HP 3000 Manuals HP 3000 Manuals
Enabling Different Levels of Optimization on HP-UX
There may be times when you want more or less optimization than what is
provided with the basic -O option.
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 noncommented 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
System Administration Tasks.