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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.lang.c
Subject: Re: D correctly simulated by pure function H cannot possibly reach its, own line 06
Date: Sun, 26 May 2024 19:04:42 +0300
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On 2024-05-26 13:12:08 +0000, olcott said:

> On 5/26/2024 3:05 AM, Mikko wrote:
>> 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.
>> 
> 
> I had to make the subject line of the thread a little less accurate
> so that people could get the gist of what I was saying. Now I have
> made it precise.

Subject lines typically are as they are limited to one line. That's
not a problem.

-- 
Mikko