Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: RISC is NOT micro-code
Message-ID: <1989Jul19.165456.20210@utzoo.uucp>
Organization: U of Toronto Zoology
References: <753@bnr-fos.UUCP> <1989Jul17.035238.6497@utzoo.uucp> <756@bnr-fos.UUCP> <2203@taux01.UUCP>
Date: Wed, 19 Jul 89 16:54:56 GMT

In article <2203@taux01.UUCP> cjosta@tasu77.UUCP (Jonathan Sweedler) writes:
>... I was always under the
>impression that RISC machines need more complex compilers, not just
>because of the need for more optimization, but because RISC chips are
>'dumber' than CISC chips...

As has been pointed out before (sigh), this is just not true.  Most
compilers can make very little use of the extra complexity of CISCs;
simple code generation for a RISC may not exploit the full power of
the machine, but the result is often reasonably fast code.

>... Don't forget that MIPS, one of the early RISC
>chips, stands for 'Microprocessor without Interlocked Pipe Stages.'
>This means that interlocks must be handled in the compiler...

Things like software pipe interlocks, delayed branches, etc. can almost
always be deferred to a cleanup pass that is very stupid and simple.  (On
some RISC chips, this sort of thing is done in the assembler, not the
compiler at all.)  One of the surprises that came out of RISC work is
that moving some of these things into the software really costs very
little.  You may occasionally miss an optimization opportunity by not
having the earlier passes aware of the final rearrangement, but it's
not really very frequent.

>Correct me if I'm wrong, but I remember looking at some of the initial
>specs for the Intel 860 and that thing looked like a nightmare to write
>a compiler for...

Hey, but look who made it. :-)  I mean, you really expected the source
of the 8086 to produce a machine that was easy to generate code for?
Not everyone has made that big a mess of their RISC designs.

The other side of the coin, of course, is that if you are willing (as
John Mashey puts it) "to commit unspeakable acts", the i860 is REALLY
FAST.  You get what you pay for.
-- 
$10 million equals 18 PM       |     Henry Spencer at U of Toronto Zoology
(Pentagon-Minutes). -Tom Neff  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
