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From: ron@brl-sem.ARPA (Ron Natalie <ron>)
Newsgroups: net.lang.c
Subject: Re:  Two Birds with One Stone
Message-ID: <615@brl-sem.ARPA>
Date: Fri, 3-Jan-86 16:15:20 EST
Article-I.D.: brl-sem.615
Posted: Fri Jan  3 16:15:20 1986
Date-Received: Sun, 5-Jan-86 01:15:06 EST
References: <874@dataioDataio.UUCP> <864@kuling.UUCP> <881@dataioDataio.UUCP>
Organization: Ballistic Research Lab
Lines: 26

> In article <864@kuling.UUCP> thomas@kuling.UUCP (Thomas H{meenaho) writes:
> >In article <874@dataioDataio.UUCP> bright@dataio.UUCP (Walter Bright writes:
> >>Almost but not quite true. A compiler CANNOT normally replace a divide
> >>by a right-shift if it is an integer divide. This is because a right
> >>shift of a negative integer is not the same as a divide.
> >However most useable processors provide arithmetic shifts which will give
> >the right result even if it is a signed divide.
> 
> Ok, try this program:
> 
> main()
> {
> 	printf("-3/2 = %d\n-3 >> 1 = %d\n",(-3)/2,(-3) >> 1);
> }
> 
> I think you'll find the results are different, even when the compiler
> does an arithmetic right shift! (The difference is the direction in
> which rounding occurs.)

Actually, K&R states that the truncation from an integer divide of negative
numbers is machine dependant -3/2 could correctly be -2 or -1 depending on
the machine.  The only guarantee that they make is that

	(a/b)*b + a%b must be equal to a

.