O

O (Linux* OS and Mac OS* X)

This is the same as specifying O2.

O0 (Linux OS and Mac OS X)

Disables all optimizations.

This option may set other options. This is determined by the compiler, depending on which operating system and architecture you are using. The options that are set may change from release to release.

This option causes certain warn options to be ignored. This is the default if you specify option -debug (with no keyword).

O1

Enables optimizations for speed and disables some optimizations that increase code size and affect speed.
To limit code size, this option:

• Enables global optimization; this includes data-flow analysis, code motion, strength reduction and test replacement, split-lifetime analysis, and instruction scheduling.

This option may set other options. This is determined by the compiler, depending on which operating system and architecture you are using. The options that are set may change from release to release.

The O1 option may improve performance for applications with very large code size, many branches, and execution time not dominated by code within loops.

O2

Enables optimizations for speed. This is the generally recommended optimization level.
Vectorization is enabled at O2 and higher levels.

On systems using IA-32 architecture, some basic loop optimizations such as Distribution, Predicate Opt, Interchange, multi-versioning, and scalar replacements are performed.

This option also enables:

• Inlining of intrinsics

• Intra-file interprocedural optimization, which includes:

  • inlining

  • constant propagation

  • forward substitution

  • routine attribute propagation

  • variable address-taken analysis

  • dead static function elimination

  • removal of unreferenced variables

• The following capabilities for performance gain:

  • constant propagation

  • copy propagation

  • dead-code elimination

  • global register allocation

  • global instruction scheduling and control speculation

  • loop unrolling

  • optimized code selection

  • partial redundancy elimination

  • strength reduction/induction variable simplification

  • variable renaming

  • exception handling optimizations

  • tail recursions

  • peephole optimizations

  • structure assignment lowering and optimizations

  • dead store elimination

This option may set other options, especially options that optimize for code speed. This is determined by the compiler, depending on which operating system and architecture you are using. The options that are set may change from release to release.

On Windows systems, this option is the same as the Ox option.

On Linux and Mac OS X systems, if -g is specified, O2 is turned off and O0 is the default unless O2 (or O1 or O3) is explicitly specified in the command line together with -g.

Many routines in the shared libraries are more highly optimized for Intel® microprocessors than for non-Intel microprocessors.

O3

Performs O2 optimizations and enables more aggressive loop transformations such as Fusion, Block-Unroll-and-Jam, and collapsing IF statements.

This option may set other options. This is determined by the compiler, depending on which operating system and architecture you are using. The options that are set may change from release to release.

On systems using IA-32 architecture or Intel® 64 architecture, when O3 is used with options -ax or -x (Linux OS) or with options /Qax or /Qx (Windows OS), the compiler performs more aggressive data dependency analysis than for O2, which may result in longer compilation times.

The O3 optimizations may not cause higher performance unless loop and memory access transformations take place. The optimizations may slow down code in some cases compared to O2 optimizations.

The O3 option is recommended for applications that have loops that heavily use floating-point calculations and process large data sets.

Many routines in the shared libraries are more highly optimized for Intel® microprocessors than for non-Intel microprocessors.