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From: Bonita Montero <Bonita.Montero@gmail.com>
Newsgroups: comp.lang.c,comp.lang.c++
Subject: Re: Can someone please verify the execution trace of this?
Date: Sun, 19 May 2024 20:08:46 +0200
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Am 18.05.2024 um 23:40 schrieb olcott:
> People are saying that they have no idea what this code does
> because they do not believe it conforms to c11 or c17.
> 
> typedef int (*ptr)();  // ptr is pointer to int function
> 00 int H(ptr x, ptr y);
> 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 }
> 
> 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);
> 
> *keeps repeating* (unless aborted)
> Line 01:
> Line 02:
> Line 03: simulated D(D) invokes simulated H(D,D) that simulates D(D)
> 
> *Simulation invariant*
> D correctly simulated by H cannot possibly reach past its own line 03.
> 
> The key thing to note is that no D correctly simulated by any H of every
> H/D pair specified by the above template ever reaches its own line 06
> and halts.
> 

Other people think 30s about this, you think years about that.