A few examples of different loops,
loopy: DO i = 1, 30, 2
... ! 15 iterations
END DO loopy DO j = 30, 1, -2
... ! 15 iterations
END DO DO k = 30, 1, 2
... ! 0 iterations
... ! loop skipped
END DO DO l = 30,1
... ! zero-trip loop
END DO
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