c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division

[Falk Hueffner]

"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