XL Fortran Enterprise Edition for AIX : User's Guide - IBM

-O5 Provides all of the functionality of the -O4 option, but also provides the
functionality of the -qipa=level=2 option.
Note: Combining -O2 and higher optimizations with -qsmp=omp invokes
additional optimization algorithms, including interprocedural analysis (IPA).
IPA optimizations provide opportunities for the compiler to generate
additional fmadd instructions.
To obtain the same floating-point accuracy for optimized and non-optimized
applications, you must specify the -qfloat=nomaf compiler option. In cases
where differences in floating-point accuracy still occur after specifying
-qfloat=nomaf, the -qstrict compiler option allows you to exert greater
control over changes that optimization can cause in floating-point semantics.
Restrictions
Generally, use the same optimization level for both the compile and link steps. This
is important when using either the -O4 or -O5 optimization level to get the best
run-time performance. For the -O5 level, all loop transformations (as specified via
the -qhot option) are done at the link step.
Increasing the level of optimization may or may not result in additional
performance improvements, depending on whether the additional analysis detects
any further optimization opportunities.
An optimization level of -O3 or higher can change the behavior of the program
and potentially cause exceptions that would not otherwise occur. Use of the
-qstrict option can eliminate potential changes and exceptions.
If the -O option is used in an @PROCESS statement, only an optimization level of
0, 2, or 3 is allowed.
Compilations with optimization may require more time and machine resources
than other compilations.
The more the compiler optimizes a program, the more difficult it is to debug the
program with a symbolic debugger.
Related Information
“-qessl Option” on page 155 allows the use of ESSL routines.
“-qstrict Option” on page 241 shows how to turn off the effects of -O3 that might
change the semantics of a program.
“-qipa Option” on page 182, “-qhot Option” on page 171, and “-qpdf Option” on
page 210 turn on additional optimizations that may improve performance for some
programs.
“Optimizing XL Fortran Programs” on page 305 discusses technical details of the
optimization techniques the compiler uses and some strategies you can use to get
maximum performance from your code.
-qOPTimize is the long form of -O.
XL Fortran Compiler-Option Reference 115