Re: 64 bit code

From	BGB <cr88192@hotmail.com>
Newsgroups	comp.programming
Subject	Re: 64 bit code
Date	2012-10-12 16:33 -0500
Organization	albasani.net
Message-ID	<k5a2hk$dve$1@news.albasani.net> (permalink)
References	(2 earlier) <k5488o$cbv$1@dont-email.me> <k54j49$cns$1@speranza.aioe.org> <162dnU-Rw5IpVOrNnZ2dnUVZ8qCdnZ2d@bt.com> <k59snb$oqm$1@speranza.aioe.org> <k59un2$qem$1@dont-email.me>

Show all headers | View raw

On 10/12/2012 3:30 PM, BartC wrote:
> "Rui Maciel" <rui.maciel@gmail.com> wrote in message
> news:k59snb$oqm$1@speranza.aioe.org...
>> Chris Uppal wrote:
>>
>>> Rui Maciel wrote:
>>>
>>>> And let's not forget about the ever elusive 80-bit extended precision
>>>> floating point format, also known as "why bother".
>>>
>>> For one answer, people might like to read:
>>>
>>> http://www.cs.berkeley.edu/~wkahan/Stnfrd50.pdf
>
>> In other words, in this example, Matrix A and vectors q and b generally
>> are
>> themselves the result of an approximate calculation, and their error
>> isn't
>> small. To put things in perspective, It's not uncommon for these
>> errors to
>> be in the 1%-0.1% range.  In some commercial number crunching
>> applications
>> that implement this class of methods, the error even goes up to 3%-4% in
>> come applications.
>>
>> Meanwhile, discussing whether floats or doubles should be used is
>> equivalent
>> to discussing the significance of an error in the range of
>> 0.0001%-0.00001%.
>>
>> Hence, why bother?
>
> I didn't understand any of your argument. Using more precision is always
> going to help some types of calculations. The difference between float and
> double is a matter of 29 or so bits, about 500 million times more
> precision,
> not the 10 to 100 million your figures suggest.
>
> And the real reason for 80 bits, is probably that the mantissa is
> exactly 64
> bits; that makes it attractive for the same sorts of reasons that 64-bit
> processors are more popular than 52-bit ones. An 80-bit floating-point
> value can also represent a 64-bit integer exactly (signed *or* unsigned
> I believe). It's worth bothering with!
>

but it is an inconvenient size in that it requires 80 bits to store, 
which is not much more than 64 bits, but kills power-of-2 alignment, 
whereas a 64-bit double is still aligned by a power-of-2.

the next power-of-2 size is 128 bits, but using a 128-bit space to store 
an 80-bit value isn't very good, as nearly 1/2 of it is wasted (this is 
often what ends up in compilers though).

a float128 value is better, but then the drawback is a lack of hardware 
support, making it slow.

so, the cost/benefit tradeoffs point to double, which can exactly 
represent a value up to 52 bits, still goes pretty fast, and has a 
convenient storage size.

or such...

Thread

64 bit code bob <bob@coolfone.comze.com> - 2012-10-10 08:22 -0700
  Re: 64 bit code Jongware <jongware@no-spam.plz> - 2012-10-10 17:35 +0200
    Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-10 17:37 +0100
      Re: 64 bit code Robert Wessel <robertwessel2@yahoo.com> - 2012-10-10 12:54 -0500
      Re: 64 bit code Fritz Wuehler <fritz@spamexpire-201210.rodent.frell.theremailer.net> - 2012-10-10 21:37 +0200
        Re: 64 bit code BGB <cr88192@hotmail.com> - 2012-10-10 21:46 -0500
      Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-10 20:43 +0100
        Re: 64 bit code "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-10-12 08:40 +0100
          Re: 64 bit code BGB <cr88192@hotmail.com> - 2012-10-12 12:36 -0500
          Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-12 20:57 +0100
            Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-12 21:30 +0100
              Re: 64 bit code BGB <cr88192@hotmail.com> - 2012-10-12 16:33 -0500
                Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-13 00:44 +0100
                Re: 64 bit code Ben Pfaff <blp@cs.stanford.edu> - 2012-10-12 20:07 -0700
              Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-13 11:11 +0100
                Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-13 13:11 +0100
                Re: 64 bit code Willem <willem@turtle.stack.nl> - 2012-10-13 12:34 +0000
                Re: 64 bit code BGB <cr88192@hotmail.com> - 2012-10-13 09:50 -0500
                Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-13 17:25 +0100
                Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-13 17:53 +0100
                Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-13 17:05 +0100
          Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-13 10:37 +0200
            Re: 64 bit code "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-10-27 12:18 +0100
              Re: 64 bit code "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-10-27 12:54 +0100
              Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-27 15:13 +0200
                Re: 64 bit code Robert Miles <milesrf@Usenet-News.net> - 2012-10-28 04:12 -0500
                Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-28 13:10 +0100
                Re: 64 bit code "LudovicoVan" <julio@diegidio.name> - 2012-10-29 04:38 +0000
                Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-29 09:30 +0100
                Re: 64 bit code "LudovicoVan" <julio@diegidio.name> - 2012-10-30 22:09 +0000
                Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-31 09:47 +0100
                Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-31 09:55 +0100
                Re: 64 bit code "LudovicoVan" <julio@diegidio.name> - 2012-10-31 14:32 +0000
            Re: 64 bit code Patricia Shanahan <pats@acm.org> - 2012-10-27 08:51 -0700
              Re: 64 bit code "LudovicoVan" <julio@diegidio.name> - 2012-10-27 17:23 +0100
              Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-27 20:43 +0200
                Re: 64 bit code "LudovicoVan" <julio@diegidio.name> - 2012-10-30 22:16 +0000
                Re: 64 bit code "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-10-31 09:53 +0100
                Re: 64 bit code "LudovicoVan" <julio@diegidio.name> - 2012-10-31 14:33 +0000
                Re: 64 bit code Fritz Wuehler <fritz@spamexpire-201211.rodent.frell.theremailer.net> - 2012-11-01 00:25 +0100
      Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-10 21:04 +0100
        Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-10 21:52 +0100
          Re: 64 bit code Ian Collins <ian-news@hotmail.com> - 2012-10-11 10:50 +1300
            Re: 64 bit code BGB <cr88192@hotmail.com> - 2012-10-10 21:54 -0500
              Re: 64 bit code "BartC" <bc@freeuk.com> - 2012-10-11 13:11 +0100
                Re: 64 bit code BGB <cr88192@hotmail.com> - 2012-10-11 11:17 -0500
      Re: 64 bit code Ben Pfaff <blp@cs.stanford.edu> - 2012-10-10 13:17 -0700
        Re: 64 bit code Fritz Wuehler <fritz@spamexpire-201210.rodent.frell.theremailer.net> - 2012-10-11 19:36 +0200
          Re: 64 bit code Ben Pfaff <blp@cs.stanford.edu> - 2012-10-12 08:13 -0700
    Re: 64 bit code Rui Maciel <rui.maciel@gmail.com> - 2012-10-10 20:38 +0100
    Re: 64 bit code Robin Vowels <robin.vowels@gmail.com> - 2012-10-10 15:22 -0700
  Re: 64 bit code Robert Miles <milesrf@Usenet-News.net> - 2012-10-12 02:13 -0500

csiph-web