linux-kernel - Re: [PATCH] iio: magnetometer: Only declare ACPI table when ACPI is enable

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Message-ID: <20170706164238.GB95735@google.com>
Date:   Thu, 6 Jul 2017 09:42:38 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH] iio: magnetometer: Only declare ACPI table when ACPI is
 enable

El Sat, Jul 01, 2017 at 01:11:12PM +0100 Jonathan Cameron ha dit:

> On Thu, 29 Jun 2017 14:51:56 +0200
> Linus Walleij <linus.walleij@...aro.org> wrote:
> 
> > On Tue, Jun 27, 2017 at 2:25 AM, Matthias Kaehlcke <mka@...omium.org> wrote:
> > 
> > > Don't inflate the kernel size with data that isn't used. The conditional
> > > declaration also fixes the following warning when building with clang:
> > >
> > > drivers/iio/magnetometer/ak8975.c:704:36: error: variable 'ak_acpi_match'
> > >   is not needed and will not be emitted
> > >   [-Werror,-Wunneeded-internal-declaration]
> > >
> > > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>  
> > 
> > Acked-by: Linus Walleij <linus.walleij@...aro.org>
> > 
> > Yours,
> > Linus Walleij
> Applied to the togreg branch of iio.git and pushed out as testing
> for the autobuilders to play with it.

Thanks!

> I'm curious though... Can the compiler not optimize this away? 
> I'd always assumed the main reason for these warnings was that the
> presence of unused data might represent a bug rather than it actually
> making any difference to the size...

clang actually optimizes it away: "variable 'ak_acpi_match' is not
needed and will not be emitted", my argument about inflating the
kernel size is not valid.