linux-kernel - Re: [RFC PATCH 2/2] Kbuild: avoid partial linking of drivers/built-in.o

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Date:	Tue, 31 Mar 2015 17:27:22 +0100
From:	Dave Martin <Dave.Martin@....com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Michal Marek <mmarek@...e.cz>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, linux-kbuild@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 2/2] Kbuild: avoid partial linking of
 drivers/built-in.o

On Tue, Mar 31, 2015 at 11:22:29AM -0400, Nicolas Pitre wrote:
> On Mon, 30 Mar 2015, Ard Biesheuvel wrote:
> 
> > On 30 March 2015 at 16:13, Michal Marek <mmarek@...e.cz> wrote:

[...]

> > > What you could do is to add a Kconfig option to arch/arm/Kconfig adding
> > > -ffunction-sections to the compiler flags. Then allyesconfig would
> > > select it and work around the problem in a somewhat elegant way.
> > >
> > 
> > Excellent idea! Arnd hasn't chimed in yet, but he is the one doing
> > lots and lots of randconfig builds and other test builds, so I will
> > wait for him to confirm that this is a useful thing to have.
> 
> I'm using -ffunction-sections as well for the kernel size reduction work 
> I'm currently doing.  The linker script has to be adapted so .text.* is 
> specified along .text otherwise those functions end up appended at the 
> end of the binary.

Interesting ... do you also mean using --gc-sections at link time?
We'd need to avoid pruning needed code that has no explicit caller,
and functions that are part of the kernel/module ABI but not used
within vmlinux.

The GCC docs suggest that -ffunction-sections may impact performance
and/or increase code size, but I don't know by how much.  Maybe it
interferes with inling.

Cheers
---Dave

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