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

v Partitioning and layout of static data according to reference
affinity. (Data that is frequently referenced together will be
located closer together in memory.)
This is the default level when -qipa is specified.
level=2 Global alias analysis, specialization, interprocedural data flow.
v Whole-program alias analysis. This level includes the
disambiguation of pointer dereferences and indirect function
calls, and the refinement of information about the side effects of
a function call.
v Intensive intraprocedural optimizations. This can take the form
of value numbering, code propagation and simplification, code
motion into conditions or out of loops, elimination of
redundancy.
v Interprocedural constant propagation, dead code elimination,
pointer analysis.
v Procedure specialization (cloning).
inline=inline-options
Provides precise user control of inlining.
fine_tuning
Other values for -qipa= provide the ability to specify the behavior
of library code, tune program partioning, read commands from a
file, and so on.
Getting the Most from -qipa
It is not necessary to compile everything with -qipa, but try to apply it to as much
of your program as possible. Here are some suggestions.
v When specifying optimization options in a makefile, remember to use the
compiler command (xlf, xlf90, and so on) to link, and to include all compiler
options on the link step.
Optimizing Subprogram Calls
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-qipa works when building executables or shared objects, but always compile
main and exported functions with -qipa.
When compiling and linking separately, use -qipa=noobject on the compile step
for faster compilation.
Ensure that there is enough space in /tmp (at least 200 MB), or use the TMPDIR
environment variable to specify a different directory with sufficient free space.
The level suboption is a throttle. Try varying it if link time is too long.
Compiling with -qipa=level=0 can be very beneficial for little additional link
time.
Look at the generated code after compiling with -qlist or -qipa=list. If too few
or too many functions are inlined, consider using -qipa=inline or
-qipa=noinline. To control inlining of a specific function, use -Q+ and -Q−.
If your application contains a mixture of Fortran and C or C++ code compiled
with IBM XL C/C+ compilers, you can achieve additional optimization by
compiling all your code with the -qipa option.
If a program has many subprogram calls, you can use the -qipa=inline option to
turn on inlining, which reduces the overhead of such calls. Consider using the -p
or -pg option with prof or gprof, respectively, to determine which subprograms are
called most frequently and to list their names on the command line.
To make inlining apply to calls where the calling and called subprograms are in
different scopes, include the -qipa option.
Optimizing XL Fortran Programs 317