Messages in this thread | | | Date | Fri, 16 Dec 2011 09:54:59 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size |
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* 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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