Optimization-level Options

Each of the command-line options: -O,-O1, -O2 and -O3 turn on several compiler capabilities. -O and -O1 are practically the same and mentioned both for compatibility with other compilers. The following table summarizes the optimizations that the compiler applies when you invoke -O1 and-or -O2, and -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 architectures, the options can behave in a different way. To specify the optimizations for your program, use options depending on the target architecture as follows.

IA-32 and Itanium(TM) compilers
-O2	ON by default. Enables options -Oi and -fp. Confines optimizations to the procedural level. -O2 turns ON intrinsics inlining.
-O3	Enables -O2 option with more aggressive optimization, for example, prefetching, scalar replacement, and loop transformations. Optimizes for maximum speed, but may not improve performance for some programs.
IA-32 compiler
-O,-O1 or -O2	Enable options -Oi, and -fp. However, -O1 disables intrinsics inlining to reduce code size. In most cases, -O2 is recommended over -O1.
Itanium compiler
-O or -O1	Enable the same optimizations as -O2 except for loop unrolling. In most cases, -O2 is recommended over -O1.