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Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
- From: Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>
- To: nobody at gcc dot gnu dot org
- Cc: gcc-prs at gcc dot gnu dot org,
- Date: 12 Jul 2002 16:26:01 -0000
- Subject: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
- Reply-to: Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>
The following reply was made to PR c/7284; it has been noted by GNATS.
From: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
To: "Al Grant" <AlGrant@myrealbox.com>
Cc: nathan@gcc.gnu.org, algrant@acm.org, gcc-bugs@gcc.gnu.org,
gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Subject: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
Date: 12 Jul 2002 18:18:14 +0200
"Al Grant" <AlGrant@myrealbox.com> writes:
> > On 12/07/2002 15:12:01 nathan wrote:
> > >Synopsis: incorrectly simplifies leftshift followed by signed power-of-2
> > >division
> > >
> > >State-Changed-From-To: open->closed
> > >State-Changed-By: nathan
> > >State-Changed-When: Fri Jul 12 07:12:01 2002
> > >State-Changed-Why:
> > >not a bug. for signed types, if 'n << c' overflows, the
> > >behaviour is undefined.
> >
> > There is no "overflow" in my sample code. The operation of shifting 128 24 bits to the left on a
> > 32-bit machine produces the bit pattern 0x80000000.
> > No bits overflow.
> >
> > The fact that a positive number may become negative when
> > left-shifted is a property of the twos complement representation.
> > The standard does not define signed left shift in terms of
> > multiplication and certainly doesn't say that it is undefined when
> > the apparently equivalent multiplication would be undefined.
>
> >Before refering to the standard, you should probably >read it.
>
> I read the C89 standard (and the C++ standard).
> You are referring to C99. gcc was not defining __STDC_VERSION__, so
> C89, not C99, is surely the relevant standard. The behaviour
> happens even if I explicitly set -std=c89, or if I use g++ 3.1, and
> you cannot justify either of those by reference to C99.
Right, I just assumed it to be very unlikely that this was changed to
be undefined in C99. I don't have the C89 standard; could you perhaps
cite the passage that shows this was defined behaviour in C89?
--
Falk