The continuing floating point saga

So, I had a couple people email me due to my floating point posts
(which required them to google for my email address, though that isn’t
difficult with the uniqueness of my name). It appears that I probably
would have gotten even more comments had my blog been set up to accept
them. Does anyone know if pyblosxom can be set up to accept comments?
Any pointers?

In particular, Laurent Boulard went to a lot of work to help me track
the problem down and find a solution/workaround. Very cool. Also,
solnul AT gmx de just sent me an alternate solution with is rather
ingenious: use the volatile keyword. That’s so cute it just
made me smile. Very clever.


Miguel
: very awesome document. Many thanks for pointing it out to
me. It’s similar to other chapters and documents I had read before,
but is more thorough. I especially liked this paragraph on page 82
(or page 252 depending on which counting method you trust in the
document). It was in reference to a nearly trivial 8-line program
which tends to produce different results depending on whether
optimizations are used:

Thus, on these systems, we can’t predict the behavior of the program
simply by reading its source code and applying a basic understanding
of IEEE 754 arithmetic. Neither can we accuse the hardware or the
compiler of failing to provide an IEEE 754 compliant environment: the
hardware has delivered a correctly rounded result to each destination,
as it is required to do, and the compiler has assigned some
intermediate results to destinations that are beyond the user’s
control, as it is allowed to do.

That was exactly the problem I was being bit by. The “destination” of
my floating point expressions in my program were put beyond my control
and thus I couldn’t force my program to provide consistent or expected
behavior. Luckily for me, gcc has an -ffloat-store option which
allows me to force the destination to be a certain desired precision,
thus allowing me to put to use my knowledge of IEEE 754. However, I’m
tempted to spurn that solution and go with the volatile
solution (which effectively does the same thing but is localized)
instead. :-)

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