[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 10/06/2011 11:13 AM, H. Peter Anvin wrote:
> On 10/06/2011 11:10 AM, Jason Baron wrote:
>> It would be cool if we could make the total width 2-bytes, when
>> possible.  It might be possible by making the initial 
>> 'JUMP_LABEL_INITIAL_NOP'
>> as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
>> time or link time - letting the compiler pick the width. In that way we could
>> get the optimal width...
>>
> Yes, that would be a win just based on icache footprint alone.

I'm not sure it would be a win, necessarily.  My test with back-to-back
jmp2 was definitely slower than with the nop padding it out to 5 bytes;
I suspect that's a result of having too many jmps within one cacheline. 
Of course, there's no reason why the CPU would optimise for jumps to
jumps, so perhaps its just hitting a "stupid programmer" path.

    J

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