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

H. Peter Anvin
2009-11-13T07:30:23Z

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.