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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.theory
Subject: Re: Every D(D) simulated by H presents non-halting behavior to H ###
Date: Tue, 21 May 2024 13:06:21 +0300
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On 2024-05-20 17:56:54 +0000, olcott said:

> On 5/20/2024 3:21 AM, Mikko wrote:
>> On 2024-05-19 14:03:01 +0000, olcott said:
>> 
>>> On 5/19/2024 8:48 AM, Mikko wrote:
>>>> On 2024-05-19 12:34:08 +0000, olcott said:
>>>> 
>>>>> On 5/19/2024 2:53 AM, Mikko wrote:
>>>>>> On 2024-05-18 15:34:36 +0000, James Kuyper said:
>>>>>> 
>>>>>>> On 5/18/24 09:02, Mikko wrote:
>>>>>>>> On 2024-05-17 17:14:01 +0000, olcott said:
>>>>>>> 
>>>>>>> I recommend ignoring olcott - nothing good ever comes from paying
>>>>>>> attention to him.
>>>>>>> 
>>>>>>>>> On 5/17/2024 5:53 AM, Mikko wrote:
>>>>>>>>>> On 2024-05-16 14:50:19 +0000, olcott said:
>>>>>>>>>> 
>>>>>>>>>>> On 5/16/2024 5:48 AM, Mikko wrote:
>>>>>>>>>>>> On 2024-05-15 15:24:57 +0000, olcott said:
>>>>>>> ...
>>>>>>>>>>>>> typedef int (*ptr)();  // ptr is pointer to int function
>>>>>>>>>>>>> 00 int H(ptr x, ptr x);
>>>>>>>>>>>>> 01 int D(ptr x)
>>>>>>>>>>>>> 02 {
>>>>>>>>>>>>> 03   int Halt_Status = H(x, x);
>>>>>>>>>>>>> 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 }
>>>>>>>>>>>> 
>>>>>>>>>>>> Can you find any compiler that is liberal enough to accept that?
>>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> It has been fully operational code under Windows and
>>>>>>>>>>> Linux for two years.
>>>>>>>>>> 
>>>>>>>>>> If your compiler does not reject that program it is not a conforming
>>>>>>>>>> C compiler. The semantics according to C standard is that a diagnostic
>>>>>>>>>> message must be given. The standard does not specify what happens if
>>>>>>>>>> you execute that program anyway.
>>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> It is not nit picky syntax that is the issue here.
>>>>>>>>> The SEMANTICS OF THE C PROGRAMMING LANGUAGE SPECIFIES
>>>>>>>>> 
>>>>>>>>> No D simulated correctly by any H of every H/D pair specified
>>>>>>>>> by the above template ever reaches its own line 06 and halts.
>>>>>>>> 
>>>>>>>> The standard allows that an program is executed but does not
>>>>>>>> specify what happens when an invalid program is executed.
>>>>>>> 
>>>>>>> You've cross-posted this to comp.lang.c after a long-running discussion
>>>>>>> solely on comp.theory. Presumably you're doing that because you want
>>>>>>> some discussion about what the standard says about this code. For the
>>>>>>> sake of those of us who have not been following that discussion on
>>>>>>> comp.theory, could you please identify what it is that you think renders
>>>>>>> this code invalid? Offhand, I don't see anything wrong with it, but I'm
>>>>>>> far more reliable when I say "I see an error" than when I say "I don't
>>>>>>> see an error".
>>>>>>> 
>>>>>>> 
>>>>>>>>> Fully operational software that runs under Widows and Linux
>>>>>>>>> proves that the above is true EMPIRICALLY.
>>>>>>>> 
>>>>>>>> No, it does not. As the program is not strictly comforming
>>>>>>>> and uses a non-standard extension some implementation may
>>>>>>>> execute it differently or refuse to execute.
>>>>>>> 
>>>>>>> Which non-standard extension does it use?
>>>>>> 
>>>>>> The main question is whether both arguments of H on the line 00 can have
>>>>>> the same name.
>>>>> 
>>>>> That was a typo that I did not believe when told because so may people
>>>>> continue to lie about the behavior of D correctly simulated by H.
>>>> 
>>>> How does the D that is correctly simulated by H different from any
>>>> D that is incorrectly simulated by H nor not simulated by H?
>> 
>> Oops, I made a typo on the last line. Pro "nor" lege "or".
>> Fortunately most of the typos are harmless but this one
>> might be a problem.
>> 
>>> typedef int (*ptr)();  // ptr is pointer to int function
>>> 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 }
>>> 
>>> 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.
>> 
>> As far as I can see, that does not say anything that was not already
>> said (but there is a minor presentational imporvement) and in particular
>> does not answer my question.
>> 
> 
> For two or three years everyone has been claiming that the
> above measures of correct simulation are incorrect. Their
> "ultimate" measure of a "correct" simulation is that D does
> whatever they expect D to do.

I don't think I can meaningfully coomment that before my question
is asnwered.

-- 
Mikko