Re: [PATCH v1] sched/topology: Make compiler happy about unused constant definitions

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	linux-kernel@vger.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH v1] sched/topology: Make compiler happy about unused constant definitions
Date: Tue, 25 Aug 2020 14:26:05 +0300	[thread overview]
Message-ID: <20200825112605.GV1891694@smile.fi.intel.com> (raw)
In-Reply-To: <20200825112405.GU1891694@smile.fi.intel.com>

On Tue, Aug 25, 2020 at 02:24:05PM +0300, Andy Shevchenko wrote:
> On Tue, Aug 25, 2020 at 11:12:21AM +0100, Valentin Schneider wrote:
> > On 25/08/20 10:03, Valentin Schneider wrote:
> > > On 25/08/20 09:26, Andy Shevchenko wrote:
> > >> On Mon, Aug 24, 2020 at 06:09:41PM +0100, Valentin Schneider wrote:
> > >>> On 24/08/20 16:39, Andy Shevchenko wrote:
> > >>> > Compilation of almost each file ends up with
> > >>> >
> > >>> >  In file included from .../include/linux/energy_model.h:10,
> > >>> >                 from .../include/linux/device.h:16,
> > >>> >                 from .../drivers/spi/spi.c:8:
> > >>> >  .../include/linux/sched/topology.h:30:27: warning: ‘SD_DEGENERATE_GROUPS_MASK’ defined but not used [-Wunused-const-variable=]
> > >>> >     30 | static const unsigned int SD_DEGENERATE_GROUPS_MASK =
> > >>> >        |                           ^~~~~~~~~~~~~~~~~~~~~~~~~
> > >>> >  ...
> > >>> >
> > >>> > Make compiler happy by annotating the static constants with __maybwe_unused.
> > 
> > > Sorry, that's what I get for trying to be too succinct; what I tried to say
> > > was that SD_DEGENERATE_GROUPS_MASK should very much be used for SMP. If the
> > > build is !SMP, it shouldn't even be defined, IOW I'm perplexed as to where
> > > this is coming from.
> > 
> > So I see how having this as a constvar rather than a constexpr is somewhat
> > daft (we get an instance per compilation unit), but none of my compilers
> > seem to complain (even with W=1). AFAIA the kernelbot didn't catch any of
> > it either.
> > 
> > Out of curiosity, what's your compiler & compiler version?
> 
> % gcc --version
> gcc (Debian 10.2.0-5) 10.2.0
> 
> % make O=... W=1 C=1 CF=-D__CHECK_ENDIAN__ -j64
> 
> > Alternatively we can make this a "proper" constant expression with the
> > below.
> 
> It doesn't fix another static const in the same file, though that one requires
> DEBUG_SCHED to be enabled.
> 
> Do you want me to split this fix and fix only the other case?

Just checked and the other case has been brought by

b6e862f38672 ("sched/topology: Define and assign sched_domain flag metadata")

with your authorship.

-- 
With Best Regards,
Andy Shevchenko