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

From: Andrew Morton
Date: Fri Dec 16 2011 - 03:46:17 EST


On Fri, 16 Dec 2011 09:19:16 +0100 Ingo Molnar <mingo@xxxxxxx> 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.
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