HP C Programmer's Guide (92434-90009)

Chapter 4 63
Optimizing HP C Programs
Enabling Different Levels of Optimization
Enabling Different Levels of Optimization
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 HP System Administration Tasks.