9.3 Loop Table Messages

The Loop Table listing (-listoptions=l ) includes an entry for each loop indicating whether it was optimized, or why it was not. This section lists the possible messages and gives a brief explanation for each. The two most common reasons for a loop to be left serial are that the iterations were not independent (the listing should give a Data Dependence message) and the loop contained I/O statements.

• Bad intrinsic call - Code rearrangement by KAP may give unpredictable results.

• Bad operator - Code rearrangement by KAP may give unpredictable results.

• Branches out of/into loop - Code may be iteration- order dependent.

• CALL with alternate returns - Program flow too uncertain.

• IO statements in loop - Input/Output assumed order- dependent.

• Cannot allocate temporary array - Usually, a Data Dependence could have been lifted by expanding a scalar into an array, but there was a name conflict.

• Distributed - Loop divided into smaller loops, some of which may be otherwise optimized.

• DO statement not optimizable - For example, iteration count of a DO WHILE loop is not known.

• Fused with other loops - Adjacent loops joined into a larger loop to reduce overhead.

• GOTO in loop - Possible flow patterns too complicated for KAP.

• Left as DO loop - Usually, a Data Dependence prevented optimization.

• Left as a serial loop - Usually, too much synchronization required.

• Nested IFs - The branching pattern was too complicated.

• No optimizable statements - For example, no arithmetic in loop.

• Not enough work - Optimization overhead greater than expected gain.

• Optimization disabled - By a directive or assertion.

• Scalar directive - A directive or assertion turned off optimization.

• Scalar mode preferable - Usually, too much synchronization needed.

• Too many inner loops: LIMIT > integer - VERY complicated loop nest; KAP will try to optimize inner loops.

• Too many stmts/DD arcs - VERY complicated loop nest; try increasing -arclimit .

• Unoptimizable call - SUBROUTINE or FUNCTION with unknown side effects.

• Unoptimizable intrinsic call - Unknown side effects.

• Unoptimizable operation - For example, character string manipulation.

• Unoptimizable statements in loop - Usually I/O.

• Unoptimizable symbol use - Variable used in a complicated way, such as with COMMON or EQUIVALENCE.

• Unoptimizable variable or operation type - Only numerical work is easily optimized by DIGITAL KAP for DEC Fortran for DIGITAL UNIX.

• Unrolled - Loop would run more efficiently than rolled up (see explanations in Chapter 4 and Chapter 8).

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