Comparing floating point: == vs is

[Jonathan M Davis]

On Saturday 16 July 2011 19:03:51 Daniel Murphy wrote:
> "Jonathan M Davis" <jmdavisProg at gmx.com> wrote in message
> news:mailman.1687.1310805329.14074.digitalmars-d-learn at puremagic.com...
> 
> > Under what circumstances would a comparison of floating point values
> > return
> > true for is but not ==? If I understand corcectly, is does a bit-by-bit
> > comparison. So, I would expect that if is returns true, == would return
> > true.
> > However, that does not seem to always be the case. Does anyone know why
> > that
> > would be?
> > 
> > In particular, I'm trying to write a function for swapping the
> > endianness
> > of
> > floating point values, and while it seems to be generally be returning
> > true for
> > is, == keeps failing. So, it sounds like I have two floating point
> > values
> > which
> > have the exact same bits (since is succeeds) but which aren't equal
> > according
> > to ==. I don't understand what's going on at all. Does anyone have a
> > clue
> > what
> > might be happening?
> > 
> > - Jonathan M Davis
> 
> As of the last release, 'is' for floating point types is broken. (before it
> just did the same as ==)
> At compile time, a is b will be true if a and b have the same bit pattern,
> or if a and b are both nans.
> At run time, a is b will return true if they have the same bit pattern only,
> but may sometimes return false on platforms other than windows as it
> includes the padding bytes in the comparison.

Hmm. Good to know. But assuming that two floating point values have the same 
bit pattern, shouldn't == return true for them? So, if == is failing, then two 
floating point values aren't identical, correct?

- Jonathan M Davis