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Sets the optimization level for the link time optimization feature.



Where level is one of the following:

Minimum optimization for the performance of the compiled binary. Turns off most optimizations. When debugging is enabled, this option generates code that directly corresponds to the source code. Therefore, this optimization might result in a significantly larger image.

Restricted optimization. When debugging is enabled, this option selects a good compromise between image size, performance, and quality of debug view.

Arm recommends -O1 rather than -O0 for the best trade-off between debug view, code size, and performance.


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 linker might perform optimizations that the debug information cannot describe.

This optimization is the default optimization level.

Very high optimization. When debugging is enabled, this option typically gives a poor debug view. Arm recommends debugging at lower optimization levels.
Enables all the optimizations from level 3 including those optimizations that are performed with the -ffp-mode=fast armclang option. This level also performs other aggressive optimizations that might violate strict compliance with language standards.
Maximum optimization. Specifically targets performance optimization. Enables all the optimizations from level fast, together with other aggressive optimizations.


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


  • 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.
Performs optimizations to reduce code size, balancing code size against code speed.
Performs optimizations to minimize image size.


If you do not specify Olevel, the linker assumes O2. This level is different from the default armclang optimization level, -O0. Arm recommends that you always specify a --lto_level=Olevel to match the optimzation level that is used with armclang, instead of relying on the default.