Re: C/C++ pitfalls related to 64-bits (unsigned long & double)

From	Miles Bader <miles@gnu.org>
Newsgroups	comp.lang.c, comp.lang.c++
Subject	Re: C/C++ pitfalls related to 64-bits (unsigned long & double)
Date	2012-02-14 13:25 +0900
Organization	NEC Electronics
Message-ID	<buod39i59dt.fsf@dhlpc061.dev.necel.com> (permalink)
References	(8 earlier) <jhc9n6$8ni$1@news.albasani.net> <0.a4992fde206ad25190a0.20120214014547GMT.8739ae427o.fsf@bsb.me.uk> <jhcf13$6jj$1@news.albasani.net> <87obt23z6c.fsf@catnip.gol.com> <jhcl3k$huq$1@news.albasani.net>

Cross-posted to 2 groups.

Show all headers | View raw

BGB <cr88192@hotmail.com> writes:
>> Integer arithmetic with doubles _is_ exact though, if the integers can
>> be exactly-represented as doubles (which appears to be the case here).
>
> yes.
>
> the issue may be partly a matter of HW though, as I am seeing it on my
> HW (using an AMD chip), but apparently someone else is not seeing it
> (with an Intel chip), but there does seem to be a pattern in the values
> (apparently: 0x4, 0x40, 0x400, 0x4000, ..., so for whatever reason
> integer results which should land on one of these values is off by a
> tiny amount...).

Any conventional PC-type system these days is going to use IEEE FP,
and if the system claims to support IEEE FP, it has to be exact.  If
it isn't, it's a bug.

I did run your program on my AMD system (phenom I), and it showed no
output.  It would be interesting to see somebody with an identical CPU
to yours try it...

> it could be a minor issue of the "arithmetic with doubles may not be
> exact even if the doubles represent integers" variety, which is odd, but
> whatever (it can be compensated for by fudging the value).

I'm not sure you could call it a minor issue.  A lot of software
assumes that FP arithmetic is exact for integer values within a
certain range, and isn't going to do any fudging (because it shouldn't
be necessary, and would have a severe performance impact), so such a
system where fudging is necessary would have ... problems.

-miles

-- 
Clarionet, n. An instrument of torture operated by a person with cotton in his
ears. There are two instruments that are worse than a clarionet -- two
clarionets.

Thread

C/C++ pitfalls related to 64-bits (unsigned long & double) Alex Vinokur <alex.vinokur@gmail.com> - 2012-02-13 04:41 -0800
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Leigh Johnston <leigh@i42.co.uk> - 2012-02-13 12:49 +0000
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Alex Vinokur <alex.vinokur@gmail.com> - 2012-02-13 04:59 -0800
      Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Leigh Johnston <leigh@i42.co.uk> - 2012-02-13 13:07 +0000
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Goran <goran.pusic@gmail.com> - 2012-02-13 04:54 -0800
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Alex Vinokur <alex.vinokur@gmail.com> - 2012-02-13 05:00 -0800
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-13 13:27 +0000
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Goran <goran.pusic@gmail.com> - 2012-02-13 07:14 -0800
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-13 08:14 -0500
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-13 08:17 -0500
      Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-13 21:52 +0000
        Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-13 17:08 -0500
        Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-13 20:44 -0500
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-13 13:16 +0000
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 11:18 -0700
      Re: C/C++ pitfalls related to 64-bits (unsigned long & double) "MikeWhy" <boat042-nospam@yahoo.com> - 2012-02-13 15:18 -0600
        Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-13 16:26 -0500
          Re: C/C++ pitfalls related to 64-bits (unsigned long & double) "MikeWhy" <boat042-nospam@yahoo.com> - 2012-02-13 16:05 -0600
            Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-13 17:13 -0500
              Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-13 23:35 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 17:25 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-14 01:45 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 18:55 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Miles Bader <miles@gnu.org> - 2012-02-14 11:51 +0900
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-14 03:13 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 20:39 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Miles Bader <miles@gnu.org> - 2012-02-14 13:25 +0900
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 23:22 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) "MikeWhy" <boat042-nospam@yahoo.com> - 2012-02-14 02:56 -0600
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-14 09:13 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) "MikeWhy" <boat042-nospam@yahoo.com> - 2012-02-14 11:16 -0600
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-14 12:27 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-13 23:04 -0500
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Miles Bader <miles@gnu.org> - 2012-02-14 14:00 +0900
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Robert Wessel <robertwessel2@yahoo.com> - 2012-02-14 00:45 -0600
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-14 14:22 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Robert Wessel <robertwessel2@yahoo.com> - 2012-02-14 13:06 -0600
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-14 12:40 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-13 23:05 -0500
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Miles Bader <miles@gnu.org> - 2012-02-14 14:01 +0900
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Keith Thompson <kst-u@mib.org> - 2012-02-14 03:01 -0800
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Lowell Gilbert <lgusenet@be-well.ilk.org> - 2012-02-14 12:53 -0500
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-14 14:39 -0500
            Re: C/C++ pitfalls related to 64-bits (unsigned long & double) jgk@panix.com (Joe keane) - 2012-02-14 20:45 +0000
              Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-14 22:57 +0000
              Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Keith Thompson <kst-u@mib.org> - 2012-02-14 15:38 -0800
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-15 00:22 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Robert Wessel <robertwessel2@yahoo.com> - 2012-02-15 00:39 -0600
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-15 08:00 +0000
      Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-13 22:00 +0000
        Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 17:04 -0700
          Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-13 16:15 -0800
          Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-14 00:46 +0000
            Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 18:31 -0700
              Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 18:41 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-14 02:33 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 20:20 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-14 03:26 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 20:49 -0700
              Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Robert Wessel <robertwessel2@yahoo.com> - 2012-02-14 00:24 -0600
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-13 23:42 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-14 06:48 -0500
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-14 13:51 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-14 08:42 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-14 19:58 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-14 14:27 +0000
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) BGB <cr88192@hotmail.com> - 2012-02-14 09:10 -0700
                Re: C/C++ pitfalls related to 64-bits (unsigned long & double) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-14 20:02 +0000
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Noob <root@127.0.0.1> - 2012-02-13 15:22 +0100
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) legalize+jeeves@mail.xmission.com (Richard) - 2012-02-13 18:49 +0000
    Re: C/C++ pitfalls related to 64-bits (unsigned long & double) James Kuyper <jameskuyper@verizon.net> - 2012-02-13 13:55 -0500
      Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Juha Nieminen <nospam@thanks.invalid> - 2012-02-13 19:14 +0000
  Re: C/C++ pitfalls related to 64-bits (unsigned long & double) Barry Schwarz <schwarzb@dqel.com> - 2012-02-13 12:14 -0800

csiph-web