linux-kernel - Re: [PATCH] media: atomisp: silence "dubious: !x

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Message-ID: <20210418215931.kbgme42kgnpqbwn4@mail>
Date:   Sun, 18 Apr 2021 23:59:31 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Mauro Carvalho Chehab' <mchehab@...nel.org>,
        Ashish Kalra <eashishkalra@...il.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] media: atomisp: silence "dubious: !x | !y" warning

On Sat, Apr 17, 2021 at 09:31:32PM +0000, David Laight wrote:
> > > Upon running sparse, "warning: dubious: !x | !y" is brought to notice
> > > for this file.  Logical and bitwise OR are basically the same in this
> > > context so it doesn't cause a runtime bug.  But let's change it to
> > > logical OR to make it cleaner and silence the Sparse warning.
> 
> The old code is very likely to by slightly more efficient.
> 
> It may not matter here, but it might in a really hot path.
> 
> Since !x | !y and !x || !y always have the same value
> why is sparse complaining at all.

They both will have the same value here and any half-decent
compiler know that and thus generate the same code, so no
worries about efficiency.

Sparse complains because the programmer's intention is not clear.
Was a boolean context or a bitwise context that was meant?
Maybe '||' was meant and the RHS had to be short cut?
Maybe what was meant was '~x | ~y'?

-- Luc