Comparing floating point: == vs is

[Daniel Murphy]

"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.