🚀 go-pugleaf

In article <15694@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:

>(This is what a compiler typically generates for a Modula loop with
>indeterminate bounds---actually, it is more like
>
>	if (a > b) goto out;
>	c = a;
>   top: ...;
>	if (c==b) goto out;
>	c++;
>	goto top;
>   out: ;

There is a trick that works on most machines; it gives a rather tighter
loop than the above:

	IF a>b THEN GOTO done END;
	c := a-1;
    loop:
	c := c+1;
	<body>
	IF c#b THEN GOTO loop END;
    done:

This even works when 'a' is the minimum of the range; the value wraps around
on the decrement but immediately wraps back again on the first increment.
For a zero-times loop this is one conditional branch taken; for an N-times
loop it is one conditional branch not taken and (N-1) taken, plus the
possible overhead of the extra "-1" in the control initialisation.