From: Gabriel Dos Reis <gdr@integrable-solutions.net>
To: Joe Buck <Joe.Buck@synopsys.COM>
Cc: gcc@gcc.gnu.org
Subject: Re: tr1::unordered_set<double> bizarre rounding behavior (x86)
Date: Tue, 05 Jul 2005 18:32:00 -0000 [thread overview]
Message-ID: <m34qb9qf4a.fsf@uniton.integrable-solutions.net> (raw)
In-Reply-To: <20050705181025.GB2315@synopsys.com>
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.
next prev parent reply other threads:[~2005-07-05 18:32 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-07-05 13:05 Michael Veksler
2005-07-05 13:32 ` Paolo Carlini
2005-07-05 15:43 ` Michael Veksler
2005-07-05 15:57 ` Gabriel Dos Reis
2005-07-05 16:04 ` Paolo Carlini
2005-07-05 20:18 ` Michael Veksler
2005-07-05 20:22 ` Paolo Carlini
2005-07-05 20:36 ` Michael Veksler
2005-07-05 20:41 ` Paolo Carlini
2005-07-05 20:47 ` Paolo Carlini
2005-07-05 21:12 ` Michael Veksler
2005-07-05 21:21 ` Gabriel Dos Reis
2005-07-05 14:28 ` Gabriel Dos Reis
2005-07-05 17:04 ` Paolo Carlini
2005-07-05 18:06 ` Gabriel Dos Reis
2005-07-05 18:10 ` Joe Buck
2005-07-05 18:32 ` Gabriel Dos Reis [this message]
2005-07-05 18:42 ` Paolo Carlini
2005-07-05 19:18 ` Gabriel Dos Reis
2005-07-05 19:52 ` Paolo Carlini
2005-07-05 20:24 ` Gabriel Dos Reis
2005-07-05 19:01 ` Michael Veksler
2005-07-05 19:24 ` Gabriel Dos Reis
2005-07-06 12:38 ` Avi Kivity
2005-07-06 12:54 ` Gabriel Dos Reis
2005-07-06 12:54 ` Michael Veksler
2005-07-06 13:01 ` Avi Kivity
2005-07-06 13:50 ` Gabriel Dos Reis
2005-07-05 18:08 ` Joe Buck
2005-07-05 18:12 ` Paolo Carlini
2005-07-05 19:58 ` Joe Buck
2005-07-05 19:59 ` Paolo Carlini
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