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Date: Tue, 05 Jul 2005 18:42:00 -0000
From: Paolo Carlini <pcarlini@suse.de>
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To: Gabriel Dos Reis <gdr@integrable-solutions.net>
Cc: Joe Buck <Joe.Buck@synopsys.COM>, gcc@gcc.gnu.org
Subject: Re: tr1::unordered_set<double> bizarre rounding behavior (x86)
References: <OF340AFDFE.4774C997-ON43257035.003F59A4-43257035.0047DBFD@il.ibm.com>	<42CABDCC.3070508@suse.de>	<m3hdf9b024.fsf@uniton.integrable-solutions.net>	<20050705181025.GB2315@synopsys.com> <m34qb9qf4a.fsf@uniton.integrable-solutions.net>
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Gabriel Dos Reis wrote:

>Joe Buck <Joe.Buck@synopsys.COM> writes:
>
>| On Tue, Jul 05, 2005 at 08:05:39PM +0200, Gabriel Dos Reis wrote:
>| > 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.
>| 
>| I disagree; on an ILP32 machine, we pull out only 32 bits for the hash
>| value, and if you aren't careful, your approach will wind up using the
>| least significant bits of the mantissa.  This will cause all values that
>| are exactly representable as floats to collide.
>
>I'm not sure we're talking about the same thing.  With the
>representations I'm talking about, value repsentation == object representation.
>
... still, the hash functions for float, double and long double that we
need for TR1 must return a size_t (6.3.3). How do you get a size_t from
an objrep_t? (fulfilling the various requirements briefly reminded by
Joe in his first message)

Paolo.