The doc says "DUP before IF WHILE UNTIL are optimized away" . When would a DUP before an IF be useful?

[Amt]

Hi,
I'm somewhat new to Forth. The documentation says that a DUP before an IF would be optimized but I don't understand a situation where a DUP before an IF would be useful. Could you help me with some examples?

The only situation where I'd use DUP before an IF would be to check for zero/non-zero, but this is already handled with the 0= condition.

[Mikael Nordman]

You would use DUP if you need a copy of the test value for other use later in the code.
The optimization makes the test non-destructive of the top of stack.