Messages in this thread | | | Date | Thu, 06 Oct 2011 14:39:49 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out |
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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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