Re: [PATCH RFC] [X86] performance improvement for memcpy_64.S byfast string.

[H. Peter Anvin]

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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