[Python-Dev] Caching float(0.0)

Josiah Carlson jcarlson at uci.edu
Wed Oct 4 03:38:43 CEST 2006


Alastair Houghton <alastair at alastairs-place.net> wrote:
> On 3 Oct 2006, at 17:47, James Y Knight wrote:
> 
> > On Oct 3, 2006, at 8:30 AM, Martin v. Löwis wrote:
> >> As Michael Hudson observed, this is difficult to implement, though:
> >> You can't distinguish between -0.0 and +0.0 easily, yet you should.
> >
> > Of course you can. It's absolutely trivial. The only part that's even
> > *the least bit* sketchy in this is assuming that a double is 64 bits.
> > Practically speaking, that is true on all architectures I know of,
> 
> How about doing 1.0 / x, where x is the number you want to test?  On  
> systems with sane semantics, it should result in an infinity, the  
> sign of which should depend on the sign of the zero.  While I'm sure  
> there are any number of places where it will break, on those  
> platforms it seems to me that you're unlikely to care about the  
> difference between +0.0 and -0.0 anyway, since it's hard to otherwise  
> distinguish them.

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...

We can observe it directly, for example on x86:
    >>> import struct
    >>> struct.pack('d', -0.0)
    '\x00\x00\x00\x00\x00\x00\x00\x80'
    >>> struct.pack('d', 0.0)
    '\x00\x00\x00\x00\x00\x00\x00\x00'
    >>>

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 */
            ...
        }
    }


 - Josiah



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