Question

im am writing a simple emulator in go (should i? or should i go back to c?).
anyway, i am fetching the instruction and decoding it. at this point i have a byte like 0x81, and i have to execute the right function.

should i have something like this

func (sys *cpu) eval() {
    switch opcode {
    case 0x80:
        sys.add(sys.b)
    case 0x81:
        sys.add(sys.c)
    etc
    }
}

or something like this

var fnTable = []func(*cpu) {
    0x80: func(sys *cpu) {
        sys.add(sys.b)
    },
    0x81: func(sys *cpu) {
        sys.add(sys.c)
    }
}
func (sys *cpu) eval() {
    return fnTable[opcode](sys)
}

1.which one is better?
2.which one is faster?
also
3.can i declare a function inline?
4.i have a cpu struct in which i have the registers etc. would it be faster if i have the registers and all as globals? (without the struct)

thank you very much.

Answer 1

1. The first version looks better to me, YMMV.

2. Benchmark it. Depends how good is the compiler at optimizing. The “jump table” version might be faster if the compiler doesn’t try hard enough to optimize.

3. Depends on your definition of what is “to declare function inline”. Go can declare and define functions/methods at the top level only. But functions are first class citizens in Go, so one can have variables/parameters/return values and structured types of function type. In all this places a function literal can [also] be assigned to the variable/field/element…

4. Possibly. Still I would suggest to not keep the cpu state in a global variable. Once you possibly decide to go emulating multicore, it will be welcome 😉