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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c++,comp.lang.c
Subject: Re: Can you see that D correctly simulated by H remains stuck in
 recursive simulation?
Date: Fri, 24 May 2024 15:37:29 +0200
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On 24/05/2024 15:08, olcott wrote:
> On 5/24/2024 7:10 AM, Richard Harnden wrote:
>> On 23/05/2024 17:52, olcott wrote:
>>> typedef int (*ptr)();  // ptr is pointer to int function in C
>>> 00       int H(ptr p, ptr i);
>>> 01       int D(ptr p)
>>> 02       {
>>> 03         int Halt_Status = H(p, p);
>>> 04         if (Halt_Status)
>>> 05           HERE: goto HERE;
>>> 06         return Halt_Status;
>>> 07       }
>>> 08
>>> 09       int main()
>>> 10       {
>>> 11         H(D,D);
>>> 12         return 0;
>>> 13       }
>>>
>>> The above template refers to an infinite set of H/D pairs where D is
>>> correctly simulated by pure function H. This was done because many
>>> reviewers used the shell game ploy to endlessly switch which H/D was
>>> being referred to.
>>>
>>> *Correct Simulation Defined*
>>> This is provided because every reviewer had a different notion of
>>> correct simulation that diverges from this notion.
>>>
>>> In the above case a simulator is an x86 emulator that correctly emulates
>>> at least one of the x86 instructions of D in the order specified by the
>>> x86 instructions of D.
>>>
>>> This may include correctly emulating the x86 instructions of H in the
>>> order specified by the x86 instructions of H thus calling H(D,D) in
>>> recursive simulation.
>>>
>>> *Execution Trace*
>>> Line 11: main() invokes H(D,D); H(D,D) simulates lines 01, 02, and 03 of
>>> D. This invokes H(D,D) again to repeat the process in endless recursive
>>> simulation.
>>
>>
>> So, you have: main -> H -> D -> H -> D -> ... -> H -> D until you run 
>> out of stack?
>>
>> No return statement is ever reached.
>> Line 3 never completes.
>> Halt_Status at line 3 never gets a value.
>>
>> </shrug>
>>
>>
> Thanks.
> 
> Proving that D correctly simulated by H never reaches its final
> state at line 06 and halts.

No, it does not.

As Richard says, you have main -> H -> D -> H -> ...

For any finite system, you will run out of stack space.  This is 
undefined behaviour in C.  /Anything/ can happen - including halting, 
returning a halt status of 1, or a halt status of 0, or a not halting, 
or printing out the works of Shakespeare.  Or it could cause the program 
to jump directly to line 6.  Once you hit undefined behaviour, you 
cannot prove /anything/.

> Thus proving that the halting problem's
> counter-example input D would be correctly determined to be non-halting
> by its simulating termination analyzer H.
>