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Optimization options

Options that control optimization behavior and performance.

Compiler optimization options

Option

Description

-O0

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.

Usage

armclang -O0

-O1

Restricted optimization. When debugging is enabled, this option gives the best debug view for the trade-off between image size, performance, and debug.

Usage

armclang -O1

-O2

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.

Usage

armclang -O2

-O3

Very high optimization. When debugging is enabled, this option typically gives a poor debug view. Arm recommends debugging at lower optimization levels.

Usage

armclang -O3

-Ofast

Enable 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.

Usage

armclang -Ofast

-ffast-math

Allow aggressive, lossy floating-point optimizations.

Usage

armclang -ffast-math

-ffinite-math-only

Enable optimizations that ignore the possibility of NaN and +/-Inf.

Usage

armclang -ffinite-math-only

-ffp-contract={fast|on|off}

Controls when the compiler is permitted to form fused floating-point operations (such as FMAs).

fast: Always (default).

on: Only in the presence of the FP_CONTRACT pragma.

off: Never.

Usage

armclang -ffp-contract={fast|on|off}

-finline

-fno-inline

Enable or disable inlining (enabled by default).

Usage

armclang -finline

(enable)

armclang -fno-inline

(disable)

-fstrict-aliasing

Tells the compiler to adhere to the aliasing rules defined in the source language.

In some circumstances, this flag allows the compiler to assume that pointers to different types do not alias. Enabled by default when using -Ofast.

Usage

armclang -fstrict-aliasing

-funsafe-math-optimizations

-fno-unsafe-math-optimizations

This option enables reassociation and reciprocal math optimizations, and does not honor trapping nor signed zero.

Usage

armclang -funsafe-math-optimizations

(enable)

armclang-fno-unsafe-math-optimizations

(disable)

-fvectorize

-fno-vectorize

Enable/disable loop vectorization (enabled by default).

Usage

armclang -fvectorize

(enable)

armclang -fno-vectorize

(disable)

-mcpu=<arg>

Select which CPU architecture to optimize for. Choose from:

  • native: Auto-detect the CPU architecture from the build computer.

  • cortex-a72: Optimize for Cortex-A72-based computers.

  • thunderx2t99: Optimize for Cavium ThunderX2-based computers.

  • generic: Generates portable output suitable for any Armv8-A computer.

Usage

armclang -mcpu=<arg>

-march=<arg>

Specifies the name of the target architecture. Choose from:

  • armv8-a: Arm®v8-A architecture.

  • armv8-a+sve: Armv8-A SVE-enabled architecture.

    Note

    When linking to the SVE libary of Arm Performance Libraries, you must also include the -armpl=sve option. For more information, see the Linker options options.

Usage

armclang -march=<arg>

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