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

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Date:	Tue, 15 Apr 2014 20:29:22 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Sam Ravnborg <sam@...nborg.org>
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 2014.04.15 at 20:19 +0200, Sam Ravnborg wrote:
> 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)).

BTW using --gc-sections during vmlinux link time will not work, because
it will "garbage collect" the whole kernel away.

> 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?

That is a good question. Because one would expect that adding
-ffunction-sections should increase the size (and it does with my
config).

-- 
Markus
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