[issue3166] Make conversions from long to float correctly rounded.

[Mark Dickinson]

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

> floating-point variable "x" has an exact nonnegative integer value 
> between 0 and 2**DBL_MANT_DIG. 

Hmm.  On closer inspection that's not quite true.  After the line

x = x * PyLong_BASE + (dig & (PyLong_BASE - pmask));

x has a value of the form n * pmask, where pmask is a power of 2 and
n is in the range [0, 2**DBL_MANT_DIG).  It's still exactly represented, 
provided that FLT_RADIX is 2.  (It's the multiplications by powers of 2 
that get hairy when FLT_RADIX is 16, since they *can* lose information.)

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue3166>
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