9.1.1.4 Optimization With Runtime Profile Feedback

The compiler applies its optimization strategies at level O3 and above much more efficiently if combined with -xprofile=use. With this option, the optimizer is directed by a runtime execution profile produced by the program (compiled with -xprofile=collect) with typical input data. The feedback profile indicates to the compiler where optimization will have the greatest effect. This may be particularly important with -O5. Here’s a typical example of profile collection with higher optimization levels:

demo% f95 -o prg -fast -xprofile=collect prg.f ...
demo% prg 
demo% f95 -o prgx -fast -O5 -xprofile=use:prg.profile prg.f ...
demo% prgx

The first compilation in the example generates an executable that produces statement coverage statistics when run. The second compilation uses this performance data to guide the optimization of the program.

(See the Fortran User’s Guide for details on -xprofile options.)