From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 26787 invoked by alias); 12 Jul 2002 15:16:02 -0000 Mailing-List: contact gcc-prs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Archive: List-Post: List-Help: Sender: gcc-prs-owner@gcc.gnu.org Received: (qmail 26773 invoked by uid 71); 12 Jul 2002 15:16:01 -0000 Date: Fri, 12 Jul 2002 08:16:00 -0000 Message-ID: <20020712151601.26772.qmail@sources.redhat.com> To: nobody@gcc.gnu.org Cc: gcc-prs@gcc.gnu.org, From: Falk Hueffner Subject: Re: c/7284: incorrectly simplifies leftshift followed by signed power-of-2 division Reply-To: Falk Hueffner X-SW-Source: 2002-07/txt/msg00378.txt.bz2 List-Id: The following reply was made to PR c/7284; it has been noted by GNATS. From: Falk Hueffner To: "Al Grant" 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" 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