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Good debug experience without code bloat -O1 Better correlation between source code and generated code -O0 no optimization Faster compile and build time -O0 no optimization Balanced code size reduction and fast performance -Os If you use a higher optimization level for performance, it has a higher impact on the other goals such as degraded debug experience, increased code size, and increased build time.
If your optimization goal is code size reduction, it has an impact on the other goals such as degraded debug experience, slower performance, and increased build time.
Consider whether code size reduction or faster performance is the goal which matters most for your application, and then choose an option which matches your goal.
Optimization level -O0 -O0 disables all optimizations.
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This optimization level is the default. Using -O0 results in a faster compilation and build time, but produces slower code than the other optimization levels. Code size and stack usage are significantly higher at -O0 than at other optimization levels.
The generated code closely correlates to the source code, but significantly more code is generated, including dead code.
Optimization level -O1 -O1 enables the core optimizations in the compiler. This optimization level provides a good debug experience with better code quality than -O0.
Also the stack usage is improved over -O0. Arm recommends this option for a good debug experience. The differences when using -O1, as compared to -O0 are: Optimizations are enabled, which might reduce the fidelity of debug information. Inlining and tail calls are enabled, meaning backtraces might not give the stack of open function activations which might be expected from reading the source.
If the result is not needed, a function with no side-effects might not be called in the expected place, or might be optimization of options.
Values of variables might not be available within their scope after they are no longer used. For example, their stack location might have been reused. Optimization level -O2 -O2 is a higher optimization for performance compared to -O1. It adds few new optimizations, and changes the heuristics for optimizations compared to -O1.
This level is the first optimization level at which the compiler might automatically generate vector instructions. It also degrades the debug experience, and might result in an increased code size compared to -O1. The differences when using -O2 as compared to -O1 are: The threshold at optimization of options the compiler believes that it is profitable to inline a call site might increase. The amount of loop unrolling that is performed might increase.
Vector instructions might be generated for simple loops and for correlated sequences of independent scalar operations. The creation of vector instructions can be inhibited with the armclang command-line option -fno-vectorize.
Optimization level -O3 -O3 is a higher optimization for performance compared to -O2. This optimization level enables optimizations that require significant compile-time analysis and resources, and changes the heuristics for optimizations optimization of options to -O2.
It also degrades the debug experience compared to -O2.
For every SCoP we generate the polyhedral representation and transform it back to gimple. Some minimal optimizations are also performed by the code generator CLooG, like index splitting and dead code elimination in loops. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism.
The differences when using -O3 as compared to -O2 are: The threshold at which the compiler believes that it is profitable to inline a call site increases. The amount of loop unrolling that is performed is increased. More aggressive instruction optimizations are enabled late in the compiler pipeline.
Optimization Options Summary
Optimization level -Os -Os aims to provide high performance without a significant increase in code size. Depending on your application, the performance provided by -Os might be similar to -O2 or -O3.
It also degrades the debug experience compared to -O1. The differences when using -Os as compared to -O3 are: The threshold at which the compiler believes it is profitable to inline a call site is lowered. The amount of loop unrolling that is performed is significantly lowered.
Arm recommends this option for best code size if LTO is not appropriate for your application. This optimization level degrades the debug experience compared to -O1.
The differences when binary options trades table -Oz as compared to -Os are: Instructs the compiler to optimize for code size only and disregard the performance optimizations, which might result in slower code. Function inlining is not disabled.
There are instances where inlining might reduce code size overall, for example if a function is called only once.
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The inlining heuristics are tuned to inline only when code size is expected to decrease as a result. Optimizations that might increase code size, such as Loop unrolling and loop vectorization are disabled.
Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Loops are generated as while loops instead of do-while loops. Optimization level -Omin -Omin aims to provide the smallest possible code size. Arm recommends this option for best code size. The differences when using -Omin as compared to -Oz are: -Omin enables a basic set of Link-Time Optimizations LTO aimed at removing unused code and data, while also trying to optimize global memory accesses.
If you want to compile at -Omin and use separate compile and link steps, then you must also include -Omin on your armlink command line. Optimization level -Ofast -Ofast performs optimizations from level -O3, including those optimizations performed with the -ffast-math armclang option. This level also performs other aggressive optimizations that might violate strict compliance with language standards.
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This level degrades the debug experience, and might result in increased code size compared to -O3. Optimization level -Omax -Omax performs maximum optimization, and specifically targets performance optimization. At this optimization level, Arm Compiler might violate strict compliance with language standards.
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- Optimization Options Summary
Use this optimization level for the fastest performance.