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Re: GUB and mpfr/mpc
From: |
David Kastrup |
Subject: |
Re: GUB and mpfr/mpc |
Date: |
Fri, 05 Dec 2014 12:22:48 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux) |
Masamichi HOSODA <address@hidden> writes:
>>>> Also, the mention of optimization reminds me of one of the horrors
>>> of floating point types: a value in a register can quietly be
>>> changed when it is written to memory. Take a look at some of the
>>> “myths” in section 1 of
>>> https://hal.archives-ouvertes.fr/hal-00128124/en/ .
>> […]
>>> I think that floating point can be used as long as it's used correct.
>>>
>>> operator== shouldn't be used for comparison of floating point values.
>>> operator!= shouldn't be used too.
>>> operator< and operator> may be used.
>>
>> The issue is more complex than that. Sit down and try this:
> [...]
>> Add looping and you’ve got fatal issues. (And if you’re like me,
>> steam will erupt from your ears any second now...)
>
> You're right of course.
> A floating point operation has a margin of error always.
> When the optimization is different, the margin of error may be also different,
> because the bit width of floating point value is different
> in a register and a memory.
>
> Therefore, robust treating for margin of error is necessary.
That does not help all that much: it just shifts the problem somewhere
else. It will tend to fix known test cases and create a new set of
previously working failures.
Instead the algorithms have to be rewritten in a manner where their
termination does not depend on the reproducibility of floating point
operations. There have been a few attempts of rewriting skyline code
that have fixed/created some problems without any real insights into
what caused the fix and what caused the new problems.
--
David Kastrup
- Re: GUB and mpfr/mpc, (continued)
- Re: GUB and mpfr/mpc, Masamichi HOSODA, 2014/12/02
- Re: GUB and mpfr/mpc, David Kastrup, 2014/12/02
- Re: GUB and mpfr/mpc, Masamichi HOSODA, 2014/12/03
- Re: GUB and mpfr/mpc, Dan Eble, 2014/12/04
- Re: GUB and mpfr/mpc, Dan Eble, 2014/12/04
- Re: GUB and mpfr/mpc, Masamichi HOSODA, 2014/12/04
- Re: GUB and mpfr/mpc, Masamichi HOSODA, 2014/12/04
- Re: GUB and mpfr/mpc, Dan Eble, 2014/12/04
- Re: GUB and mpfr/mpc, Masamichi HOSODA, 2014/12/05
- Re: GUB and mpfr/mpc,
David Kastrup <=
- Re: GUB and mpfr/mpc, Dan Eble, 2014/12/05
- Re: GUB and mpfr/mpc, David Kastrup, 2014/12/06
- Re: GUB and mpfr/mpc, Dan Eble, 2014/12/06
- Re: GUB and mpfr/mpc, Masamichi HOSODA, 2014/12/08
- Re: GUB and mpfr/mpc, David Kastrup, 2014/12/08
- Re: GUB and mpfr/mpc, Dan Eble, 2014/12/08
- Re: GUB and mpfr/mpc, Keith OHara, 2014/12/21
Re: GUB and mpfr/mpc, Masamichi Hosoda, 2014/12/28