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

[Falk Hueffner <falk.hueffner@student.uni-tuebingen.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