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From: nather@utastro.UUCP (Ed Nather)
Newsgroups: net.arch
Subject: Re: RISC perspective
Message-ID: <26@utastro.UUCP>
Date: Fri, 9-Mar-84 10:54:44 EST
Article-I.D.: utastro.26
Posted: Fri Mar  9 10:54:44 1984
Date-Received: Sat, 10-Mar-84 12:43:30 EST
References: <27900009@ucbesvax.UUCP>
Organization: UTexas Astronomy Dept., Austin, Texas
Lines: 35

<>
	I have advocated "cache-advisor" instructions for RISC's--instructions
	that don't *do* anything, from the programmer's point of view, but which
	inform the cache-manager of highly-probable-to-certain upcoming memory
	events.  This is another trade-off--a loss of code-density with a
	possible net gain in performance.  That need not complicate the
	architecture.  It could, in fact simplify it--if cache-block-
	prefetching were more effective,
	then instructions could be looser and "fatter" (more easily decoded and
	executed) with less worry over the fetching costs.  Possible analogies:
	impurity doping of semiconductors to increase electron mobility, or
	adding minute amounts of certain minerals in steel-making to improve its
	tensile strength.  It's worth some experimentation, I think.
	---
	Michael Turner (ucbvax!ucbesvax.turner)

This sounds like a splendid idea to me.  If the programmer inserts a single
instruction that serves as "advice" to the cache-manager, then the overhead
involved is just 1 fetch-time (plus decoding time, but that's true for any
instruction).  It would act like a "pseudo-op" in an assembler program, to
help manage things but generate no output.  If such advice saves only 1 fetch
it breaks even; everything after that is gravy.  I found that the "sieve"
benchmark could be speeded up a factor of 2 on our Vax by advising the "C"
compiler about what to put into registers.  The "C" compiler is the only one
we have humble enough to accept advice.  I find the thought of a much closer
relationship between compiler and programmer, via direct "advice" where it
can really matter, a marvelous one.  Advising the computer directly  could
be even better!


-- 

                                       Ed Nather
                                       ihnp4!{ut-sally,kpno}!utastro!nather
                                       Astronomy Dept., U. of Texas, Austin