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

    * Andrew Morton <akpm@linux-foundation.org> wrote:

    > On Fri, 16 Dec 2011 09:19:16 +0100 Ingo Molnar <mingo@elte.hu> wrote:
    >
    > >
    > > This patch turns on -momit-leaf-frame-pointer on x86 builds and
    > > thus shrinks .text noticeably. On a defconfig-ish kernel:
    > >
    > > text data bss dec hex filename
    > > 9843902 1935808 3649536 15429246 eb6e7e vmlinux.before
    > > 9813764 1935792 3649536 15399092 eaf8b4 vmlinux.after
    > >
    > > That's 0.3% off text size.
    > >
    > > The actual win is larger than this percentage suggests: many
    > > small, hot helper functions such as find_next_bit(),
    > > do_raw_spin_lock() or most of the list_*() functions are leaf
    > > functions and are now shorter by 2 instructions.
    > >
    > > Probably a good chunk of the framepointers related runtime
    > > overhead on common workloads is eliminated via this patch, as
    > > small leaf functions execute more often than larger parent
    > > functions.
    > >
    > > 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.
    >
    > The only problem I can think of (apart from tickling gcc bugs) is that
    > it might break __builtin_return_address(n) for n>0 with frame pointers
    > enabled? The only code I can find which does this is
    > drivers/isdn/hardware/mISDN/ and ftrace.

    Well, AFAICS it won't really 'break' it but behave as if the
    leaf function got inlined into the parent function. I think we
    can live with that.

    Thanks,

    Ingo


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