linux-kernel - Re: [PATCH RFC] [X86] performance improvement for memcpy

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Message-ID: <4AFD0AFC.5020603@zytor.com>
Date:	Thu, 12 Nov 2009 23:30:04 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Ma, Ling" <ling.ma@...el.com>
CC:	Pavel Machek <pavel@....cz>, "mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S by
 fast string.

On 11/12/2009 11:23 PM, Ma, Ling wrote:
> Hi H. Peter Anvin
>> What it sounds to me is that for Nehalem, we want to use memcpy_c for >=
>> 1024 bytes and the old code for < 1024 bytes;
> 
> Yes, so we modify memcpy_c as memcpy_new for Nehalem, and keep old
> code for Core2 is acceptable?

No, what I think we should do is to rename the old memcpy to something
like memcpy_o, and then have the actual memcpy routine look like:

	cmpq $1024, %rcx
	ja memcpy_c
	jmp memcpy_o

... where the constant as well as the ja opcode can be patched by the
alternatives mechanism (to a jb if needed).

memcpy is *definitely* frequent enough that static patching is justified.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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