[issue1580] Use shorter float repr when possible

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

So work on the py3k-short-float-repr branch is nearing completion, and
we (Eric and I) would like to get approval for merging these changes
into the py3k branch before this month's beta.

A proposal: I propose that the short float representation should be
considered an implementation detail for CPython, not a requirement for
Python the language.  This leaves Jython and IronPython and friends free
to do as they wish.  All that should be required for Python itself is
that float(repr(x)) == x for (non-nan, non-infinite) floats x.  Does
this seem reasonable to people following this issue?  If it's
controversial I'll take it to python-dev.

Eric's summarized his changes above.  Here are mine (mostly---some
of this has bits of Eric's work in too):

 - change repr(my_float) to produce output with minimal number of
digits, on IEEE 754 big- and little-endian systems (which includes all
systems we can actually get our hands on right now).

 - there's a new file Python/dtoa.c (and a corresponding file
Include/dtoa.h) with a cut-down version of Gay's dtoa and strtod in it.
There are comments at the top of that file indicating the changes that
have been made from Gay's original code.

 - one minor semantic change:  repr(x) changes to exponential format at
1e16 instead of 1e17.  This avoids a strange situation where Gay's code
produces a 16-digit 'shortest' integer string for a particular float and
Python's formatting code then pads with an incorrect '0':

>>> x = 2e16+8.  # 2e16+8. is exactly representable as a float
>>> x
20000000000000010.0

There's no way that this padding with bogus digits can happen for
numbers less than 1e16.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue1580>
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