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Path: utzoo!mnetor!seismo!uwvax!cottage!lm
From: lm@cottage.WISC.EDU (Larry McVoy)
Newsgroups: comp.arch
Subject: Re: Japanese 32-bit CPUs ( NEC V70
Message-ID: <3571@spool.WISC.EDU>
Date: Tue, 12-May-87 14:31:13 EDT
Article-I.D.: spool.3571
Posted: Tue May 12 14:31:13 1987
Date-Received: Fri, 15-May-87 01:06:01 EDT
References: <372@winchester.UUCP> <28200037@ccvaxa>
Sender: news@spool.WISC.EDU
Reply-To: lm@cottage.WISC.EDU (Larry McVoy)
Organization: U of Wisconsin CS Dept
Lines: 30

In article <28200037@ccvaxa> preece@ccvaxa.UUCP writes:
>I find it a little amusing that the same people who say "complex
>feature x just isn't used very much" tend to be the same people who
>say "not to worry, a sufficiently clever compiler will take care of
>our ship's need for X".  If compilers can be made smart enough to
>handle some of the special things that RISCs need, they could be
>made smart enough to make better use of the complex features in
>CISCs.

This is hogwash.  As someone who has a certain amount of compiler experience,
I can say that e RISC compiler is likely to be much less intelligent than
a CISC one.  The reason (hold the flames a bit, ok?) is that I have yet
to see an orthogonal CISC machine.  The 32000 series is the closest.  Vax
and 68000 don't come very close.   The problem is this:  you're generating
code for a particular action, right?  And this special instruction looks
like just the ticket.  And then (or maybe three months later) you realize
that you needed a signed displacement and they give you an unsigned 
displacement.  Or something similar.  So you end up going in and 
generating the code in the "stupid" straightforward manner.  Or - maybe
you're really dedicated and you add another 200 lines of code to the
compiler to catch this special case.  And in another week you find out...

The problem can be summarized as follows:

Provide 0, 1 or infinity.  No exceptions.  CISC is trying to approximate
infinity.  As it gets closer the chip gets slower.  The infinity choice
is clearly wrong.  Admit it.


Larry McVoy 	        lm@cottage.wisc.edu  or  uwvax!mcvoy