Re: [rc4-amd64] RC4 optimized for AMD64

[dean gaudet]

On Mon, 1 Nov 2004, Marc Bevand wrote:

> I have just published a small paper about optimizing RC4 for
> AMD64 (x86-64). A working implementation is also provided:
> 
>   http://epita.fr/~bevand_m/papers/rc4-amd64.html
> 
> Kernel people may be interested given the fact that Linux
> already implements RC4.

you've made a non-portable flags assumption:

>       dec     %r11b
>       ror     $8,             %r8             # (ror does not change ZF)
>       jnz 1b

the contents of ZF are undefined after a rotation... most importantly they 
differ between p4 (ZF is set according to result) and k8 (ZF unchanged).

do you really measure a perf improvement from this assumption?  note that 
p4 would prefer "sub $1, %r11b" here instead of dec... but the difference 
is likely minimal.

-dean
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