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ARM Compiler armclang Reference Guide : -O

-O

Specifies the level of optimization to use when compiling source files.

Syntax

-Olevel

Where level is one of the following:

0

Minimum optimization for the performance of the compiled binary. Turns off most optimizations. When debugging is enabled, this option gives the best possible debug view because the structure of the generated code directly corresponds to the source code.

This is the default optimization level.

1

Restricted optimization. When debugging is enabled, this option gives a generally satisfactory debug view with good code density.

2

High optimization. When debugging is enabled, the debug view might be less satisfactory because the mapping of object code to source code is not always clear. The compiler might perform optimizations that cannot be described by debug information.

3

Very high optimization. When debugging is enabled, this option typically gives a poor debug view. ARM recommends debugging at lower optimization levels.

fast

Enables all the optimizations from -O3, together with other aggressive ​optimizations that might violate strict compliance with language standards, including -ffast-math.

max
Maximum optimization. Specifically targets performance optimization. Enables all the optimizations from -Ofast, together with other aggressive optimizations.

Caution

This option is not guaranteed to be fully standards-compliant for all code cases.

Note

  • Code-size, build-time, and the debug view, can each be adversely affected when using this option.
  • ARM cannot guarantee that the best performance optimization is achieved in all code cases.
s

Performs optimizations to reduce code size, balancing code size against code speed.

z

Performs optimizations to minimize image size.

Default

If you do not specify -Olevel, the compiler assumes -O0.

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