The Using interprocedural analysis section contains more information on IPA including details on IPA suboptions.
Enables more aggressive HOT transformations to optimize loop constructs and array language.
Optimizes your application to execute on a hardware architecture identical to your build machine. If the architecture of your build machine is incompatible with your application's execution environment, you must specify a different -qarch suboption after the -O4 option. This overrides -qarch=auto.
Optimizes your cache configuration for execution on specific hardware architecture. The auto suboption assumes that the cache configuration of your build machine is identical to the configuration of your execution architecture. Specifying a cache configuration can increase program performance, particularly loop operations by blocking them to process only the amount of data that can fit into the data cache.
If you want to executing your application on a different machine, specify correct cache values.