#include <bits/stdc++.h>

uint64_t stack[10000];

uint64_t FibCustomStack(int initial_n)
{
	uintptr_t stacktop = reinterpret_cast<uintptr_t>(stack);
	uint64_t ret;
	int n;
	
	*reinterpret_cast<void**>(stacktop) = &&end;
	*reinterpret_cast<int*>(stacktop + 8) = initial_n;
	goto entry;
	
end:
	return ret;

entry:
	n = *reinterpret_cast<int*>(stacktop + 8);
	if (n <= 2)
	{
		ret = 1;
		goto *reinterpret_cast<void**>(stacktop);
	}
	
	// This file demonstrates a GCC bug (for v9.3.0 at least)
	//
	// if you keep the things below, the executable crashes 
	// despite the code below is not even executed (since initial_n == 2)
	// (and you can confirm by disassembly that gcc generated faulty code)
	// 
	// however, if you remove everything below, 
	// the executable runs successfully and prints out the correct output of 1
	// This happens even if you compile with -O0 -fno-strict-aliasing 
	// (so the potential strict-alias-related UB due to reinterpret_casts is not the reason)
	//
	stacktop += 24;
	*reinterpret_cast<void**>(stacktop) = &&after_call1;
	*reinterpret_cast<int*>(stacktop + 8) = n - 1;
	goto entry;
after_call1:	
	stacktop -= 24;
	*reinterpret_cast<uint64_t*>(stacktop + 16) = ret;
	
	stacktop += 24;
	*reinterpret_cast<void**>(stacktop) = &&after_call2;
	*reinterpret_cast<int*>(stacktop + 8) = n - 2;
	goto entry;
after_call2:
	stacktop -= 24;
	ret += *reinterpret_cast<uint64_t*>(stacktop + 16);
	goto *reinterpret_cast<void**>(stacktop);
}

int main()
{
	uint64_t res = FibCustomStack(2);
	printf("%llu\n", static_cast<unsigned long long>(res));
	return 0;
}