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

[Al Grant]

The following reply was made to PR c/7284; it has been noted by GNATS.

From: "Al Grant" <AlGrant@myrealbox.com>
To: falk.hueffner@student.uni-tuebingen.de
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: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division
Date: Fri, 12 Jul 2002 16:09:44 +0000

 > On 12/07/2002 15:12:01 nathan wrote:
 > >Synopsis: incorrectly simplifies leftshift followed by signed power-of-2=
 =20
 > >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.
 >=20
 > 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.
 >=20
 > 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). =20
 You are referring to C99.  gcc was not defining __STDC_VERSION__, so C89, n=
 ot C99, is surely the relevant standard.  The behaviour happens even if I=
  explicitly set -std=3Dc89, or if I use g++ 3.1, and you cannot justify e=
 ither of those by reference to C99.