Each of the command-line options: -O,-O1, -O2 and -O3 turn on several compiler optimizations. -O and -O1 are practically the same and are only mentioned for compatibility with other compilers. The following table summarizes the optimizations that the compiler applies when you invoke -O1, -O2, or -O3 optimizations.
| Option | Optimization | Affected Aspect of Program |
|---|---|---|
| -O1, -O2 | global register allocation | register use |
| -O1, -O2 | instruction scheduling | instruction reordering |
| -O1, -O2 | register variable detection | register use |
| -O1, -O2 | common subexpression elimination | constants and expression evaluation |
| -O1, -O2 | dead-code elimination | instruction sequencing |
| -O1, -O2 | variable renaming | register use |
| -O1, -O2 | copy propagation | register use |
| -O1, -O2 | constant propagation | constants and expression evaluation |
| -O1, -O2 | strength reduction-induction variable | simplification instruction, selection-sequencing |
| -O1, -O2 | tail recursion elimination | calls, further optimization |
| -O1, -O2 | software pipelining | calls, further optimization |
| -O3 | prefetching, scalar replacement, loop transformations |
memory access, instruction parallelism, predication, software pipelining |
For IA-32 and Itanium(TM) architectures, the options can behave in a different way. To specify the optimizations for your program, use options for depending on the target architecture as follows.
| IA-32 and Itanium(TM) compilers | |
|---|---|
| -O, -O1, -O2 | ON by default. Confines optimizations to the procedural level. Turns ON intrinsics inlining. All three optimizations are equal. |
| -O3 | Enables -O2 option with more aggressive optimizations,
for example:
Optimizes for maximum speed, but may not improve performance for some programs. |