HP Fortran Programmer's Guide (B3908-90031; September 2011)

Performance and optimization
Using options to control optimization
Chapter 6148
Using options to control optimization
HP Fortran includes a rich set of command-line options for controlling optimization. For most applications,
we recommend optimizing with -O, which enables the default level of optimization. (For information about
the default level of optimization, refer to Table on page 149; look up +O2 in the first column.) You can raise
or lower the level of optimization with the +Oopt-level option, and you can use the +Ooptimization
option to control the kinds of optimizations that are available at each level.
The following sections describe how to use the +Oopt-level and +Ooptimization options. For detailed
descriptions of the optimization options, see the HP Fortran Programmer’s Reference.
Using +O to set optimization levels
HP Fortran provides four levels of optimization. Each higher level is a superset of the lower levels; level 4 is
the highest level and can result in a significant increase in program performance. Level 2 is the default level
of optimization.
You invoke optimization by compiling with the +Oopt-level option, where opt-level is an integer in
the range 0 - 4. The following command line invokes the optimizer at the highest level:
$ f90 +O4 file.f90
You can invoke level 2 (the default level) by specifying the -O option.
Table 6-1 summarizes each level, giving the option that invokes that level, the advantages, disadvantages,
and recommended usages. For technical information about the specific optimizations at each level, refer to
the HP PA-RISC Compiler Optimization Technology White Paper. A PostScript version of this document is
available online in /opt/langtools/newconfig/white_papers/optimize.ps.
NOTE You can debug programs optimized up to level 2. To prepare an optimized program for
debugging, use the command line:
$ f90 -g +Oopt-level prog.f90