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RealView Compiler User's Guide
Preface Introduction Getting started with the ARM Compiler Compiler Features Coding Practices Optimizing code
Optimizing for size versus speed Optimization levels and the debug view Selecting the target device Optimizing loops Using volatile Code metrics Measuring code and data sizes Measuring stack use Reducing debug information in objects and librarie Functions Minimizing parameter passing overhead __value_in_regs __pure Placing ARM function qualifiers Inlining How the compiler decides to inline When is it practical for the compiler to inline? Managing inlining Inlining functions across multiple files Debugging data and the --[no]_inline keyword Marking functions as static Setting breakpoints on inlined functions in ROM im Aligning data About data alignment The __packed qualifier and unaligned accesses to d __packed structures versus individually __packed f Using floating-point arithmetic Support for floating-point operations VFP architectures The --fpu option Floating-point linkage The --fpmode option Using the --fpu option VFP support Trapping and identifying division-by-zero errors Integer division (Software) Floating point division Support for ARM architecture v6 Instruction generation Alignment support Endian support Diagnostic Messages Using the Inline and Embedded Assemblers SemihostingOptimization levels and the debug view
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| Home > Coding Practices > Optimizing code > Optimization levels and the debug view | |||
The precise optimizations performed by the compiler depend both on the level of optimization chosen, and whether you are optimizing for performance or code size.
The compiler supports the following optimization levels:
-O0Minimum optimization. The compiler performs simple optimizations that do not impair the debug view.
When debugging is enabled, this option gives the best possible debug view.
-O1Restricted optimization.
When debugging is enabled, this option gives a generally satisfactory debug view with good code density.
-O2High optimization. This is the default optimization level.
When debugging is enabled, this option might give a less satisfactory debug view.
-O3Maximum optimization. This is the most aggressive form of optimization available. It is weighted towards your choice of
-Ospaceor-Otime. Specifying this option enables multifile compilation by default where multiple files are specified on the command line.When debugging is enabled, this option typically gives a poor debug view.
Because optimization affects the mapping of object code to source code, the choice of optimization level and -Ospace/-Otime generally impacts the debug view. When debugging is enabled using --debug, explicitly specify the most appropriate optimization level using the -Onum command-line option.
The option -O0 is the best option to use if a simple debug view is needed. Selecting -O0 typically increases the size of the image by 7-15%. To reduce the size of your debug tables, use the --no_debug_macros option.
For more information see:
‑‑[no_]debug in the Compiler Reference Guide
‑‑[no_]debug_macros in the Compiler Reference Guide
‑‑dwarf2 in the Compiler Reference Guide
‑‑dwarf3 in the Compiler Reference Guide
‑Onum in the Compiler Reference Guide.
| Copyright © 2007 ARM Limited. All rights reserved. | ARM DUI 0375A |
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