Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!uunet!mcsun!hp4nl!phigate!ehviea!leo
From: leo@ehviea.ine.philips.nl (Leo de Wit)
Newsgroups: comp.sys.atari.st.tech
Subject: Re: Re: Floating Pt. Math with a 68881 not always faster
Message-ID: <887@ehviea.ine.philips.nl>
Date: 10 Sep 90 11:21:51 GMT
References: <8387@ncar.ucar.edu>
Reply-To: leo@ehviea.UUCP (Leo de Wit)
Organization: Philips I&E Eindhoven
Lines: 28

In article <8387@ncar.ucar.edu> moses@hao.ucar.edu (Julie Moses) writes:
|        Having looked at the Alcyon 68881 assembly source code, there
|does not seem to be much one can do to further optimize the F.P. 
|routines. Prospero's are probably based on Alcyons.  The TT's copressor
|will probably run circles around any single precision done by a 68xxx
|CPUs (I hope).

If you're looking for fast math, you could perhaps make use of
representation of real numbers by integer quotients; this is a common
technique when precision and/or range allow it (which is the case for a
lot of real-life applications). Each real (or float if you want) is
represented by a pair of integers (choose either longs or shorts),
whose quotient is the real (approximately). Especially in cases where
shorts can be used for the representation (low precision), and on a
processor that prefers integers to floats this can boost up
calculations dramatically.

Another way to possibly increase performance is to put often calculated
function values in an array, e.g. for sin(x) calculate sin(i*pi/180)
for i = 0..90. Intermediate values can be found - for example - by
linear interpolation.

For an example of quotient calculation, you can take a look at the
sources of the 3D demo program I wrote a while ago.

Cheers,

   Leo.