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Date: Tue, 27 Sep 2016 13:48:00 -0000
From: Michael Matz <matz@suse.de>
To: Jason Merrill <jason@redhat.com>
cc: Bernd Edlinger <bernd.edlinger@hotmail.de>,     "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,     Jeff Law <law@redhat.com>
Subject: Re: [PATCH] Make -Wint-in-bool-context warn on suspicious shift ops
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Hi,

On Tue, 27 Sep 2016, Jason Merrill wrote:

> On Sun, Sep 25, 2016 at 3:46 AM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
> > This patch makes -Wint-in-bool-context warn on suspicious integer left
> > shifts, when the integer is signed, which is most likely some kind of
> > programming error, for instance using "<<" instead of "<".
> >
> > The warning is motivated by the fact, that an overflow on integer shift
> > left is undefined behavior, even if gcc won't optimize the shift based
> > on the undefined behavior.
> >
> > So in absence of undefined behavior the boolean result does not depend
> > on the shift value, thus the whole shifting is pointless.
> 
> It's pointless for unsigned integers, too; why not warn for them as
> well?

Um, because left shift on unsigned integers is never undefined, so
  !!(1u << a)
is meaningful and effectively tests if a < CHAR_BITS*sizeof(unsigned) ?


Ciao,
Michael.