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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.lang.c
Subject: Re: Can you see that D correctly simulated by H remains stuck in recursive simulation?
Date: Sun, 26 May 2024 11:05:47 +0300
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On 2024-05-25 11:42:59 +0000, olcott said:

> On 5/25/2024 4:59 AM, Mikko wrote:
>> On 2024-05-24 16:57:36 +0000, olcott said:
>> 
>>> On 5/24/2024 10:01 AM, Fred. Zwarts wrote:
>>>> Op 23.mei.2024 om 18:52 schreef olcott:
>>>>> 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.
>>>> 
>>>> The case can be simplified even more (D is not needed):
>>>> 
>>> 
>>> We are ONLY asking about whether D correctly simulated by pure function 
>>> H can possibly reach its own final state at line 06 and halt.
>>> 
>>> Because H is a pure function we know that H halts.
>> 
>> We don't know that H halts. The OP said the opposite:
>> 
> 
> The above references *pure function* H thus we do know
> that the spec *requires* H to halt.
> https://en.wikipedia.org/wiki/Pure_function

In that case the spec requires halting, though OP was not clar about that.

>> On 2024-05-23 16:52:21 +0000, olcott said:
>>> *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.

Here OP is clear: H does not halt when both arguments are D.

Therefore H does not conform to the spec.

>> To repeat in endless recursve simulation makes halting impossible.
>> Apparently OP's interpretation of "pure function" does not imply
>> halting.
>> 
> 
> Every H is required to halt and return a value.
> To make things simple every H returns the meaningless 56.
> It is endless recursive simulation until H halts.

No, it isn't. "endless recursive simulation" means that H does not halt.
Thereis no "unless" in the OP.

-- 
Mikko