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

[Dave Martin <Dave.Martin@arm.com>]

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@suse.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