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	2011-04-22 09:21 -0400
Organization	A noiseless patient Spider
Message-ID	<iorvdl$pg6$1@dont-email.me> (permalink)
References	(6 earlier) <rubrum-49C519.16165221042011@news.albasani.net> <ioqegk$gvf$1@dont-email.me> <rubrum-B00AB5.17242621042011@news.albasani.net> <ioqjvt$e1$1@dont-email.me> <rubrum-F39599.21330521042011@news.albasani.net>

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On 04/22/2011 12:33 AM, Michael Press wrote:
> In article <ioqjvt$e1$1@dont-email.me>,
>  James Kuyper <jameskuyper@verizon.net> wrote:
> 
>> On 04/21/2011 08:24 PM, Michael Press wrote:

...
>> It was not my intention to suggest any certainty about why you thought
>> that code should work. I said as much, in words you chose to clip:
>>> However, I suppose that's not the only assumption that would make it work.
> 
> Since I never wrote something such as if( x < 0), there are no grounds
> for assuming I wrote about negatives or overflow. 

Yes, there were grounds for assuming that you were writing about
overflow, and more than adequate ones: your code inherently produces an
overflow. I assumed that you were aware of that fact and were, in fact
relying upon the behavior that you expected to occur when it overflowed.
If you wrote this code without realizing that overflow would occur, I
apologize for making unwarranted assumptions about your competence.

I never claimed that you wrote anything about x<0. On most
implementations I'm familiar with, x will remain positive throughout the
run of that program. It's y which will end up negative as soon as the
relevant expression overflows on those implementations, though that's
not guaranteed by the standard.

...
>> assumption. Since you seem offended by the idea that you were relying on
>> that particular assumption, I'll happily retract my guesses as to what
>> you were thinking when you wrote that code. However, that leaves me with
>> no idea why you thought it would work. Would you care to explain what
>> assumptions you were making about the results of signed integer overflow
>> when you wrote that code?
>>
>> It did work, on the machines where I tested it, and it worked precisely
>> because x+step did produce a negative value.
> 
> Here is a list of assumptions. They are as complete and consistent 
> as I could make them in the time spent; but very likely reducible.
> 
> Assumptions.

The assumptions that you list seem very familiar. About the only things
you've left out are the existence of an inverse element and
associativity for addition.

However, completeness was unnecessary; a more specific answer would have
been a lot more responsive to my question. What I was really looking for
was some idea of what you expections you had about the result of x+step
when the mathematical result of adding those two number would be greater
than INT_MAX, a situation your program is guaranteed to create.

Let me be a lot more specific, let's consider the case where x+step is
about to be calculated with x and step both equal to the largest power
of 2 less than or equal to INT_MAX, a situation your program is
guaranteed to create. Do you think that there are any restrictions, of
any kind, on the value of x+step at that point? If so, what do you think
those restrictions are?

Typical 2's complement implementations of 'int' obey all of those
assumptions, but the C standard does not mandate them. In particular,
none of the following assumptions is required to be true for 'int'
arithmetic :

> * + is commutative
> * * is commutative
> * Multiplication is  distributive over addition.
...
> * Addition is repeated addition of 1.
>   - That is if 0 < b, then there is a positive integer (not int!), n,  such that
>     b = 1 + 1 + ... + 1 (n times).
>   - If 0 < a + b, then a + b = a + [1 + 1 + 1 + ... + 1 (n times)].
> * Multiplication is repeated addition.

In every case where a+b or a*b overflows, the behavior is undefined
according to the C standard, which means that the results of those
operations are allowed to violate each and every one of the above
assumptions. C does not guarantee that any of the following are true:

	INT_MAX + 1 == 1 + INT_MAX
	INT_MIN * 2 == 2 * INT_MAX
	INT_MAX*(1+2) == INT_MAX*1 + INT_MAX*2
	INT_MAX + 1 + 1 == INT_MAX + (1 + 1)
	INT_MIN * 2 == INT_MIN + INT_MIN

I make explicit use of INT_MAX and INT_MIN only in order to portably
create values for which the relevant expressions are guaranteed to
overflow; the key issue is the overflow itself, not the macros I relied
upon to ensure it.

A fully conforming implementation could return, as the result for any
int expression that overflows, the current value stored in register 7,
which might happen to contain the machine address of 'x'. I suppose that
your program could produce the expected result on such an
implementation, but it's pretty unlikely to do so.
-- 
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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