Re: Handling numeric types in interpreter

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: Handling numeric types in interpreter
Date	2015-07-28 23:10 +0000
Organization	Aioe.org NNTP Server
Message-ID	<mp9261$7qb$1@speranza.aioe.org> (permalink)
References	(6 earlier) <lnoaj0xj0b.fsf@kst-u.example.com> <mp8fe2$kn1$1@dont-email.me> <mp8mes$hqh$1@dont-email.me> <mp8qh3$3g5$1@dont-email.me> <lnzj2glzxm.fsf@kst-u.example.com>

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Keith Thompson <kst-u@mib.org> wrote:
> jacobnavia <jacob@jacob.remcomp.fr> writes:
(big snip)
>> For people that do not do numerical work long double is useless. OK.

>> But for other people long double is a necessity.

> I disagree with both of you.

> For portable code you can't assume that long double is bigger than
> double -- but you can assume that it's at least as big.  If you
> want to use the biggest floating-point type available, that's
> long double.  On some platforms you won't get any more precision
> or range than double; on others, you will.  If the extra range and
> precision on *some* systems is worth the inconsistency then you win.
> (double isn't guaranteed to have the same range and precision across
> systems, but these days it's *usually* 64-bit IEEE.)

The idea behind the 80 bit temporary real (intel's name) format
is that it holds more precision and range for intermediate
calculations.  The registers are that size, and intermediate
results are stored in the registers.  On the other hand, you
can't guarantee that the compiler will figure it out. 

> Also, since long double is guaranteed to be the largest
> floating-point type (excluding extensions), it can be handy if
> client code might be giving you values of any floating-point type.

> On the other hand, if "long double is a necessity" (by which I
> presume you mean that range and/or precision greater than that of
> double is a necessity), then you're out of luck if you're using a
> system where double and long double have the same representation.
> Using a different compiler that has a larger long double type is
> great if you have that option.  Not everyone does.

In addition, there are surprising results when the compiler does
keep intermediate values in registers. As there are a limited 
number, it might not keep all intermediate values in registers,
so you get different values for the same expression. 

Also, optimization might keep values in registers between
statements, again surprising the user.

> BTW, I'm curious about your two different implementations of long
> double, 80 and 128 bits.  How do you specify which one you want
> to use?  If it's a compile-time option, does the linker prevent
> you from linking object files that don't use the same representation?

There is still a shortage of hardware implementations of 128 bit
formats. Maybe when the new IEEE decimal formats get more popular.
The 64 bit is supposed to be single precision, and the 128 bit
double precision.  It will be interesting to see compiler 
support for these formats.

