Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!decwrl!glacier!mips!mash
From: mash@mips.UUCP (John Mashey)
Newsgroups: net.arch
Subject: Re: Addressing modes
Message-ID: <389@mips.UUCP>
Date: Tue, 11-Mar-86 03:46:50 EST
Article-I.D.: mips.389
Posted: Tue Mar 11 03:46:50 1986
Date-Received: Thu, 13-Mar-86 07:38:31 EST
References: <946@garfield.UUCP> <1417@sdcsvax.UUCP>
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 47

Peter Ludemann 	ludemann@ubc-cs.uucp (ubc-vision!ubc-cs!ludemann) writes:
> In article <1163@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
> >In article <169@ubc-cs.UUCP> ludemann@ubc-cs.UUCP (Peter Ludemann) writes:
> >>As far as I'm concerned, the test for RISCness should be: given any
> >>piece of source code, is there only one reasonable code sequence which can be
> >>output by the compiler?
> >
> >This is not a possible objective.  Consider sequences like:
> >	(description of optimisations possible)
> 
> You miss my point.  The optimisations described (in this case common 
> expression detection) are possible *regardless* of the target 
> machine's architecture.  Any good compiler should try to optimise 
> code like this.  However, once these "source level" and flow analysis 
> optimisations are finished, there should be only one reasonable way 
> of generating the machine code.  Otherwise, the poor compiler writer 
> has to do another set of optimisation analyses, all because of the 
> target machine's instruction set.  

One more time: I urge people not to propose metrics that are exceedingly
difficult to measure or comprehend or even define.  As I said before,
I'm not sure "RISCness" is something one necessarily wants to measure
as a goal, in place of things that one really cares about like performance,
price/performance, code size, etc, etc.  However, if one wants to measure
it, for whatever reason, PLEASE 
a) Use cycles/MIPS (normalized to some common unit of work), i.e., try:
	cycles/MIPS = (clock rate in Mhz) / (MIPS rating)
so that, for example, if you think a VAX 11/780 with UNIX is a 1 Mips cpu,
at 5Mhz, it gets a 5.  If you think a good 68020 implementation is
twice as fast, then it gets 16.67/2 = 8.33, etc.  [Notice that this
isn't cycles/instruction, but cycles/equivalent work.]
OR
b) Propose a different metric that has a reasonable way to measure it.
Is it unreasonable to ask for an algorithm to compute the metric?
[I suggest that the "only one way to do it" test is, in general,
mechanically undecidable.]

(Note: Peter's comment does get at something that usually happens,
which is that simpler machines have less different obvious code selection
possibilities at some level or other, but even then, there are almost
always time-space tradeoffs to be made, which allow for different code
sequences for even the RISCiest of machines.)
-- 
-john mashey
UUCP: 	{decvax,ucbvax,ihnp4}!decwrl!mips!mash
DDD:  	408-720-1700
USPS: 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086