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From: Richard Heathfield <rjh@cpax.org.uk>
Newsgroups: comp.theory
Subject: Re: Formal systems that cannot possibly be incomplete except for
 unknowns and unknowable
Date: Wed, 7 May 2025 23:16:37 +0100
Organization: Fix this later
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On 07/05/2025 22:59, olcott wrote:
> On 5/7/2025 4:52 PM, Richard Heathfield wrote:
>> On 07/05/2025 22:46, olcott wrote:
>>> On 5/7/2025 4:30 PM, Richard Heathfield wrote:
>>
>> <snip>
>>
>>>> If the simulation can't reach code that the directly executed 
>>>> program reaches, then it's not a faithful simulation.
>>>>
>>>
>>> If is was true that it is not a faithful simulation
>>> then you would be able to show exactly what sequence
>>> of instructions would be a faithful simulation.
>>
>> If it were false, you'd be able to chop out the unreachable 
>> code without any adverse effects. Can you?
>>
>> <snip>
>>
> 
> I already know the answer.

Then you already know why your simulation code fails to simulate 
correctly... but of course you /don't/ know, so I'll explain.

Let us postulate a program that contains a function as follows:

void invisible(void)
{
   secret();
}

When directly executed, the program calls invisible(), but when 
simulated, the invisible() call is unreachable.

secret() could do nothing, or it might launch a for(;;); or it 
could catch SIGABRT with a returning handler and call abort(). 
The simulator has no way of knowing which is which.

Therefore a termination analyser that attempts to analyse this 
program but cannot reach all the code is doomed to be unable to 
run a correct simulation and is necessarily reduced to guesswork.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within