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

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Date:	Fri, 16 Dec 2011 09:19:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	linux-kernel@...r.kernel.org
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Beulich <JBeulich@...e.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>
Subject: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size

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.

Signed-off-by: Ingo Molnar <mingo@...e.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)
--
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