Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: olcott <polcott333@gmail.com>
Newsgroups: comp.lang.c,comp.lang.c++
Subject: Can someone please verify the execution trace of this?
Date: Sat, 18 May 2024 16:40:45 -0500
Organization: A noiseless patient Spider
Lines: 45
Message-ID: <v2b78t$2vima$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Sat, 18 May 2024 23:40:46 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="95afb1fc0a4871125108def5044e156a";
	logging-data="3132106"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/4AUIE7UDSRR0nbGU1avMd"
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:Md6s5OEybS4c+dCAk8+TSpVxfSU=
Content-Language: en-US
Bytes: 2225

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.

-- 
Copyright 2024 Olcott "Talent hits a target no one else can hit; Genius
hits a target no one else can see." Arthur Schopenhauer