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

Compiling and linking
Compiling with the f90 command
Chapter 2 61
NOTE You can insert (or remove) underscore characters in the names of any of the optimization
options to improve their readability. The compiler will recognize the option name with or
without underscores.
Reviewing general optimization options
The following options allow you to control how optimization affects code size, compilation time, runtime
performance, and other user-visible effects. The syntax for using these options is:
+O[no]optimization
where optimization is a parameter that specifies the class of optimization to apply to
your program. The different parameters are described below. The prefix no negates the
effect of optimization.
Except for +Oall, +Ofast and +Ofaster the options do not override a specified level of optimization,
nor do they imply a particular level. (The +Oall option automatically invokes the highest level of
optimization. +Ofast invokes +O2 level of optimization in addition to few other specific flags
described later. Whereas, +Ofaster invokes +O3 level of optimization). To use any of these options you
must also include the +On option on the same command line, where n specifies the level at which the type of
optimization is effective. Thus, if you wish to apply all optimizations available at level 3 except those that
might significantly increase code size, you would use the command line:
f90 +O3 +Osize my_prog.f90
If an option is mistakenly used at a level at which the corresponding optimization is not performed, the
compiler will issue a warning message.
The defaults specified in the following descriptions are in effect only at the specified optimization levels,
unless stated otherwise.
+O[no]aggressive
+Oaggressive enables optimizations that can result in significant performance
improvement but can also change a program’s behavior. This option is only effective at
optimization level 2 or higher.
The +Oaggressive option performs optimizations invoked by the following options:
• +Oentrysched
• +Olibcalls
• +Onofltacc
• +Onoinitcheck