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

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Date:	Tue, 15 Apr 2014 13:36:02 +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 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)).

-- 
Markus
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