RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler Command-line Options
1 Restricted optimization. Removes unused inline functions and unused
static functions. Turns off optimizations that seriously degrade the debug
view. If used with --debug, this option gives a satisfactory debug view
with good code density.
2 High optimization. If used with --debug, the debug view might be less
satisfactory because the mapping of object code to source code is not
always clear.
This is the default optimization level.
3 Maximum optimization. -O3 performs the same optimizations as -O2
however the balance between space and time optimizations in the
generated code is more heavily weighted towards space or time compared
with -O2. That is:
• -O3 -Otime aims to produce faster code than -O2 -Otime, at the risk
of increasing your image size
• -O3 -Ospace aims to produce smaller code than -O2 -Ospace, but
performance might be degraded.
In addition, -O3 performs extra optimizations that are more aggressive,
such as:
• High-level scalar optimizations, including loop unrolling, for -O3
-Otime. This can give significant performance benefits at a small
code size cost, but at the risk of a longer build time.
• More aggressive inlining and automatic inlining for -O3 -Otime.
• Multifile compilation by default.
Note
The performance of floating-point code can be influenced by selecting an appropriate
numerical model using the --fpmode option.
Note
Do not rely on the implementation details of these optimizations, because they might
change in future releases.
Default
If you do not specify -Onum, the compiler assumes -O2.
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