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From: reiter@harvard.UUCP (Ehud Reiter)
Newsgroups: net.arch
Subject: Re: VAX polyd instruction
Message-ID: <759@harvard.UUCP>
Date: Wed, 5-Mar-86 10:52:04 EST
Article-I.D.: harvard.759
Posted: Wed Mar  5 10:52:04 1986
Date-Received: Fri, 7-Mar-86 07:49:13 EST
References: <946@garfield.UUCP> <1417@sdcsvax.UUCP> <6777@boring.UUCP> <1476@lanl.ARPA> <78@cad.UUCP>
Reply-To: reiter@harvard.UUCP (Ehud reiter)
Organization: Aiken Comp Lab, Harvard
Lines: 23
Keywords: RISC, optimiser, compiler
Summary: POLYD and Trig Functions

In the various RISC vs CISC debates, the VAX POLYD instruction has often
been pointed out as the "archtype" of a bad instruction - inefficient and
difficult for compilers to handle.  Now, in article <78@cad.UUCP>, Richard
Rudell (rudell@cad.UUCP) points out that "common knowledge" to the contrary,
his tests show that the VAX POLYD instruction is faster even than unrolled
assembly language.  The point I wish to make is that it is irrelevant that
the POLYD instruction is difficult for compilers to handle, because its
main purpose was to speed up evaluation of mathematical functions (SIN, EXP,
LOG, etc.), which nearly always come down to evaluating an approximation
polynomial (see H. Levy and R. Eckhouse, COMPUTER PROGRAMMING AND ARCHITECTURE:
THE VAX-11 (Digital Press), pg 167).  Therefore, the intended "user" of the
POLYD instruction was the run time library system, not compiled code.

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.

					Ehud Reiter
					reiter@harvard.ARPA
					...seismo!harvard!reiter.UUCP