Is possible to jmp inside a created JIT?

[crazynds]

I developed a program that creates several JIT functions.
Currently when developing these JIT functions, there are cases where within a JIT x' function I want to jump from other area in x' function, in which case I use a simple jmp or call. But now I have to go from a JIT function y' to the function x' at a certain point.
Is it possible that within the function y 'implement some call or jmp that references function x' at that specific point? Is it possible to take the position of the function after it is created in memory and discover the position where I want to enter the function x '?

What I want can be exemplified by the following code snippets, but they must be functions created separately, and the function x would be created before the function y. Ex:
-function x':
0. inc rax

1. add rax,rbx
2. xor rbx,rbx
3. return

-function y':
0. dec rax

1. call x' function at 2 line
2. return

[kobalicek]

What kind of tool do you use to generate the function - Assembler or Compiler?

With Assembler this should be possible with using labels - you simply create a Label and then call that Label. If the functions are not generated in a single CodeHolder instance you can still create label and then query for its relative distance from the beginning of the code - then it's again callable (it would be base address + label offset). Use CodeHolder::labelOffset(label) method to get the relative distance.

With Compiler this is not possible as each function has prolog/epilog and you can only jump (call) to the beginning of the function.

[crazynds]

I use Assembler to generate the jit function.

I test some code, and work, but i don't know if this is the correct method.

JitRuntime rt;
Function fn1,fn2;
uint64_t position;
{
	//First function X
	CodeHolder code;
	code.init(rt.environment());
	Assembler a(&code);
	Label label = a.newLabel();
	a.xor_(rax,rax);
	a.bind(label);
	a.inc(rax);
	a.ret();
	Error err = rt.add(&fn1, &code);
	position = fn1+code.labelOffset(label); //Function address + offset position
	if (err) return 1;
}

{
	//Second function Y
	CodeHolder code;
	code.init(rt.environment());
	Assembler a(&code);
	a.mov(rax,2);
	a.jmp(position);
	Error err = rt.add(&fn2, &code);
	if (err) return 1;
}


int result = fn2();
printf("%d\n", result);

rt.release(fn1);
rt.release(fn2);

[kobalicek]

Yes that's a possibility - another possibility is to use the same CodeHolder to generate both functions and use the Label directly in the call.

I would recommend loading the address to register before the call though as JitAllocator has no guarantees about where the code will be allocated (so +- 2GB is not guaranteed).

[crazynds]

Ok, this resolve the problem.