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Re: infinities reformulated




On Thu, 16 Jun 2005, Aubrey Jaffer wrote:

> Can you give an example of a calculation where you expect
> that choosing a reduced precision will reap a large
> benefit?

Reduction in precision beyond the level of a small float
size supported by the hardware is rarely useful, even when
performing binary tricks, but:

It often happens in neural networks (read: my day job) that
being able to store a bunch of floats compactly (level-2
cache size) results in dramatic speedups, and in such cases
(in C) I use arrays of 32-bit floats rather than 64-bit
doubles.  Since R5RS strongly recommends "precision equal to
or greater than the most precise flonum format supported by
the hardware," and further because in scheme I can't in
general rely on a particular hardware representation without
indirections, tag bits, and other encapsulating structures
which will blow the cache, I can't really do this in R5RS
scheme.  I can do it using implementation- specific
extensions in Chicken and Bigloo, and I can do it in Stalin,
another Lisp-1 dialect that's largely similar to scheme.

But a couple of years ago, I had a (toy) project where I was
simulating orbits several centuries into the future in a
game where the objective was to get a hypothetical
spacecraft from L3/Earth to pluto using only 100 m/s of
delta-vee plus orbital mechanics.  And in that project,
having 512-bit precise reals (thanks to Chicken which
allowed itself to be recompiled with alternate real
precision) was *NECESSARY*, since even with scaling, using
"doubles" would have lost crucial information in the
underflow.  Of course, it took a long long time to find a
good solution, but search strategies for a good solution
were what the game was about.

			Bear