Optimizing PERFORM Statements

Using PERFORM is generally very efficient, and is a very good way of keeping the size of your program down as well as giving it an easy-to-maintain structure. The following rules enable you to use PERFORM in the most efficient ways.

• Put commonly used pieces of code in sections and perform them.

  Apart from being good coding practice, this saves space.

• When you increment or decrement a counter, terminate it with a literal value rather than a value held in a data item. For example, to execute a loop n times, set the counter to n and then decrement the counter until it becomes zero, rather than incrementing the counter from zero to n.
• Perform sections, not paragraphs. Put EXIT PROGRAM and STOP RUN statements at the end of the first (main) section.

  The range of an out-of-line PERFORM statement should not contain the end of another perform range. If it does, the program is said to trickle, that is execution is allowed to go past the end of a perform range. Applying the rule above ensures that this does not happen.

  If your program does not trickle, you can compile it with the directive NOTRICKLE which causes the Compiler to produce more efficient code since PERFORM is implemented as a machine code CALL instruction with a corresponding RET at the end of the perform range. If NOTRICKLE is used where two perform ranges overlap, results are unpredictable.

  For example, do not use code in the following form:

• Only use GO TO for paragraphs in the same section.
• Do not use PERFORM .. THRU statements.
• Do not use ALTER statements. The presence of an ALTER statement in a program prevents optimization of PERFORM statements.