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From: hrubin@pop.stat.purdue.edu (Herman Rubin)
Newsgroups: comp.arch,comp.lang.misc
Subject: Compilers and efficiency
Message-ID: <9782@mentor.cc.purdue.edu>
Date: 7 Apr 91 19:04:12 GMT
References: <27fa3350.6bc2@petunia.CalPoly.EDU> <1991Apr5.172533.6717@agate.berkeley.edu>
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In article <1991Apr5.172533.6717@agate.berkeley.edu>, doug@eris.berkeley.edu (Doug Merritt) writes:
> In article <1991Apr4.125122.1@capd.jhuapl.edu> waltrip@capd.jhuapl.edu writes:
> >	He observes that "Few compilers use any of the nifty instructions that
> >	a CISC has."  I believe I read recently that RISC was based on the
> >	observation that, in fact, only about 30% of the instructions in CISC
> >	computers were used by compilers.  The rest of the instructions, for
> >	all practical purposes, were just excess baggage.

			.........................

> Anyway, the point is that even if compilers *do* use every possible feature
> of a CISC, it still usually won't make the CISC s/w competitive with RISC
> s/w. CISC cpus almost never put sufficient h/w optimization into the support
> of the fancy instructions. (There are exceptions to this, of course, and I
> haven't been watching the 030 and 040 to see how they do in this regard.
> CISC may yet rise again, but not until after superscalar RISC has been
> completely exploited.)

If the languages do not allow the user to communicate  the use of those
features of the CISC architecture which can make the object code considerably
faster, the compiler cannot take advantage of them.  This IS the situation.
For example, suppose there was an instruction, which could be effectively
used, which would divide a float by a float, obtaining an integer quotient
and a float remainder.  Now just how is the compiler to recognize that this
is in fact wanted?  

There is, at present, no language designed to take into account the collection
of "natural" useful hardware which the present users can find uses for, and
which are far more expensive in software than in hardware.

-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907-1399
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (Internet, bitnet)   {purdue,pur-ee}!l.cc!hrubin(UUCP)