Enable Implied-DO Loop Collapsing

DO loop collapsing reduces a major overhead in I/O processing. Normally, each element in an I/O list generates a separate call to the Compaq Fortran RTL. The processing overhead of these calls can be most significant in implied-DO loops.

Compaq Fortran reduces the number of calls in implied-DO loops by replacing up to seven nested implied-DO loops with a single call to an optimized run-time library I/O routine. The routine can transmit many I/O elements at once.

Loop collapsing can occur in formatted and unformatted I/O, but only if certain conditions are met:

• The control variable must be an integer. The control variable cannot be a dummy argument or contained in an EQUIVALENCE or VOLATILE statement. Compaq Fortran must be able to determine that the control variable does not change unexpectedly at run time.
• The format must not contain a variable format expression.

For more information:

• See the VOLATILE attribute and statement.
• On loop optimizations, see Optimization Levels: the /optimize:num Option.