Messages in this thread | | | Date | Tue, 11 Jul 2017 10:40:56 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 4/8] objtool: add undwarf debuginfo generation |
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* Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> Anyway, I used some linker magic to temporarily move the unwinder code to the > end of .text, so that unwinder changes don't add unexpected side effects to the > microbenchmark behavior. Now I'm getting more consistent results: the packed > struct is measuring ~2% slower. The slight slowdown might just be explained by > the fact that GCC generates some extra instructions for extracting the fields > out of the packed struct.
Yeah, the 16-bit field accesses versus a zero-extended 32-bit field are more complex to access even on x86 that has a fair amount of 16-bit legacy.
> In the meantime, I found a ~10% speedup by making the "fast lookup table" block > size a power-of-two (256) to get rid of the need for a slow 'div' instruction. > > I think I'm done performance tweaking for now. I'll keep the packed struct, and > add the code for the 'div' removal, and hope to submit v3 soon.
Sounds good to me!
~2% slowdown for ~30% RAM savings for a debug data structure that is about as large as a typical kernel's total .text is a decent trade-off.
Thanks,
Ingo
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