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

Profile-directed Feedback (PDF)
Profile-directed feedback is a two-stage compilation process that lets you provide
the compiler with data characteristic of typical program behavior. An instrumented
executable is run in a number of different scenarios for an arbitrary amount of
time, producing as a side effect a profile data file. A second compilation using the
profile data produces an optimized executable.
PDF should be used mainly on code that has rarely executed conditional error
handling or instrumentation. The technique has a neutral effect in the absence of
firm profile information, but is not recommended if insufficient or uncharacteristic
data is all that is available.
The following diagram illustrates the PDF process.
Source
code
Compile with
-qpdf1
Instrumented
executable
Compile with
-qpdf2
Sample runs
Profile data
Optimized
executable
The two stages of the process are controlled by the compiler options -qpdf1 and
-qpdf2. Stage 1 is a regular compilation using an arbitrary set of optimization
options and -qpdf1, that produces an executable or shared object that can be run in
a number of different scenarios for an arbitrary amount of time. Stage 2 is a
recompilation using the same options, except -qpdf2 is used instead of -qpdf1,
during which the compiler consumes previously collected data for the purpose of
path-biased optimization.
Using Profile-directed Feedback (PDF)
You can optimize an application based on an analysis of how often it executes
different sections of code, as follows:
1. Compile the application with -qpdf1.
2. Run the application using a typical data set or several typical data sets. When
the application exits, it writes profiling information in the PDF file.
3. Compile the application with -qpdf2.
If you want to see which functions are used the most often, do the following:
1. Compile the application with -qpdf1 -qshowpdf.
2. Run the application using a typical data set or several typical data sets. The
application writes more detailed profiling information in the PDF file.
3. Use the showpdf utility to view the information in the PDF file.
You can take more control of the PDF file generation, as follows:
1. Compile the application with -qpdf1.
Optimizing XL Fortran Programs 315