Messages in this thread | | | Date | Fri, 16 Dec 2011 11:46:03 +0000 | From | "Jan Beulich" <> | Subject | Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size |
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>>> On 16.12.11 at 10:23, Jeremy Fitzhardinge <jeremy@goop.org> wrote: > On 12/16/2011 12:53 AM, Ingo Molnar wrote: >> * Ingo Molnar <mingo@elte.hu> wrote: >> >>> [...] >>> >>> The call-chains are still intact for quality backtraces and >>> for call-chain profiling (perf record -g), as the backtrace >>> walker can deduct the full backtrace from the RIP of a leaf >>> function and the parent chain.
Are you sure about that even if the leaf function uses rBP for a different purpose?
>> Hm, noticed one complication while looking at annotated assembly >> code in perf top. Code doing function calls from within asm() is >> incorrectly marked 'leaf' by GCC: >> >> ffffffff812b82d8 <arch_local_save_flags>: >> ffffffff812b82d8: ff 14 25 00 d9 c1 81 callq *0xffffffff81c1d900 >> ffffffff812b82df: c3 retq >> >> So all the paravirt details will have to be fixed, so that GCC >> is able to see that there's a real function call done inside. >> Jeremy, Konrad?
If the above is not a problem, wouldn't this simply result in a skipped function layer?
Also, iirc it's not just pv-ops that uses calls within asm()-s.
> Um. So the issue is that a function that contains only pvops looks like > it's a leaf to gcc and it does some leaf-function optimisation? > > How can we tell gcc the asm contains a call, or otherwise suppress the > "leaf function" classification?
I'm afraid you can't without adding code (i.e. a dummy function call).
Jan
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