memcpy().

As mentioned above , we will optimize further memcpy_c soon.
Two reasons : 
  1. movs instruction need long lantency to startup
  2. movs instruction is not good for unaligned case.
  
>> BTW the improvement is only from core2 shift register optimization,
>> but for most previous cpus shift register is very sensitive because of decode stage. 
>> I have test Atom, Opteron, and Nocona, new patch is still better.

>I think we can add a flag to make this improvement only valid for Core2 or other CPU like it,
>just like X86_FEATURE_REP_GOOD.

We should optimize core2 in memcpy_c function in future, I think.

Thanks
Ling


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