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From: dik@cwi.nl (Dik T. Winter)
Newsgroups: comp.arch
Subject: Re: Why The Move To RISC Architectures?  ('386 vs. RISC)
Message-ID: <8912@boring.cwi.nl>
Date: 23 Mar 90 01:05:33 GMT
References: <28012@cup.portal.com> <1990Mar20.175843.2612@utzoo.uucp> <5303@scolex.sco.COM> <1268@m3.mfci.UUCP> <1990Mar22.184122.7917@ultra.com>
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In article <1990Mar22.184122.7917@ultra.com> shj@ultra.com (Steve Jay) writes:
 >                                     By 1970, however, CDC had a new
 > compiler, FTN, which did rearrange instructions to optimize usage
 > of the multiple functional units.  The technology of both local and
 > global optimization in the FTN compiler was continously improved,
 > and by mid to late 70's, it was difficult to beat the compiler even
 > with hand tuned assembly language.
And then came the problem.  CDC came with newer versions of their machine,
and newer versions of their compiler.  The problem was that different
machines had different requirements with respect to scheduling.  So a
program fully optimized for a 7600 was not optimal for a 170/750.  There
were switches in the compiler to tune for the different models, but at
least on the 170/750 it was possible to take the compiler generated
assembler code, hand tune it by simple peep-hole optimization, and
gain a factor of 2 (but of course not for all programs).

This is in general a problem if the compiler has too much to do.
Newer models of the machine require a different compiler.  And not
only newer models, but if you have a range of models differing only in
price and performance, you may have introduced different scheduling
requirements for the different models.  Although your architecture can
be such that object code compiled for one model is valid for another
model, it may be sub-optimal.  And think next about the hassle to
maintain different versions of the compiler!

 > I don't think the unavailability of an optimizing compiler when the
 > 6600 first came out in any way detracts from the RISCness of the
 > machine.  You can read articles written around 1965 which justify
 > the design decisions for the 6600 in terms almost identical to those
 > used today to justify RISC over CISC.

I agree here.  And do not take me wrong; I like the (60 bit) Cybers and
the Crays.

Although this belongs more to comp.compilers it is also of significance in
this group, because there is a strong interaction between compiler and
machine.
-- 
dik t. winter, cwi, amsterdam, nederland
dik@cwi.nl