-- glen

Thread

Handling numeric types in interpreter luser droog <luser.droog@gmail.com> - 2015-07-13 19:33 -0700
  Re: Handling numeric types in interpreter jacobnavia <jacob@jacob.remcomp.fr> - 2015-07-14 07:31 +0200
  Handling numeric types in interpreter asetofsymbols@gmail.com - 2015-07-14 00:35 -0700
  Re: Handling numeric types in interpreter David Kleinecke <dkleinecke@gmail.com> - 2015-07-14 08:55 -0700
  Re: Handling numeric types in interpreter Bartc <bc@freeuk.com> - 2015-07-15 23:18 +0100
    Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-24 00:21 -0700
      Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-24 08:36 -0700
        Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-24 11:04 -0700
          Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-24 12:25 -0700
            Re: Handling numeric types in interpreter James Kuyper <jameskuyper@verizon.net> - 2015-07-24 22:21 -0400
              Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 02:28 -0700
                Re: Handling numeric types in interpreter Bartc <bc@freeuk.com> - 2015-07-25 15:16 +0100
                Re: Handling numeric types in interpreter Richard Damon <Richard@Damon-Family.org> - 2015-07-25 11:03 -0400
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-25 10:05 -0700
              Re: Handling numeric types in interpreter gazelle@shell.xmission.com (Kenny McCormack) - 2015-07-25 12:32 +0000
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 06:12 -0700
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-25 10:08 -0700
                Re: Handling numeric types in interpreter ais523 <ais523@nethack4.org> - 2015-07-28 17:50 +0000
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-28 11:52 -0700
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-28 12:25 -0700
                Re: Handling numeric types in interpreter Richard Heathfield <rjh@cpax.org.uk> - 2015-07-28 20:52 +0100
                Re: Handling numeric types in interpreter jacobnavia <jacob@jacob.remcomp.fr> - 2015-07-28 23:01 +0200
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-28 14:44 -0700
                Re: Handling numeric types in interpreter Ian Collins <ian-news@hotmail.com> - 2015-07-29 09:43 +1200
                Re: Handling numeric types in interpreter glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-28 23:10 +0000
                Re: Handling numeric types in interpreter Ian Collins <ian-news@hotmail.com> - 2015-07-29 12:04 +1200
                Re: Handling numeric types in interpreter Melzzzzz <mel@zzzzz.com> - 2015-07-29 02:39 +0200
                Re: Handling numeric types in interpreter jacobnavia <jacob@jacob.remcomp.fr> - 2015-07-29 12:04 +0200
                Re: Handling numeric types in interpreter glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-29 14:58 +0000
                Re: Handling numeric types in interpreter jacobnavia <jacob@jacob.remcomp.fr> - 2015-07-29 14:14 +0200
                Re: Handling numeric types in interpreter Bartc <bc@freeuk.com> - 2015-07-29 13:29 +0100
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-29 08:48 -0700
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-29 08:45 -0700
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-28 16:28 -0700
                Re: Handling numeric types in interpreter jacobnavia <jacob@jacob.remcomp.fr> - 2015-07-29 12:02 +0200
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-29 08:51 -0700
                Re: Handling numeric types in interpreter jacobnavia <jacob@jacob.remcomp.fr> - 2015-07-29 18:35 +0200
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-29 10:47 -0700
                Re: Handling numeric types in interpreter Robert Wessel <robertwessel2@yahoo.com> - 2015-07-25 15:46 -0500
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 14:08 -0700
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-25 15:42 -0700
                Re: Handling numeric types in interpreter Robert Wessel <robertwessel2@yahoo.com> - 2015-07-26 00:31 -0500
                Re: Handling numeric types in interpreter James Kuyper <jameskuyper@verizon.net> - 2015-07-25 17:48 -0400
                Re: Handling numeric types in interpreter Robert Wessel <robertwessel2@yahoo.com> - 2015-07-25 17:31 -0500
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 15:57 -0700
                Re: Handling numeric types in interpreter Bartc <bc@freeuk.com> - 2015-07-26 01:07 +0100
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 17:26 -0700
                Re: Handling numeric types in interpreter Richard Damon <Richard@Damon-Family.org> - 2015-07-25 22:25 -0400
                Re: Handling numeric types in interpreter James Kuyper <jameskuyper@verizon.net> - 2015-07-25 19:04 -0400
                Re: Handling numeric types in interpreter Robert Wessel <robertwessel2@yahoo.com> - 2015-07-26 00:23 -0500
                Re: Handling numeric types in interpreter James Kuyper <jameskuyper@verizon.net> - 2015-07-26 08:24 -0400
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-26 06:36 -0700
                Re: Handling numeric types in interpreter Robert Wessel <robertwessel2@yahoo.com> - 2015-07-26 16:30 -0500
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-26 15:42 -0700
                Re: Handling numeric types in interpreter Robert Wessel <robertwessel2@yahoo.com> - 2015-07-26 16:28 -0500
                Re: Handling numeric types in interpreter James Kuyper <jameskuyper@verizon.net> - 2015-07-26 18:57 -0400
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-26 16:20 -0700
                Re: Handling numeric types in interpreter Keith Thompson <kst-u@mib.org> - 2015-07-25 15:58 -0700
              Re: Handling numeric types in interpreter James Kuyper <jameskuyper@verizon.net> - 2015-07-25 12:56 -0400
                Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 14:18 -0700
      Re: Handling numeric types in interpreter glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-24 18:06 +0000
        Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-24 11:16 -0700
          Re: Handling numeric types in interpreter glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-24 23:06 +0000
            Re: Handling numeric types in interpreter Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-24 16:21 -0700
  Re: Handling numeric types in interpreter Michael Angelo Ravera <maravera@prodigy.net> - 2015-07-15 19:13 -0700
    Re: Handling numeric types in interpreter luser droog <luser.droog@gmail.com> - 2015-07-23 08:36 -0700

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