Re: Performance of signed vs unsigned types

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Performance of signed vs unsigned types
Date	2011-04-23 18:14 -0400
Organization	A noiseless patient Spider
Message-ID	<iovj0t$tv5$1@dont-email.me> (permalink)
References	(12 earlier) <rubrum-5A668D.14224322042011@news.albasani.net> <iostlt$6u4$1@dont-email.me> <rubrum-B7BCA0.17334522042011@news.albasani.net> <iouijs$5ut$1@dont-email.me> <rubrum-3E4135.14075423042011@news.albasani.net>

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On 04/23/2011 05:07 PM, Michael Press wrote:
> In article <iouijs$5ut$1@dont-email.me>,
>  James Kuyper <jameskuyper@verizon.net> wrote:
> 
>> The alternative is not simply having your program blow up. Other,
>> subtler possibilities allowed by the fact that the behavior of your
>> program is undefined include:
>> 1. (y = x+step) > x might return true, even though y is actually less
>> than x.
>> 2. (y = 2*step) > step might return true, even though y is actually less
>> than step; in particular, y might be negative, causing step to become
>> negative.
>> 3. step might also, arbitrary, become negative for no particularly good
>> reason other than the fact that the behavior is undefined.
>> 4. The lines of your program might start executing in an arbitrary
>> order, unrelated to the order that's required when the behavior is
>> actually defined by the standard.
> 
> Who benefits from having these implementations available?

Those were mere examples of possibilities, and I can't claim to know of
any specific benefit associated any of those particular possibilities.

However, the committee decided to leave the behavior on overflow
undefined, by failing to define it. It was not an accidental omission.
It is in fact cited, in the standard itself, as the prime example of how
behavior can be undefined by reason of a lack of an applicable
definition. The committee could, instead, have stated that the result on
overflow is unspecified, in which case your program would be guaranteed
to work perfectly. The fact that they chose to leave the behavior
undefined strongly suggests that they knew of particular contexts in
which it would have been difficult to implement C, if the integer
overflow was only allowed to generate an unspecified result.

In general, not having to worry about making the compiled code work
properly when there is an overflow (for any particular definition of
properly), removes a barrier to producing better optimization. I have no
idea whether there's any worthwhile optimizations on the other side of
that barrier, but I wouldn't recommend betting against the possibility
that there are.
-- 
James Kuyper

