Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 16 Dec 2011 15:06:38 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size |
| |
On Fri, Dec 16, 2011 at 09:19:16AM +0100, Ingo Molnar 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.
In the case of linked stacks (like frame pointer linking interrupt to exception stack) we may also miss a leaf function (and not its parent).
Now perhaps we can live with all that, I don't know.
> > Signed-off-by: Ingo Molnar <mingo@elte.hu> > --- > arch/x86/Makefile | 8 ++++++++ > 1 file changed, 8 insertions(+) > > Index: linux/arch/x86/Makefile > =================================================================== > --- linux.orig/arch/x86/Makefile > +++ linux/arch/x86/Makefile > @@ -72,6 +72,14 @@ else > KBUILD_CFLAGS += -maccumulate-outgoing-args > endif > > +# > +# This shrinks many small functions, we don't actually > +# need their frame pointer, in backtraces the RIP will > +# identify the function and the stack frame walker will > +# find the parent function: > +# > +KBUILD_CFLAGS += $(call cc-option,-momit-leaf-frame-pointer) > + > ifdef CONFIG_CC_STACKPROTECTOR > cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh > ifeq ($(shell $(CONFIG_SHELL) $(cc_has_sp) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
| ![\](/images/icornerr.gif) |