Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!uakari.primate.wisc.edu!sdd.hp.com!decwrl!sgi!karsh@trifolium.esd.sgi.com
From: karsh@trifolium.esd.sgi.com (Bruce Karsh)
Newsgroups: comp.sys.sgi
Subject: Re: detecting integer overflow
Message-ID: <67704@sgi.sgi.com>
Date: 25 Aug 90 05:00:25 GMT
References: <GLOBUS.90Aug21114837@wk208.nas.nasa.gov> <67688@sgi.sgi.com> <67702@sgi.sgi.com>
Sender: guest@sgi.sgi.com
Reply-To: karsh@trifolium.sgi.com (Bruce Karsh)
Distribution: comp.sys.sgi
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 49

In article <67702@sgi.sgi.com> karsh@trifolium.sgi.com (Bruce Karsh) writes:

>This should be

> 	if( (~a^b &  a^c) < 0) handle_overflow();

I can't get anything right today.  Bruce Holloway pointed out the precedence
problem.  It should be:

        if( ((~a^b) & (a^c)) < 0) handle_overflow();

because the precedence of & is higher than the precedence of ^.

Just to make sure this is right this time, a test case is attached below.

/* Test of software overflow handling. */
main()
{
	ofltst(0x80000000,0x80000000);
	ofltst(0x80000000,0x00000000);
	ofltst(0x80000000,0x7fffffff);
	ofltst(0x00000000,0x80000000);
	ofltst(0x00000000,0x00000000);
	ofltst(0x00000000,0x7fffffff);
	ofltst(0x7fffffff,0x80000000);
	ofltst(0x7fffffff,0x00000000);
	ofltst(0x7fffffff,0x7fffffff);
}
ofltst(a,b)
int a,b;
{
	int c;
	c=a+b;
        if( ((~a^b) & (a^c)) < 0) handle_overflow(a,b,c);
	else handle_nooverflow(a,b,c);
}
handle_overflow(a,b,c)
int a,b,c;
{
	printf("%11d = %11d + %-11d overflow\n",c,a,b);
}
handle_nooverflow(a,b,c)
int a,b,c;
{
	printf("%11d = %11d + %-11d no overflow\n",c,a,b);
}

			Bruce Karsh
			karsh@sgi.com