Handle invalid function calls

test/opcode_test.go

@@ -16,11 +16,13 @@ func TestCal(t *testing.T) {
 functions:
 - - [cal, +1]
   - [add, g0, 1]
+  - [cal, +2] # Function doesn't exist, immediate implicit return
+  - [add, g0, 2]
 
 - - [mov, g0, 5]
 `)
 
-	expectGlobalMemory(t, state, 0, 6)
+	expectGlobalMemory(t, state, 0, 8)
 }
 
 func TestRet(t *testing.T) {

vm/state.go

@@ -71,6 +71,10 @@ func (state *State) processInstruction() {
 	state.instructionIndex += 1
 	instr.opHandler(state, instr)
 
+	if state.functionIndex >= int64(len(state.functions)) {
+		state.ret()
+	}
+
 	if state.instructionIndex >= int64(len(fnc)) {
 		state.ret()
 	}
@@ -132,11 +136,6 @@ func (state *State) writeSigned(op *Operand, value int64) {
 }
 
 func (state *State) call(functionOffset int64) {
-	if state.functionIndex+functionOffset >= int64(len(state.functions)) {
-		state.setError(fmt.Errorf("Invalid function call index: %d + %d = %d", state.functionIndex, functionOffset, state.functionIndex+functionOffset))
-		return
-	}
-
 	stackFrame := newStackFrame(state)
 	state.stack = append(state.stack, stackFrame)
 	state.functionIndex += functionOffset