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-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 generates code that directly corresponds to the source code. Therefore, this might result in a significantly larger image.

This is the default optimization level.

1
Restricted optimization. When debugging is enabled, this option gives the best debug view for the trade-off between image size, performance, and debug.
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 level 3 including those performed with the -ffp-mode=fast armclang option. This level also performs other aggressive optimizations that might violate strict compliance with language standards.
max
Maximum optimization. Specifically targets performance optimization. Enables all the optimizations from level fast, together with other aggressive optimizations.

CAUTION:

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

CAUTION:

-Omax automatically enables the armclang -flto option and the generated object files are not suitable for creating static libraries. When -flto is enabled, you cannot build ROPI or RWPI images.

Note

When using -Omax:

  • Code-size, build-time, and the debug view can each be adversely affected.
  • Arm cannot guarantee that the best performance optimization is achieved in all code cases.
  • It is not possible to output meaningful disassembly when the -flto option is enabled, which is turned on by default at -Omax, because this generates files containing bitcode.
  • If you are trying to compile at -Omax and have separate compile and link steps, then also include -Omax on your armlink command line.

Note

Link Time Optimization does not honor the armclang -mexecute-only option. If you use the armclang -flto or -Omax options, then the compiler cannot generate execute-only code and produces a warning.
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. For the best debug view, Arm recommends -O1 rather than -O0 for the best trade-off between image size, performance, and debug.

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