8.3 Loop Table Messages

The Loop Table (/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 and that the loop contained I/O statements.

Messages referring to DO loops apply to C for loops:

bad intrinsic call - Loop optimization may give unpredictable results.
bad operator - Loop optimization may give unpredictable results.
branches out of-into loop - Code may be iteration- 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.
concurrentized - Should not appear. If it does, indicates miscellaneous optimization.
distributed - Loop divided into smaller loops, some of which may be further optimized.
DO statement not optimizable - For example, the iteration count of a while loop is not known.
ELSE IF - Some loops inside nested IFs require more information to optimize than KAP can deduce.
empty loop - Loop with no executable statements inside (nothing to optimize), or loop whose body will never be executed.
fused with other loops - Adjacent loops joined into a larger loop to reduce overhead.
GOTO in loop - Possible flow patterns are too complicated for KAP.
I/O statement in loop - Input/Output is assumed to be order-dependent.
left as DO loop - Usually, a data dependence prevented concurrentization.
left as DOALL - Loop was already explicitly concurrent in the input dialect of C.
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 the expected gain.
optimization disabled - By a directive or assertion.
replaced by calls - Some loop nests can be replaced by a call to a library routine.
scalar directive - A directive or assertion turned off optimization.
scalar inner loop - Inside a parallel loop already.
scalar mode preferable - Usually, too much synchronization needed.
too many inner loops - LIMIT > integer very complicated loop nest; KAP will try to optimize the inner loops. Try increasing /limit .
too many stmts/DD arcs - Very complicated loop nest; try increasing /arclimit.
too much synchronization - Loop iterations depend on each other in a complicated way; the overhead from untangling them would be greater than expected gain.
unoptimizable call - Function with unknown side effects.
unoptimizable operation - Character string manipulation.
unoptimizable statements in loop - Usually I/O.
unoptimizable symbol use - Variable used in a complicated way.
unoptimizable variable or operation type - Only numerical work is easily concurrentized.
unrolled - Loop could not be concurrentized, so KAP unrolled it (see explanations of unrolling in Chapter 4 and Chapter 7).
unrolled completely, fused or removed - Loop has been turned into straight-line code, joined to another loop, or dropped entirely.
unrolled; already in a parallel loop - Loop could be concurrentized, but target machine does not support nested parallelism; KAP unrolled it.
vectorized - Can be used for optimizations which enhance the vectorization performed by the C compiler.

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