Accelerator Manual (G06.27+, H06.04+, J06.03+)

Accelerator Manual—527303-003
5-1
5 Setting Accelerator Options
This section describes the Accelerator elementary and super options. Accelerator
elementary options determine whether or not the Accelerator generates code to
account for specific program variances or constructs. For programs or procedures that
contain one of these variances or constructs, you must set to “on” the appropriate
elementary option to produce an accelerated program that runs correctly. When you
set “on” any of the elementary options, the Accelerator generates additional code that
can decrease program performance. The descriptions of each elementary option
explain when you can safely set these options “off” to increase program performance.
These Accelerator options affect the control of:
•
Atomicity of Instructions on page 5-2
•
Inheritance of Condition Codes on page 5-3
•
Overflow Checking on page 5-5
•
Safe Aliasing Rules on page 5-6
•
Truncation in Address Calculations on page 5-6
For ease of use, the Accelerator elementary option settings have been packaged into
three sets called the Accelerator super options. The super options are:
•
Common
Select Common unless you know that your program contains TAL procedures that
rely on the atomic execution of certain TNS operations or on the inheritance of
conditions codes. Common is correct for all other programs. Common corresponds
to the default setting for each of the elementary options; however, you should set
this option explicitly to make sure that you and other programmers know the
acceleration strategy that has been chosen.
•
Fast
Select the Fast super option if you know that your program does not require
overflow trapping, condition-code inheritance, truncation of indexed address-field
values, instruction atomicity, or the ability to make piecemeal modifications to
pointers or array indexes by means of indirect addressing. The Fast option was
designed for accelerating the operating system. If you select Fast, you must be
extremely thorough in testing your program to make sure it does not rely on any of
the program variances or constructs that are not reproduced for a program
accelerated with the Fast option selected.
•
Safe
Select the Safe super option if you have tried Common and found that your
program does not run correctly. Programs accelerated with the Safe option can
experience poor performance.