Path: utzoo!attcan!uunet!husc6!think!ames!elroy!cit-vax!mangler
From: mangler@cit-vax.Caltech.Edu (Don Speck)
Newsgroups: comp.arch
Subject: Re: negative addresses (really unsigned arithmetic)
Summary: efficiency tricks
Message-ID: <6575@cit-vax.Caltech.Edu>
Date: 17 May 88 08:32:54 GMT
References: <2393@uvacs.CS.VIRGINIA.EDU> <9485@apple.Apple.Com>, <1988May15.220044.12987@utzoo.uucp>
Organization: California Institute of Technology
Lines: 10

One should use unsigned numbers when mixing arithmetic and logical
operators, e.g. division by shifting right, modulus by masking, etc.

There are certain optimizations along those lines that are only safe
if the compiler can be sure that the number cannot be negative
(stemming largely from the convention of rounding towards zero).

Array indices can be range-checked with a single unsigned compare.

Don Speck   speck@vlsi.caltech.edu  {amdahl,ames!elroy}!cit-vax!speck