Re: tr1::unordered_set<double> bizarre rounding behavior (x86)

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Avi Kivity <avi@argo.co.il> writes:

| On Tue, 2005-07-05 at 20:05 +0200, Gabriel Dos Reis wrote:
| > Paolo Carlin
| > It is definitely a good thing to use the full bits of value
| > representation if we ever want to make all "interesting" bits part of
| > the hash value.  For reasonable or sane representations it suffices to
| > get your hand on the object representation, e.g.:
| > 
| >    const int objsize = sizeof (double);
| >    typedef unsigned char objrep_t[objsize];
| >    double x = ....;
| >    objrep_t& p = reintepret_cast<objrep_t&>(x);
| >    // ...
| > 
| > and let frexp and friends only for less obvious value representation.
| 
| most architectures have different bit representations for +0.0 and -0.0,
| yet the two values compare equal.

Indeed.  Paolo, Michael and I have been discussing that offline.
the signed zero representation is not the a serious one (say compare
to having padding bits or how to handle NaNs properly) -- just
compare to 0.0 and return hash value 0 before going mocking around
with bytes.

-- Gaby