• sprucely (31/31) Oct 29 2009 So I have this optimization in which I jump from function to function to...

sprucely <timberdig gmail.com> writes:

So I have this optimization in which I jump from function to function to avoid
the overhead of setting up and tearing down identical stack frames. Now I want
to be able to jump from a member function of one object to a member function of
another object. I accomplished this with g++ by replacing the this pointer that
is passed in as a hidden parameter. After some difficulty with dmd I realized
that the this pointer is actually stored in EAX. But simply updating EAX to
hold the new this pointer doesn't seem to be working. I've verified that EAX
contains the address of the destination object, but it is still behaving as if
the this pointer had not changed...

class Test()
{
	static byte* jumpTo;
	void* nextObject;

	int count;

	void Run()
	{
		if (count++ > 5)
			return;
	        asm
	        {
	                mov ECX, EAX;
			mov EAX, nextObject[ECX];
	                jmp dword ptr jumpTo;
	        }
	}
}

int main(char[][] args)
{
	//14 is offset for parameterless member function with no stack variables
	Test.jumpTo = cast(byte*)(&Test.Run) + 14;

	auto t1 = new Test();
	auto t2 = new Test();

	t1.nextObject = cast(void*)t2;
	t2.nextObject = cast(void*)t1;

	t1.Run();

	Stdout(t1.count, t2.count);

	return 0;
}

...This gives me the result "7, 0". I don't have the actual disassembled code
in front of me but IIRC member variables are referenced via offset from the
location stored in EAX. I'm not sure what else to try.