Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!hao!gatech!mcnc!duke!dfk
From: dfk@duke.cs.duke.edu (David Kotz)
Newsgroups: comp.arch
Subject: Re: RISC is a nasty no-no!
Message-ID: <11199@duke.cs.duke.edu>
Date: 29 Feb 88 16:44:15 GMT
References: <179@wsccs.UUCP: <696@nuchat.UUCP> <284@scdpyr.UUCP> <25699@linus.UUCP>
Organization: Duke University CS Dept.; Durham, NC
Lines: 20
Summary: array indices don't necessarily use multiplication

> There's something about RISC architectures in general that I find 
> confusing. Since they (read SPARC or equivalent) have no integer multiply
> instructions, any code which has a fair number of these is going to
> be slow. This would include any program which had access to 2-D arrays
> since one must do multiplications (unless the array sizes are a convenient
> power of 2) to get the array indices right. ...
> Bob Silverman

Multiplication is not necessary to access 2-D arrays if the array is
set up like most arrays in C, where each row is a typical vector and
the 2-D array is just a vector of pointers to each row vector. Then
double-indirection is necessary, rather than multiplication. I won't
say that's any better, but you don't *need* multiplication. (It might
not be so bad once the first two pointers are cached, and a good
programmer puts them in registers anyway). 

David Kotz
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