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

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– If any automatic variables in the called procedures are involved in an
EQUIVALENCE statement
– If the same variable argument is passed more than once in the same call: for
example, CALL SUB(X,Y,X)
Some procedures that use computed GO TO statements, where any of the
corresponding statement labels are also used in an ASSIGN statement, might
not be inlined.
To change the size limits that control inlining, you can specify -qipa=limit=n,
where n is 0 through 9. Larger values allow more inlining.
It is possible that C/C++ functions can be inlined into Fortran programs and
visa-versa during link-time optimizations. The C/C++ code would have to be
compiled using the IBM XL C/C++ compilers with -qipa and a compatible option
set to that used in the XLF compilation.
Shared-memory Parallelism (-qsmp)
Some IBM processors are capable of shared-memory parallel processing. Compile
with -qsmp to generate the threaded code needed to exploit this capability. The
option implies a -O2 optimization level. The default behavior for the option
without suboptions is to do automatic parallelization with optimization.
The most commonly used -qsmp suboptions are summarized in the following
table.
Commonly used -qsmp suboptions
Suboption
Behavior
auto
Instructs the compiler to automatically generate parallel code where possible
without user assistance. This option also recognizes all the SMP directives.
omp
Enforces compliance with the OpenMP Fortran API for specifying explicit
parallelism. Note that -qsmp=omp is currently incompatible with -qsmp=auto.
opt
Instructs the compiler to optimize as well as parallelize. The optimization is
equivalent to -O2 –qhot in the absence of other optimization options. The default
setting of -qsmp is -qsmp=auto:noomp:opt.
suboptions
Other values for the suboption provide control over thread scheduling, nested
parallelism, locking, and so on.
Getting the Most out of -qsmp
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Use -qsmp=omp:noauto if you are compiling an OpenMP program and do not
want automatic parallelization. By default, the parallelization performed is both
explicit and automatic.
Before using -qsmp with automatic parallelization, test your programs using
optimization and -qhot in a single-threaded manner.
Always use the reentrant compiler invocations (the _r command invocations, like
xlf_r) when using -qsmp.
By default, the runtime uses all available processors. Do not set the
XLSMPOPTS=PARTHDS or OMP_NUM_THREADS variables unless you want
to use fewer than the number of available processors. You might want to set the
number of executing threads to a small number or to 1 to ease debugging.
If you are using a dedicated machine or node, consider setting the SPINS and
YIELDS variables (suboptions of XLSMPOPTS) to 0. Doing so prevents the
operating system from intervening in the scheduling of threads across
synchronization boundaries such as barriers.
When debugging an OpenMP program, try using -qsmp=noopt (without -O) to
make the debugging information produced by the compiler more precise. You
Optimizing XL Fortran Programs 319