Thread

Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 14:21 +0100
  Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-20 09:49 -0400
    Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 15:21 +0100
    Re: Performance of signed vs unsigned types Peter Nilsson <airia@acay.com.au> - 2011-04-20 15:34 -0700
      Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-21 17:57 +0100
        Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-21 19:26 +0100
    Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-20 21:45 -0700
      Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-21 06:20 -0400
        Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-21 13:54 -0700
          Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-21 18:50 -0400
            Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-21 16:16 -0700
              Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-21 19:27 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-21 17:24 -0700
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-21 21:00 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-21 21:33 -0700
                Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-22 08:22 +0100
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-22 14:25 -0700
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-22 09:21 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-22 14:22 -0700
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-22 17:58 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-22 17:33 -0700
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-23 09:01 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-23 14:07 -0700
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-23 18:14 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 11:42 -0700
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-25 22:34 -0400
                Re: Performance of signed vs unsigned types gordonb.nw63u@burditt.org (Gordon Burditt) - 2011-04-26 10:59 -0500
                Re: Performance of signed vs unsigned types "madamemandm@yahoo.com" <madamemandm@yahoo.com> - 2011-04-22 07:55 -0700
                Re: Performance of signed vs unsigned types Noob <root@127.0.0.1> - 2011-04-22 17:03 +0200
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-22 08:42 -0700
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-22 14:15 -0700
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-22 15:23 -0700
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-22 17:37 -0700
                Re: Performance of signed vs unsigned types Seebs <usenet-nospam@seebs.net> - 2011-04-23 02:46 +0000
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-23 14:04 -0700
                Re: Performance of signed vs unsigned types Barry Schwarz <schwarzb@dqel.com> - 2011-04-23 14:28 -0700
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 11:43 -0700
                Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-23 23:38 +0100
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 11:46 -0700
                Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-25 22:43 +0100
                Re: Performance of signed vs unsigned types Seebs <usenet-nospam@seebs.net> - 2011-04-24 11:34 +0000
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 11:47 -0700
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-25 12:32 -0700
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 20:25 -0700
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-26 07:45 -0700
                Re: Performance of signed vs unsigned types lawrence.jones@siemens.com - 2011-04-24 23:50 -0400
                Re: Performance of signed vs unsigned types Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-04-25 02:13 -0700
                Re: Performance of signed vs unsigned types Ian Collins <ian-news@hotmail.com> - 2011-04-25 21:36 +1200
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 11:48 -0700
                Re: Performance of signed vs unsigned types lawrence.jones@siemens.com - 2011-04-25 16:32 -0400
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-25 15:22 -0700
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 20:32 -0700
                Re: Performance of signed vs unsigned types "io_x" <a@b.c.invalid> - 2011-04-26 09:42 +0200
                Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 12:04 +0100
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-26 08:15 -0700
                Re: Performance of signed vs unsigned types Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-04-26 08:35 -0700
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-26 08:52 -0700
                Re: Performance of signed vs unsigned types Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-04-26 09:15 -0700
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-26 10:22 -0700
                Re: Performance of signed vs unsigned types gordonb.s68qo@burditt.org (Gordon Burditt) - 2011-04-26 19:59 -0500
                Re: Performance of signed vs unsigned types "madamemandm@yahoo.com" <madamemandm@yahoo.com> - 2011-04-27 14:15 -0700
                Re: Performance of signed vs unsigned types "io_x" <a@b.c.invalid> - 2011-04-28 12:15 +0200
                Re: Performance of signed vs unsigned types Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-04-28 03:26 -0700
                Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-28 12:09 +0100
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-28 08:03 -0400
                Re: Performance of signed vs unsigned types Seebs <usenet-nospam@seebs.net> - 2011-04-28 16:44 +0000
                Re: Performance of signed vs unsigned types Gordon Burditt <gordon@hammy.burditt.org> - 2011-04-29 05:42 -0500
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 20:27 -0700
                Re: Performance of signed vs unsigned types Ian Collins <ian-news@hotmail.com> - 2011-04-26 15:30 +1200
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-26 00:14 -0700
                Re: Performance of signed vs unsigned types Ian Collins <ian-news@hotmail.com> - 2011-04-26 20:23 +1200
                Re: Performance of signed vs unsigned types Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-04-26 04:17 -0700
                Re: Performance of signed vs unsigned types Ike Naar <ike@sverige.freeshell.org> - 2011-04-22 06:46 +0000
                Re: Performance of signed vs unsigned types James Kuyper <jameskuyper@verizon.net> - 2011-04-22 09:37 -0400
                Re: Performance of signed vs unsigned types Michael Press <rubrum@pacbell.net> - 2011-04-25 11:49 -0700
              Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-22 09:09 -0700
                Re: Performance of signed vs unsigned types "madamemandm@yahoo.com" <madamemandm@yahoo.com> - 2011-04-22 09:46 -0700
                Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-22 20:21 +0100
  Re: Performance of signed vs unsigned types "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2011-04-20 10:06 -0400
    Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 15:28 +0100
      Re: Performance of signed vs unsigned types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-20 16:11 +0100
      Re: Performance of signed vs unsigned types Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-04-20 08:14 -0700
        Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 16:24 +0100
          Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 17:09 +0100
            Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 17:19 +0100
              Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-20 12:09 -0700
                Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 21:39 +0100
                Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-20 15:01 -0700
                Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-21 11:16 +0100
      Re: Performance of signed vs unsigned types "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2011-04-20 13:06 -0400
        Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-20 18:16 +0100
  Re: Performance of signed vs unsigned types gordonb.bfd7y@burditt.org (Gordon Burditt) - 2011-04-21 11:22 -0500
    Re: Performance of signed vs unsigned types Thomas Scrace <tom@scrace.org> - 2011-04-21 19:27 +0100
      Re: Performance of signed vs unsigned types Keith Thompson <kst-u@mib.org> - 2011-04-21 12:09 -0700

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