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From: thomas@kuling.UUCP (Thomas H{meenaho)
Newsgroups: net.lang.c
Subject: Re: division
Message-ID: <873@kuling.UUCP>
Date: Thu, 9-Jan-86 23:11:24 EST
Article-I.D.: kuling.873
Posted: Thu Jan  9 23:11:24 1986
Date-Received: Sun, 12-Jan-86 06:06:00 EST
References: <874@dataioDataio.UUCP> <864@kuling.UUCP> <199@myrias.UUCP> <6263@utzoo.UUCP>
Reply-To: thomas@kuling.UUCP (Thomas H{meenaho)
Organization: The Royal Inst. of Techn., Stockholm
Lines: 26

In article <6263@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) 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.
>> 
>> The first is correct, the second is WRONG.  -1/2 is 0 not -1.
>

	[long story about different ways to divide negative numbers]

Enough about my comment on arithmetic shifts. I thought the problem was
the sign bit which is lost if one uses a logical shift.

			    x					       x
However the idea of adding 2  -1 when dividing a negative number with 2  seems
like a win.



-- 
Thomas Hameenaho, Dept. of Computer Science, Uppsala University, Sweden
Phone: +46 18 138650
UUCP: thomas@kuling.UUCP (...!{seismo,mcvax}!enea!kuling!thomas)