This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
Re: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
- From: "Al Grant" <AlGrant at myrealbox dot com>
- To: falk dot hueffner at student dot uni-tuebingen dot de
- Cc: nathan at gcc dot gnu dot org,algrant at acm dot org,gcc-bugs at gcc dot gnu dot org,gcc-prs at gcc dot gnu dot org,nobody at gcc dot gnu dot org,gcc-gnats at gcc dot gnu dot org
- Date: Fri, 12 Jul 2002 16:09:44 +0000
- Subject: Re: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
> 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.