Date: Tue, 15 Apr 2014 13:36:02 +0200
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: Sam Ravnborg <sam@ravnborg.org>
Cc: Ralf Baechle <ralf@linux-mips.org>, Josh Triplett <josh@joshtriplett.org>,
        Ingo Molnar <mingo@kernel.org>, Andi Kleen <ak@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Michal Marek <mmarek@suse.cz>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        John Crispin <blogic@openwrt.org>
Subject: Re: [GIT] kbuild/lto changes for 3.15-rc1
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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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