linux-kernel - Re: [PATCH] clocksource: Fix build in non-OF case

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Date:	Thu, 28 Mar 2013 14:47:51 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Rob Herring <robherring2@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Tomasz Figa <tomasz.figa@...il.com>,
	Axel Lin <axel.lin@...ics.com>
Subject: Re: [PATCH] clocksource: Fix build in non-OF case

On Thursday 28 March 2013, Mark Brown wrote:
> On Thu, Mar 28, 2013 at 01:08:22PM +0000, Arnd Bergmann wrote:
> > On Thursday 28 March 2013, Mark Brown wrote:
> 
> > > That still looks like it'll reference the function?
> 
> > Yes, that is intentional. The idea is to create a reference to the
> > function so gcc doesn't complain about unused symbols if the function
> > gets marked static, but at the same time mark the data structure we
> > define as unused so gcc can drop the structure as well as the function
> > if they are not referenced from anywhere else.  This should let us
> > get away with fewer #ifdef hacks in the code, better build-time coverage
> > but without producing larger object code.
> 
> So GCC is supposed to be smart enough to figure this out and users need
> to not do the ifdefs?  I have to say this does seem a bit surprising
> from a user point of view but it does make sense from a general niceness
> point of view.

Yes, I'm pretty sure that all gcc-4.x versions can do this right at -Os
and -O2 levels. The new gcc-4.8 -Og level may get it wrong but is also
broken for many other things we do in the kernel, just like building with
gcc -O0.

Since we recently introduced the IS_ENABLED() macro to test for preprocessor
symbols, I think there is a general trend away from any #ifdefs in driver
code.

	Arnd
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