From mboxrd@z Thu Jan  1 00:00:00 1970
From: peterz@infradead.org (Peter Zijlstra)
Date: Fri, 20 Apr 2018 18:51:39 +0200
Subject: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG
 case
In-Reply-To: <34572fee-36d0-36e1-ba6d-f098b145aba4@gmx.de>
References: <20180416085426.24157-1-Phil_K97@gmx.de>
 <20180420075717.GB4064@hirez.programming.kicks-ass.net>
 <34572fee-36d0-36e1-ba6d-f098b145aba4@gmx.de>
Message-ID: <20180420165139.GP4064@hirez.programming.kicks-ass.net>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On Fri, Apr 20, 2018 at 06:29:07PM +0200, Philipp Klocke wrote:
> The gain is stopping a warning that clutters the output log of clang.

Well, you should not be using clang anyway. It is known to miscompile
the kernel.

> To improve readability, one can drop the ifdef-structure and just keep
> the right shift version, like Nicholas suggested. This will have a (very
> small)
> impact on performance in CONFIG_SCHED_DEBUG case, but when
> debugging, performance is no problem anyways.

See that is two bad choices.

> > Also, if sysctl_sched_features is a constant, the both expressions
> > _should_ really result in a constant and clang should still warn about
> > it.
> No, because clang only warns if the constant is neither 1 nor 0.
> (These being the 'best' integer representations of booleans)

Then won't something like so work?

#define sched_feat(x) !!(sysctl_sched_features & (1UL << __SCHED_FEAT_##x))

That forces it into _Bool space (0/1) and per the above rule will no
longer warn.