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:
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:
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:
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:
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.
