Computed gotos on Reddit

[Dmitry Olshansky]

On 26-Jul-12 11:56, Don Clugston wrote:
> But doing that screws up the CPU"s stack prediction so badly that it
> will dominate the timing
> At least do something like:
>
> jump_table:
>        move EAX, [ESP]
>        ret
>

BTW this seems to be a roundabout way to get address of label
that I can use do a threaded code interpreter.
Basically every branch is:

L_opcode_1:
	asm { mov EAX, [ESP];
		ret;
	}
	... real code here
L_opcode_2:
	asm { mov EAX, [ESP];
		ret;
	}
	... real code here

Then "compile" step looks like this:
while(not_last_opcode(code[pc]){
	size_t c = code[pc];
	switch(c){
	case op_1:
		asm{ call L_opcode_1; add EAX, 4; mov c, EAX; }
		break;
	case op_2:
		...
	}
	code[pc] = c; //now we have label address
	pc++;
}

//interpret:
pc = 0;
size_t op = code[pc];
asm { mov EAX, op; jump eax; } //here we go, almost computed goto

Obviously it's backwards and awful. Makes me wonder why can't we take it 
directly, what's limitation ?
How about allowing it, at least in inline assembly?


-- 
Dmitry Olshansky