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From: Peter Zijlstra <peterz@infradead.org>
To: Philipp Klocke <Phil_K97@gmx.de>
Cc: sil2review@lists.osadl.org, kernelnewbies@kernelnewbies.org,
	llvmlinux@lists.linuxfoundation.org,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	der.herr@hofr.at, lukas.bulwahn@gmail.com
Subject: Re: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG case
Date: Fri, 20 Apr 2018 09:57:17 +0200	[thread overview]
Message-ID: <20180420075717.GB4064@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180416085426.24157-1-Phil_K97@gmx.de>

On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote:

> This patch is motivated by the clang warning Wconstant-logical-operand,
> issued when logically comparing a variable to a constant integer that is
> neither 1 nor 0.  It happens for sched_feat(x) when sysctl_sched_features
> is constant, i.e., CONFIG_SCHED_DEBUG is not set.
> 
> kernel/sched/fair.c:3927:14: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]
>         if (initial && sched_feat(START_DEBIT))
>                     ^  ~~~~~~~~~~~~~~~~~~~~~~~
> kernel/sched/fair.c:3927:14: note: use '&' for a bitwise operation
>         if (initial && sched_feat(START_DEBIT))
>                     ^~
>                     &
> kernel/sched/fair.c:3927:14: note: remove constant to silence this warning
>         if (initial && sched_feat(START_DEBIT))
>                    ~^~~~~~~~~~~~~~~~~~~~~~~~~~


> @@ -1305,7 +1305,11 @@ static const_debug __maybe_unused unsigned int sysctl_sched_features =
>  	0;
>  #undef SCHED_FEAT
>  
> +#ifdef CONFIG_SCHED_DEBUG
>  #define sched_feat(x) (sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
> +#else
> +#define sched_feat(x) ((sysctl_sched_features >> __SCHED_FEAT_##x) & 1UL)
> +#endif

So this is extra ugly, for no gain?

WTH does clang complain about a constant? Can't you just disable that
stupid warning?

Also, if sysctl_sched_features is a constant, the both expressions
_should_ really result in a constant and clang should still warn about
it.

I'm really not seeing why we'd want to do this. Just fix clang to not be
stupid.

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  parent reply	other threads:[~2018-04-20  7:57 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-16  8:54 Philipp Klocke
2018-04-18 13:49 ` Nicholas Mc Guire
2018-04-20  7:57 ` Peter Zijlstra [this message]
2018-04-20 16:29   ` Philipp Klocke
2018-04-20 16:51     ` Peter Zijlstra
2018-04-20 21:29       ` Lukas Bulwahn
2018-04-23  9:45         ` Peter Zijlstra
2018-04-20 10:34 ` Peter Zijlstra

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