Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!umd5!uvaarpa!mcnc!ecsvax!tpmsph
From: tpmsph@ecsvax.UUCP (Thomas P. Morris)
Newsgroups: comp.arch
Subject: Re: RISC is a nasty no-no!
Message-ID: <4699@ecsvax.UUCP>
Date: 1 Mar 88 12:21:21 GMT
References: <179@wsccs.UUCP: <696@nuchat.UUCP> <284@scdpyr.UUCP> <11199@duke.cs.duke.edu>
Organization: UNC Educational Computing Service
Lines: 30
Summary: good programmers vs good compilers


  In response to  a posting about RISC having "bad" performance due to
array indexing which might require heavy integer multiplication, David
Kotz points out:
> 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). 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Two points: (1) a good programmer shouldn't have to put those pointers
into registers. That's what good compilers are for! ;-)  (2) If your
array elements are being accessed sequentially, a good _optimizing_
compiler ought to be changing those array-index computations with
addititively computed offsets from a base. Strength reduction, code
hoisting, and elimination or reduction of invariant computations can
do wonders for code!

Of course, a good programmer _would_ indicate to his/her "C" compiler that 
the indices or the pointers ought to go in registers, too!


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Tom Morris                                 BITNET: TOM@UNCSPHVX
UNC School of Public Health                UUCP  : ...!mcnc!ecsvax!tpmsph

        "It's a joke, boy! I say, it's a joke! Don't ya get it?"
                                                -Foghorn Leghorn
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