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

-qpdf Option
Syntax
-qpdf{1|2}
Tunes optimizations through profile-directed feedback (PDF), where results from
sample program execution are used to improve optimization near conditional
branches and in frequently executed code sections.
To use PDF, follow these steps:
1. Compile some or all of the source files in a program with the -qpdf1 option.
You need to specify the -O2 option, or preferably the -O3, -O4, or -O5 option,
for optimization. Pay special attention to the compiler options that you use to
compile the files, because you will need to use the same options later.
In a large application, concentrate on those areas of the code that can benefit
most from optimization. You do not need to compile all of the application’s
code with the -qpdf1 option.
2. Run the program all the way through using a typical data set. The program
records profiling information when it finishes. You can run the program
multiple times with different data sets, and the profiling information is
accumulated to provide an accurate count of how often branches are taken and
blocks of code are executed.
Important: Use data that is representative of the data that will be used during
a normal run of your finished program.
3. Relink your program using the same compiler options as before, but change
-qpdf1 to -qpdf2. Remember that -L, -l, and some others are linker options, and
you can change them at this point. In this second compilation, the accumulated
profiling information is used to fine-tune the optimizations. The resulting
program contains no profiling overhead and runs at full speed.
For best performance, use the -O3, -O4, or -O5 option with all compilations when
you use PDF (as in the example above). If your application contains C or C++ code
compiled with IBM XL C/C+ compilers, you can achieve additional PDF
optimization by specifying the -qpdf1 and -qpdf2 options available on those
compilers. Combining -qpdf1/-qpdf2 and -qipa or -O5 options (that is, link with
IPA) on all Fortran and C/C++ code will lead to maximum PDF information being
available for optimization.
Rules
The profile is placed in the current working directory or in the directory that the
PDFDIR environment variable names, if that variable is set.
To avoid wasting compilation and execution time, make sure that the PDFDIR
environment variable is set to an absolute path. Otherwise, you might run the
application from the wrong directory, and it will not be able to locate the profile
data files. When that happens, the program may not be optimized correctly or may
be stopped by a segmentation fault. A segmentation fault might also happen if you
change the value of the PDFDIR variable and execute the application before
finishing the PDF process.
Background Information
Because this option requires compiling the entire application twice, it is intended
to be used after other debugging and tuning is finished, as one of the last steps
before putting the application into production.
210 XL Fortran Enterprise Edition for AIX : User’s Guide