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

    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?

    The alternative is to just make it a plain C-level indirect call, but
    then we'd lose all the patching and callee-save optimisations.

    Any suggestions?

    Thanks,
    J



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