Re: Parsing timestamps?

minforth <minforth@gmx.net> writes:

Am 10.07.2025 um 21:33 schrieb Paul Rubin:
Kahan was also overly critical of dynamic Unum/Posit formats.
>
Time has shown that he was partially wrong:
https://spectrum.ieee.org/floating-point-numbers-posits-processor

What is supposed to be partially wrong?

FP numbers have a number of not-so-nice properties, and John L,
Gustafson uses that somewhat successfully to sell his alternatives to
the gullible.  The way to do that is to give some examples where
traditional FP numbers fail and his alternative under consideration
works.  I have looked at a (IIRC) slide deck by Kahan where he shows
examples where the altenarnative by Gustafson (don't remember which
one he looked at in that slide deck) fails and traditional FP numbers
work.

Where does that leave us?  Kahan makes the good argument that
numerical analysts have worked out techniques to deal with the
shortcomings of traditional FP numbers for over 70 years.  For
Gustafson's number formats these techniques are not applicable; maybe
one can find new ones for these number formats, but that's not clear.

For Posits (Type III Unums), which are close to traditional FP in many
respects, one can see how that would work out; while traditional FP
has a fixed division between mantissa and exponents, in Posits the
division depends on the size of the exponent.  This means that
reasoning about the accuracy of the computation would have to consider
the size of the exponent, and is therefore more complex than for
traditional FP; with a little luck you can produce a result that gives
an error bound based on the smallest mantissa size, but that error
bound will be worse than for tranditional FP.

- anton
--
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