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

Performance and optimization
Conservative vs. aggressive optimization
Chapter 6162
Conservative vs. aggressive optimization
At optimization level 2 or higher, the optimizer makes a number of assumptions about the program it is
optimizing—for example, that re-ordering an expression for improved instruction scheduling will not
change its results. In general, these assumptions relate to how closely the target program conforms to the
Fortran 90 Standard. For programs that conform to the Standard, it is safe for the optimizer to apply certain
optimizations that can significantly improve performance. For nonstandard-conforming programs, these
same optimizations could change the results or behavior of the program in ways that may not be acceptable
to the programmer.
The +Oconservative and +Oaggressive options enable you to set the optimizer’s assumptions about
which optimizations it can and cannot apply to a program. Each option invokes a subset of the fine-tuning
options that balances safety and performance according to the coding style of the target program. You can
use either option at optimization level 2 or higher.
NOTE +Oaggressive and +Oconservative are incompatible and must not appear on the same
command line.
Table on page 160 lists the assumptions that the optimizer makes about your program when you compile
with +Oconservative, +Oaggressive, or neither option (the default). The table also lists the
fine-tuning options that are invoked by +Oconservative and +Oaggressive. The options listed for the
default case are the subset of the ones invoked by +Oconservative and +Oaggressive. For information
about the fine-tuning options listed in the third column, see Table on page 152.
Table 6-4 Conservative, aggressive, and default optimizations
Specified options Assumptions Invoked options
+Onoconservative
+Onoaggressive
(the default)
• Standard-conforming +Onoentrysched
+Omoveflops
+Onoparmsoverlap
+Onovectorize