DO loops in Fortran 77 are not executed if the initial value of the DO control variable is beyond the final value. These are called zero-trip loops. IF statements can be removed when they are used only to ensure that a DO statement is executed only when the upper bound of the loop is greater than or equal to the lower bound, for example:
IF (N .GE. 1) THEN DO 10 I = 1,N A(I) = A(I-1) + B(I) 10 CONTINUE ENDIF
Becomes:
DO 10 II1=1,N A(II1) = A(II1-1) + B(II1) 10 CONTINUE