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Re: Does this program have the specified behavior?

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On 12/10/21 4:40 AM, Tim Rentsch wrote:
> Mark Bluemel <mark.bluemel@gmail.com> writes:
> 
>> On Wednesday, 17 November 2021 at 05:13:08 UTC, Chris M. Thomasson wrote:
>>
>>> On 11/16/2021 8:46 PM, olcott wrote:
>>>
>>>> #include <stdint.h>
>>>> #include <stdio.h>
>>>> typedef int (*ptr)();
>>>>
>>>> int H(ptr x, ptr y)
>>>> {
>>>>    x(y); // direct execution of P(P)
>>>>    return 1;
>>>> }
>>>>
>>>> int P(ptr x)
>>>> {
>>>>    H(x, x);
>>>>    return 1;
>>>> }
>>>>
>>>> int main(void)
>>>> {
>>>>    H(P, P);
>>>> }
>>>
>>> Stack overflow...
>>
>> Surely a compiler can optimise this to remove the recursion, and
>> for that matter, remove any functionality at all, as the code has
>> no observable behaviour, as far as I can see (I'd be happy to be
>> corrected on this!).
> 
> A program that simply does a 'return 0;' from main() does have
> observable behavior:  it modifies no files (and doesn't perform
> any volatile accesses or do any I/O to interactive devices).  The
> empty set is still a set.
> 
> By contrast, a program that sits and does nothing and simply runs
> forever does /not/ have observable behavior.  The reason is
> simple:  the predicate condition for what happens with files is
> evaluated only at program termination.  No termination means no
> way to assess observable behavior.
> 
> The question of whether the C standard allows (or intends to
> allow) the program transformation you describe is a thorny
> problem, and not one I'm planning on researching further.  But
> considering just "observable behavior" in the two cases, ISTM
> that the two programs clearly are not identical, because one
> does have observable behavior and the other one does not.
> 

I think the key point is that the infinite recursion means the program 
exceeds the environment bounds for defined operation and thus the 
behavior of the program has become officially 'Undefined Behavior'. Once 
the compiler can see that this will happen, it is freed of all restrictions.

Any observable behavior that would happen before the program enters 
Undefined Behavior needs to happen, but there is none, so the compiler, 
if it is smart enough to recognize the behavior, is totally free to do 
what ever it wants with the program.

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Do this program have the specified behavior? olcott <NoOne@NoWhere.com> - 2021-11-16 21:33 -0600
  Does this program have the specified behavior? olcott <NoOne@NoWhere.com> - 2021-11-16 22:46 -0600
    Re: Does this program have the specified behavior? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-11-16 21:12 -0800
      Re: Does this program have the specified behavior? Mark Bluemel <mark.bluemel@gmail.com> - 2021-11-17 00:28 -0800
        Re: Does this program have the specified behavior? olcott <NoOne@NoWhere.com> - 2021-11-17 08:33 -0600
        Re: Does this program have the specified behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-10 01:40 -0800
          Re: Does this program have the specified behavior? Richard Damon <Richard@Damon-Family.org> - 2021-12-10 07:56 -0500
            Re: Does this program have the specified behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-11 09:40 -0800
              Re: Does this program have the specified behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-11 16:37 -0800
                Re: Does this program have the specified behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-12 09:29 -0800
          Re: Does this program have the specified behavior? antispam@math.uni.wroc.pl - 2021-12-11 19:38 +0000
            Re: Does this program have the specified behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-11 16:45 -0800
            Re: Does this program have the specified behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-12 08:53 -0800
    Re: Does this program have the specified behavior? Richard Damon <Richard@Damon-Family.org> - 2021-11-17 06:53 -0500
      Re: Does this program have the specified behavior? olcott <NoOne@NoWhere.com> - 2021-11-17 08:49 -0600
        Re: Does this program have the specified behavior? Juha Nieminen <nospam@thanks.invalid> - 2021-11-18 06:32 +0000
          Re: Does this program have the specified behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-18 11:15 +0000
            Re: Does this program have the specified behavior? olcott <NoOne@NoWhere.com> - 2021-11-18 08:59 -0600
          Re: Does this program have the specified behavior? olcott <NoOne@NoWhere.com> - 2021-11-18 08:58 -0600
    Re: Does this program have the specified behavior? Anurag Ranjan <anuragranjan630@gmail.com> - 2021-11-17 22:00 +0000

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