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    SubjectRe: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size
    >>> 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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