Enabling Different Levels of Optimization

Level 1 Optimization

To enable level 1 optimization, use the +O1 option, as follows:

cc +O1 sourcefile.c

Level 1 optimization compiles quickly, but still provides some run-time speedup.

Level 2 Optimization

To enable level 2 optimization, use the +O2 option, as follows:

cc +O2 sourcefile.c

Level 2 (equivalent to -O) takes more time to compile, but produces greatly improved run-time speed.

Level 3 Optimization

To enable level 3 optimization, use the +O3 option, as follows:

cc +O3 sourcefile.c

Level 3 does full optimization of all subprograms within a single file.

Level 4 Optimization

To enable level 4 optimization, use the +O4 option, as follows:

cc +O4 sourcefile.c

Level 4 can potentially produce the greatest improvements in speed by performing optimizations across multiple object files. Level 4 does optimizations at link time, so compiles will be faster, but links will be longer.

Depending on the size and number of the modules, compiling at level 4 can consume a large amount of virtual memory. Level 4 may consume roughly 1.25 megabytes per 1000 lines of non-commented source. When you use level 4 on a large application, it is a good idea to increase the system swap space. For information on increasing system swap space, see the book Managing Systems and Workgroups.