On 4 Oct 2006, at 02:38, Josiah Carlson wrote:
> Alastair Houghton <[EMAIL PROTECTED]> wrote:
>
> There is, of course, the option of examining their representations in
> memory (I described the general technique in another posting on this
> thread). From what I understand of IEEE 764 FP doubles, -0.0 and +0.0
> have different representations, and if we look at the underlying
> representation (perhaps by a "*((uint64*)(&float_input))"), we can
> easily distinguish all values we want to cache...
Yes, though a trip via memory isn't necessarily cheap, and you're
also assuming that the machine doesn't use an FP representation with
multiple +0s or -0s. Perhaps they should be different anyway though,
I suppose.
> And as I stated before, we can switch on those values. Alternatively,
> if we can't switch on the 64 bit values directly...
>
> uint32* p = (uint32*)(&double_input)
> if (!p[0]) { /* p[1] on big-endian platforms */
> switch p[1] { /* p[0] on big-endian platforms */
> ...
> }
> }
That's worse, IMHO, because it assumes more about the
representation. If you're going to look directly at the binary, I
think all you can reasonably do is a straight binary comparison. I
don't think you should poke at the bits without first knowing that
the platform uses IEEE floating point.
The reason I suggested 1.0/x is that it's one of the few ways (maybe
the only way?) to distinguish -0.0 and +0.0 using arithmetic, which
is what people that care about the difference between the two are
going to care about.
Kind regards,
Alastair.
--
http://alastairs-place.net
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