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PDF is a compiler option to do further procedural level optimization such as directing register allocations, instruction scheduling, and basic block rearrangement.

1. Compile the source files in a program with -qpdf1 (the function main() must be compiled also). The -lpdf option is required during the link step. All the other compilation options used must also be used during step 3.
2. Run the program all the way through a typical data set. The program records profiling information when it exits into a file called .__BLOCKS in the directory specified by the PDFDIR environment variable or in the current working directory if that variable is not set. 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. It is important to use data that is representative of the data used during a typical run of your finished program.
3. Recompile the program using the same compiler options as in step 1, but change -qpdf1 to -qpdf2. Remember that -L and -l are linker options, and you can change them at this point; in particular, omit the -lpdf option. 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.