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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)

-fsimdmath

-fno-simdmath

Enable (-fsimdmath) or disable (-fno-simdmath) the vectorized libm library (libamath) to help the vectorization of loops containing calls to libm.

For more information, see https://developer.arm.com/products/software-development-tools/hpc/documentation/vector-math-routines.

Default is -fno-simdmath.

Usage

armclang -fsimdmath

armclang -fno-simdmath

-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.

  • neoverse-n1: Optimize for Neoverse N1-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 base architecture and extensions available on the target.

-march=<arg> where <arg> is constructed as name[+[no]feature+...]:

name

armv8-a: Arm®v8-A application architecture profile.

armv8.1-a: Armv8.1 application architecture profile.

armv8.2-a: Armv8.2 application architecture profile.

feature

Is the name of an optional architectural feature that can be explicitly enabled with +feature and disabled with +nofeature.

For AArch64, the following features can be specified:

  • crc - Enable CRC extension. On by default for -march=armv8.1-a or higher.

  • crypto - Enable Cryptographic extension.

  • fullfp16 - Enable FP16 extension.

  • lse - Enable Large System Extension instructions. On by default for -march=armv8.1-a or higher.

  • sve - Scalable Vector Extension (SVE). This feature also enables fullfp16. See Scalable Vector Extension for more information.

  • sve2- Scalable Vector Extension version two (SVE2). This feature also enables sve. See Arm A64 Instruction Set Architecture for SVE and SVE2 instructions.

  • sve2-aes - SVE2 Cryptographic extension. This feature also enables sve2.

  • sve2-bitperm - SVE2 Cryptographic Extension. This feature also enables sve2.

  • sve2-sha3 - SVE2 Cryptographic Extension. This feature also enables sve2.

  • sve2-sm4 - SVE2 Cryptographic Extension. This feature also enables sve2.

Note

When enabling either the sve2 or sve features, to link to the SVE-enabled version of Arm Performance Libraries, you must also include the -armpl=sve option. For more information about the supported options for -armpl, see the -armpl description.

Usage

armclang -march=<arg>

Examples

armclang -march=armv8-a

armclang -march=armv8-a+sve

armclang -march=armv8-a+sve2

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