linux-kernel - Re: [GIT] kbuild/lto changes for 3.15-rc1

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Message-ID: <20140415181945.GA22608@ravnborg.org>
Date:	Tue, 15 Apr 2014 20:19:45 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Josh Triplett <josh@...htriplett.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michal Marek <mmarek@...e.cz>,
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	John Crispin <blogic@...nwrt.org>
Subject: Re: [GIT] kbuild/lto changes for 3.15-rc1

On Tue, Apr 15, 2014 at 01:36:02PM +0200, Markus Trippelsdorf wrote:
> On 2014.04.15 at 13:19 +0200, Sam Ravnborg wrote:
> > > 
> > > And while the code size reduction is less for MIPS than what others have
> > > reported for their platforms (I'm still investigating) is still is enough
> > > that embedded developers would commit murder for.
> > 
> > I have experimented a little with a patch that links all of vmlinux in one step.
> > I compared the text size of vmlinux without and with -ffunction-sections.
> > 
> > With a defconfig build on x86 (32 bit) I got following results:
> > 
> >                         size     difference
> > singlelink          10266506
> > function-sections    9487369         779137  7,5%
> > 
> > So this is a reduction of ~800 kb by enabling -ffunction-sections which
> > allows the linker to throw away unused sections.
> > 
> > I have not boot tested the kernel so chances are that too much was thrown out by the linker.
> > But this is an option that has much smaller cost to use than lto.
> > And seems to benefit nicely in size.
> > 
> > I have not tried this wihtout my singlelink patch - but I assume similar results.
> 
> No, it wouldn't work, because you cannot mix -r and --gc-sections (or
> gold's --icf (identical code folding)).
With make allnoconfig I see a 5% decrease in text size by applying -ffunction-sections.
This is with latest mainline and no other than the following applied:

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 602f57e..51bac0a 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -171,6 +171,8 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 KBUILD_CFLAGS += $(call cc-option,-mno-sse -mno-mmx -mno-sse2 -mno-3dnow,)
 KBUILD_CFLAGS += $(call cc-option,-mno-avx,)
 
+KBUILD_CFLAGS += -ffunction-sections
+
 KBUILD_CFLAGS += $(mflags-y)
 KBUILD_AFLAGS += $(mflags-y)
 

What can then explain this size difference?

	Sam
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