Re: the language (and the role of Undefined Behavior therein)

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: the language (and the role of Undefined Behavior therein)
Date	2015-08-25 12:33 -0700
Organization	None to speak of
Message-ID	<lnd1ybxix9.fsf@kst-u.example.com> (permalink)
References	(8 earlier) <lna8thz3lo.fsf@kst-u.example.com> <d1225caa-a905-4e5c-b318-0fc3a4c7df08@googlegroups.com> <mrgjdt$k9t$1@dont-email.me> <mrhu7e$dq8$1@speranza.aioe.org> <845e7b1a-6704-4512-a635-657e7924b7dc@googlegroups.com>

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supercat@casperkitty.com writes:
> On Tuesday, August 25, 2015 at 9:31:26 AM UTC-5, glen herrmannsfeldt wrote:
>> There are pretty much two ways to make a ones complement adder.
>> One compares -0 equal to +0, the other never generates -0 when
>> the inputs are never -0. This allows for the trap representation,
>> and doesn't complicate the ALU when comparing zeros.
>
> It's also worth noting that a two's-complement C compiler could also
> make -INT_MAX-1 be a trap representation, and in some cases might save
> a little code by defining INT_MIN to be -MAX_INT rather than
> -INT_MAX-1.

(Just to mention, you wrote "MAX_INT" rather than "INT_MAX").

Note that making -INT_MAX-1 a trap representation doesn't mean that any
operation that yields that value has to "trap".  It merely means that
such an operation has undefined behavior.

An implementation with 32-bit 2's-complement signed int could have:

    #define INT_MIN (-2147483647)
    #define INT_MAX   2147483647

and then document that -2147483648 is a trap represenation, and
make no other changes.  If printf("%d\n", INT_MIN-1) happens to
"work", that's just one possible consequence of undefined behavior.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-18 16:19 -0700
  Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-18 21:15 -0400
    Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 10:11 -0700
      Re: the language (and the role of Undefined Behavior therein) Ike Naar <ike@iceland.freeshell.org> - 2015-08-19 19:58 +0000
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 14:09 -0700
      Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-19 16:10 -0400
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-19 16:01 -0700
          Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-19 20:23 -0400
            Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 10:35 -0700
              Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-20 12:32 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 13:31 -0700
                Re: the language (and the role of Undefined Behavior therein) donalhambra@gmail.com - 2015-08-20 14:23 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 16:13 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-20 17:48 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-21 01:32 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-21 18:34 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-21 22:11 -0700
                Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-22 09:41 -0400
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-22 16:59 +0000
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-22 23:30 -0400
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 03:36 +0000
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 00:25 -0400
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 22:57 -0700
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 19:49 +0000
                Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 14:43 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 16:27 -0700
                Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 21:15 -0400
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 20:18 -0700
                Re: the language (and the role of Undefined Behavior therein) raltbos@xs4all.nl (Richard Bos) - 2015-08-22 14:37 +0000
                Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-22 19:12 -0700
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-22 23:38 -0400
                Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-23 20:01 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-23 21:56 -0700
                Re: the language (and the role of Undefined Behavior therein) David Kleinecke <dkleinecke@gmail.com> - 2015-08-24 17:25 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-24 18:21 -0700
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 22:22 -0400
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-25 14:31 +0000
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-25 09:22 -0700
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-25 18:55 +0000
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-25 12:33 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-25 16:23 -0700
                Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-24 00:31 -0400
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-08-24 10:32 -0700
                Re: the language (and the role of Undefined Behavior therein) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-22 23:54 -0700
                Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-24 03:45 +0000
                Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 08:11 -0700
      Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 18:46 -0400
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 19:04 -0700
          Re: the language (and the role of Undefined Behavior therein) Richard Damon <Richard@Damon-Family.org> - 2015-08-23 23:40 -0400
            Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-23 22:48 -0700
              Re: the language (and the role of Undefined Behavior therein) Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-24 00:38 -0700
  Re: the language (and the role of Undefined Behavior therein) James Kuyper <jameskuyper@verizon.net> - 2015-08-18 21:22 -0400
  Re: the language (and the role of Undefined Behavior therein) Ike Naar <ike@iceland.freeshell.org> - 2015-08-20 21:44 +0000
    Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-08-20 16:48 -0700
    Re: the language (and the role of Undefined Behavior therein) Philip Lantz <prl@canterey.us> - 2015-08-20 23:41 -0700

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