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From: jlg@lanl.ARPA (Jim Giles)
Newsgroups: net.arch
Subject: Re: VAX polyd instruction
Message-ID: <132@lanl.ARPA>
Date: Thu, 6-Mar-86 09:16:23 EST
Article-I.D.: lanl.132
Posted: Thu Mar  6 09:16:23 1986
Date-Received: Sat, 8-Mar-86 22:16:54 EST
References: <946@garfield.UUCP> <1417@sdcsvax.UUCP> <6777@boring.UUCP> <1476@lanl.ARPA> <78@cad.UUCP> <759@harvard.UUCP>
Reply-To: jlg@a.UUCP (Jim Giles)
Organization: Los Alamos National Laboratory
Lines: 27
Keywords: RISC, optimiser, compiler

In article <759@harvard.UUCP> reiter@harvard.UUCP (Ehud reiter) writes:
>Of course, one can argue that the best way to speed up trig functions is not
>with a polynomial evaluation instruction, but rather by hard coding the trig
>functions directly in the floating point unit (some of the new floating
>point chips, like the 80287, seem to be moving in this direction), but if RISC
>types get upset at POLYD, they must really hate the thought of primitive
>instructions for SIN, ATAN, etc.

Quite the contrary.  The evaluation of these primitives in hardware is not
necessarily a bad idea.  As long as it's done in a seperate functional
unit (and the archecture is pipelined) it is not a bad idea at all.  The
problem with POLYD is not what it does, but that it slows the rest of the
machine's instructions down even if it's not used (which it often isn't).

Of course (as you can see), I'm not a RISC purist.  I don't think single
clock execution of ALL instructions is manditory.  Pipelining complicates
archecture because (among other things) it means that you have to provide
for instruction delays because of reserved registers.  I think this is
more than offset by the speed advantage of having several functional units
which can each be simple because they are only responsible for part of
the instruction set.  The most useful thing about RISC ideas is the
reduction of addressing modes.  The CRAY has the only two I've ever really
needed: immediate and indexed (I COULD get by without immediate, but it's
cheap to provide - apparently).

J. Giles
Los Alamos