Expanding procedure calls inline
Inlining involves copying referenced procedures into the code from which they are referenced. This eliminates the calling overhead for inlined routines and enables the optimizer to perform other optimizations in the inlined routines.
For FORTRAN and C programs, you can specify the -Q option (along with -O2 or -O3) to have procedures inlined into their reference points.
Inlining enhances performance in some programs, while it degrades performance in others. A program with inlining might slow down because of larger code size, resulting in more cache misses and page faults, or because there are not enough registers to hold all the local variables in some combined routines.
If you use the -Q option, always check the performance of the version of your program compiled with -O3 and -Q to that compiled only with -O3. Performance of programs compiled with -Q might improve dramatically, deteriorate dramatically, or change little or not at all.
The compiler decides whether to inline procedures based on their size. You might be able to enhance your application's performance by using other criteria for inlining. For procedures that are unlikely to be referenced in a typical execution (for example, error-handling and debugging procedures), disable inlining selectively by using the -Q-names option. For procedures that are referenced within hot spots, specify the -Q+names option to ensure that those procedures are always inlined.