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

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Date:   Tue, 20 Apr 2021 10:36:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dan Carpenter' <dan.carpenter@...cle.com>
CC:     'Mauro Carvalho Chehab' <mchehab@...nel.org>,
        Ashish Kalra <eashishkalra@...il.com>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: RE: [PATCH] media: atomisp: silence "dubious: !x | !y" warning

From: Dan Carpenter
> Sent: 20 April 2021 11:28
> 
> On Sat, Apr 17, 2021 at 09:31:32PM +0000, David Laight wrote:
> > From: Mauro Carvalho Chehab
> > > Sent: 17 April 2021 19:56
> > >
> > > Em Sat, 17 Apr 2021 21:06:27 +0530
> > > Ashish Kalra <eashishkalra@...il.com> escreveu:
> > >
> > > > 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.
> >
> 
> Smatch doesn't warn about | vs || if both sides are true/false.  But
> I've occasionally asked people if they were trying to do a fast path
> optimization but it's always just a typo.

The problem is with people blindly patching code to 'fix'
these warnings.
It might just be a fast path optimisation - which they break.

Trying to beat the compiler into submission can be hard though.
Getting it to 'or' together the outputs from a series of x86
'setne' instructions isn't for the faint hearted.
Not helped by the instruction only setting %al.

	David

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