[issue1580] Use shorter float repr when possible

[Noam Raphael]

Noam Raphael added the comment:

2007/12/13, Guido van Rossum <report at bugs.python.org>:
>
> > Ok, so if I understand correctly, the ideal thing would be to
> > implement decimal to binary conversion by ourselves. This would make
> > str <-> float conversion do the same thing on all platforms, and would
> > make repr(1.1)=='1.1'. This would also allow us to define exactly how
> > floats operate, with regard to infinities and NaNs. All this is for
> > IEEE-754 platforms -- for the rare platforms which don't support it,
> > the current state remains.
>
> Does doubledigits.c not work for non-754 platforms?

No. It may be a kind of an oops, but currently it just won't compile
on platforms which it doesn't recognize, and it only recognizes 754
platforms.
>
> > 2. Keep the binary to shortest decimal routine and use it only when we
> > know that the system's decimal to binary routine is correctly rounding
> > (we can check - perhaps Microsoft has changed theirs?)
>
> Tim says you can't check (test) for this -- you have to prove it from
> source, or trust the vendor's documentation. I would have no idea
> where to find this documented.
>
The program for testing floating point compatibility is in
http://www.cant.ua.ac.be/ieeecc754.html

To run it, on my computer, I used:
./configure -target Conversions -platform IntelPentium_cpp
make
./IeeeCC754 -d -r n -n x Conversion/testsets/d2bconvd
less ieee.log

This tests only doubles, round to nearest, and ignores flags which
should be raised to signal inexact conversion. You can use any file in
Conversions/testsets/d2b* - I chose this one pretty randomly.

It turns out that even on my gcc 4.1.3 it finds a few floats not
correctly rounded. :(

Anyway, it can be used to test other platforms. If not by the
executable itself, we can pretty easily write a python program which
uses the test data.

I don't know what exactly the errors with gcc 4.1.3 mean - is there a
problem with the algorithm of glibc, or perhaps the testing program
didn't set some flag?

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