7.6.分支

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\(7.6.\)Branching

1.Branching implementation

  Take the following program as example:

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//return x * y
int mult (int x, int y) {
	int sum = 0;
	int n = 1;

	while !(n>y) {
		sum += x;
		n++;
	}

	return sum;
}

  This program can be translated into pseudo VM code as below:

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function mult(x,y)
	push 0
	pop sum
	push 1
	pop n
label LOOP
	push n
	push y
	gt
	if-goto ENDLOOP
	push sum
	push x
	add
	pop sum
	push n
	push 1
	add
	pop n
	goto LOOP
label ENDLOOP
	push sum
	return

  Notice that we have three new symbols: label, if-goto, goto:

  • label LABEL declare the label.
  • if-goto LABEL: if the statement is true, then jump to LABEL.
  • goto LABEL: jump to LABEL with no condition.

  With these three statement, we can translate the branching expression into VM code.