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

[Falk Hueffner]

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 17:13:48 +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.
 
 6.5.7.4:
 
 "The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
 bits are filled with zeros. If E1 has a signed type and nonnegative
 value, and E1 * 2^E2 is representable in the result type, then that
 is the resulting value; otherwise, the behavior is undefined."
 
 -- 
 	Falk