User's Guide

Choosing an Optimization Level

Optimization requires additional compilation time, but usually results in a faster run time. XL Fortran allows you to select whether you want optimization to be performed at compile time. By default, the compiler performs only very minor optimizations. For the fastest compile time, we recommend not optimizing a program that you are debugging or compiling to check syntax.

After choosing an optimization level, you can investigate some of the other optimization options mentioned in the following sections. An easy way to choose options that reflect your goals is to run the command xxlf. This features a point-and-click interface to different categories of compiler options.

Figure 23. Optimization Window of the xxlf Application

Specify the -O compiler option, with an optional digit that signifies the level, to enable compiler optimization. The default level is 2.

-O2 performs control and data flow analysis for the entire program unit. This analysis allows optimizations, such as common expression elimination, strength reduction, code motion, and global register assignment. When you use this option, the compiler pays particular attention to innermost loops and to subscript address calculations. The compiler keeps variables in registers wherever possible to eliminate unnecessary loads and stores.

-O2 performs the following optimization techniques:

• Value numbering
• Branch straightening
• Common expression elimination
• Code motion
• Reassociation and strength reduction
• Global constant propagation
• Store motion
• Dead store elimination
• Dead code elimination
• Global register allocation
• Instruction scheduling

-O3 performs the -O2-level optimizations and additional optimizations that may have the following effects:

• Require more machine resources during compilation
• Take longer to compile
• Change the semantics of the program slightly

The exact optimizations that are performed depend on a number of factors:

• Whether the program can be rearranged and still execute correctly
• The relative benefit of each optimization
• The machine architecture

Although it is not strictly another optimization level, the -qipa option extends the optimizations across procedures (even if the procedures are in different files). It enhances the effectiveness of the optimizations that are done by other optimization options, particularly -O (at any level) and -Q. Because it can also increase compile time substantially, you may want to use it primarily for tuning applications that are already debugged and ready to be used. Note that the -O4 compiler option implies the -O3 compiler option.