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Re: differences between implementations of IEEE-754 basic operators

David Hough 754R work <754r@xxxxxxxxxxx> wrote:

one can validly argue that it is silly to expect absolute 
repeatable floating-point behavior

It might be unrealistic to expect it at the current time, but as a
general proposition one might well wonder why "standard" arithmetic
is so unpredictable.

One might.  And, once upon a time, the experts in the field did
so wonder.  They investigated why, and found out that it was a
fundamental and irremovable property of the floating-point model
when used in the context of any normal programming language and
programming methodology.  And that was 40 years ago ....

And many inexperienced people do so wonder.  I wouldn't call
them silly.

I would call them ignorant, because they have been misinformed
by the so-called experts.  What I would call those experts is
rather stronger than 'silly'.

Yes, getting reproducible results IS possible.  But you CAN'T do
it without defining a programming methodology intended to deliver
that property.  And that has FAR more drastic consequences than
any of its proponents admit - in particular, it is effectively
incompatible with most forms of parallelism.

Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1@xxxxxxxxx
Tel.:  +44 1223 334761    Fax:  +44 1223 334679

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