Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!amdcad!ames!pasteur!agate!ig!uwmcsd1!bbn!rochester!ur-tut!sunybcs!rutgers!umn-d-ub!umn-cs!hall!pmk
From: pmk@hall.cray.com (Peter Klausler)
Newsgroups: comp.lang.c
Subject: FP math used for int operations
Message-ID: <3337@hall.cray.com>
Date: 22 Jan 88 18:29:52 GMT
References: <2335@haddock.ISC.COM>
Organization: Cray Research, Inc., Mendota Heights, MN
Lines: 26
Summary: Supercomputer trivia

In article <2335@haddock.ISC.COM>, karl@haddock.ISC.COM (Karl Heuer) writes:
> ... Just as the "/" symbol invokes the integer-divide operator when both
> operands are integers (it most certainly does *not* invoke floating-point
> divide and then truncate the result), ...

Our architectures do not possess integer division instructions, so we're
forced to do generic integer division (i=j/k) via something like:

	i = (int) ((float) j / (float) k)

In fact, it's even more complicated, since there's no actual floating
division instruction either, but rather a "reciprocal approximation"
instruction, which generates a half-precision reciprocal that must then
be refined with the Newton-Raphson method iteration instruction.

It gets worse on Cray-1's and X-MP's, which don't have instructions
for coercing full ints into floats or vice versa. All of these
complications conspire to require 15 instructions for i=j/k.

(There's no integer multiplication instruction for full-word integers,
either, so similar tricks are also necessary for things like i=j*k.)

Moral:	Try to use floats when programming a scientific machine.
	Your generated code will use them anyway.

- Peter Klausler @ Cray Research compiler development (ihnp4!cray!hall!pmk)