CPU Build Options

Overview

NumPy provides configuration options to optimize performance based on CPU capabilities. These options allow you to specify which CPU features to support, balancing performance, compatibility, and binary size. This document explains how to use these options effectively across various CPU architectures.

Key Configuration Options

NumPy uses several build options to control CPU optimizations:

• cpu-baseline: The minimum set of CPU features required to run the compiled NumPy.

  • Default: min (provides compatibility across a wide range of platforms)

  • If your target CPU doesn't support all specified baseline features, NumPy will fail to load with a Python runtime error

• cpu-baseline-detect: controls detection of CPU baseline based on compiler flags. Default value is auto that enables detection if -march= or a similar compiler flag is used. The other possible values are enabled and disabled to respective enable or disable it unconditionally.

• cpu-dispatch: Additional CPU features for which optimized code paths will be generated.

  • Default: max (enables all available optimizations)

  • At runtime, NumPy will automatically select the fastest available code path based on your CPU's capabilities

• disable-optimization: Completely disables all CPU optimizations.

  • Default: false (optimizations are enabled)

  • When set to true, disables all CPU optimized code including dispatch, SIMD, and loop unrolling

  • Useful for debugging, testing, or in environments where optimization causes issues

These options are specified at build time via meson-python arguments

pip install . -Csetup-args=-Dcpu-baseline="min" -Csetup-args=-Dcpu-dispatch="max"
# or through spin
spin build -- -Dcpu-baseline="min" -Dcpu-dispatch="max"

cpu-baseline and cpu-dispatch can be set to specific CPU groups, features<opt-supported-features>, or special options that perform specific actions. The following sections describe these options in detail.

Common Usage Scenarios

Building for Local Use Only

When building for your machine only and not planning to distribute

python -m build --wheel -Csetup-args=-Dcpu-baseline="native" -Csetup-args=-Dcpu-dispatch="none"

This automatically detects and uses all CPU features available on your machine.

Excluding Specific Features

You may want to exclude certain CPU features from the dispatched features

# For x86-64: exclude all AVX-512 features
python -m build --wheel -Csetup-args=-Dcpu-dispatch="max -X86_V4"

# For ARM64: exclude SVE
python -m build --wheel -Csetup-args=-Dcpu-dispatch="max -SVE"

Targeting Older CPUs

On x86-64, by default the baseline is set to min which maps to X86_V2. This unsuitable for older CPUs (before 2009) or old virtual machines. To address this, set the baseline to none

python -m build --wheel -Csetup-args=-Dcpu-baseline="none"

This will create a build that is compatible with all x86 CPUs, but without any manual optimizations or SIMD code paths for the baseline. The build will rely only on dispatched code paths for optimization.

Targeting Newer CPUs

Raising the baseline improves performance for two main reasons:

• Dispatched kernels don't cover all code paths

• A higher baseline leads to smaller binary size as the compiler won't generate code paths for excluded dispatched features

For CPUs from 2015 and newer, setting the baseline to X86_V3 may be suitable

python -m build --wheel -Csetup-args=-Dcpu-baseline="min+X86_V3"

Supported CPU Features By Architecture

NumPy supports optimized code paths for multiple CPU architectures. Below are the supported feature groups for each architecture. The name of the feature group can be used in the build options cpu-baseline and cpu-dispatch.

X86

These groups correspond to CPU generations:

• X86_V2: x86-64-v2 microarchitectures (CPUs since 2009)

• X86_V3: x86-64-v3 microarchitectures (CPUs since 2015)

• X86_V4: x86-64-v4 microarchitectures (AVX-512 capable CPUs)

• AVX512_ICL: Intel Ice Lake and similar CPUs

• AVX512_SPR: Intel Sapphire Rapids and newer CPUs

On IBM/POWER big-endian

On IBM/POWER little-endian

On ARMv7/A32

On ARMv8/A64

On IBM/ZSYSTEM(S390X)

On RISCV64

Special Options

Beyond specific feature names, you can use these special values:

NONE

Enables no features (equivalent to an empty string).

NATIVE

Enables all features supported by the host CPU.

DETECT

Detects the features enabled by the compiler. This option is appended by default to cpu-baseline if -march, -mcpu, -xhost, or /QxHost is set in the environment variable CFLAGS unless cpu-baseline-detect is disabled.

MIN

Enables the minimum CPU features for each architecture:

MAX

Enables all features supported by the compiler and platform.

Operator Operators (-/+)

Remove or add specific features, useful with MAX, MIN, and NATIVE:

• Adding a feature (+) includes all implied features

• Removing a feature (-) excludes all successor features that imply the removed feature

Examples

python -m build --wheel -Csetup-args=-Dcpu-dispatch="max-X86_V4"
python -m build --wheel -Csetup-args=-Dcpu-baseline="min+X86_V4"

Usage And Behaviors

Case Insensitivity

CPU features and options are case-insensitive

python -m build --wheel -Csetup-args=-Dcpu-dispatch="X86_v4"

Mixing Features across Architectures

You can mix features from different architectures

python -m build --wheel -Csetup-args=-Dcpu-baseline="X86_V4 VSX4 SVE"

Order Independence

The order of specified features doesn't matter

python -m build --wheel -Csetup-args=-Dcpu-dispatch="SVE X86_V4 x86_v3"

Separators

You can use spaces or commas as separators

# All of these are equivalent
python -m build --wheel -Csetup-args=-Dcpu-dispatch="X86_V2 X86_V4"
python -m build --wheel -Csetup-args=-Dcpu-dispatch=X86_V2,X86_V4

Feature Combination

Features specified in options are automatically combined with all implied features

python -m build --wheel -Csetup-args=-Dcpu-baseline=X86_V4

Equivalent to

python -m build --wheel -Csetup-args=-Dcpu-baseline="X86_V2 X86_V3 X86_V4"

Baseline Overlapping

Features specified in cpu-baseline will be excluded from the cpu-dispatch features, along with their implied features, but without excluding successor features that imply them.

For instance, if you specify cpu-baseline="X86_V4", it will exclude X86_V4 and its implied features X86_V2 and X86_V3 from the cpu-dispatch features.

Compile-time Detection

Specifying features to cpu-dispatch or cpu-baseline doesn't explicitly enable them. Features are detected at compile time, and the maximum available features based on your specified options will be enabled according to toolchain and platform support.

This detection occurs by testing feature availability in the compiler through compile-time source files containing common intrinsics for the specified features. If both the compiler and assembler support the feature, it will be enabled.

For example, if you specify cpu-dispatch="AVX512_ICL" but your compiler doesn't support it, the feature will be excluded from the build. However, any implied features will still be enabled if they're supported.

Platform differences

Some exceptional conditions force us to link some features together when it come to certain compilers or architectures, resulting in the impossibility of building them separately.

These conditions can be divided into two parts, as follows:

Architectural compatibility

The need to align certain CPU features that are assured to be supported by successive generations of the same architecture, some cases:

• On ppc64le VSX(ISA 2.06) and VSX2(ISA 2.07) both imply one another since the first generation that supports little-endian mode is Power-8(ISA 2.07)

• On AArch64 NEON NEON_FP16 NEON_VFPV4 ASIMD implies each other since they are part of the hardware baseline.

For example

# On ARMv8/A64, specify NEON is going to enable Advanced SIMD
# and all predecessor extensions
python -m build --wheel -Csetup-args=-Dcpu-baseline=neon
# which is equivalent to
python -m build --wheel -Csetup-args=-Dcpu-baseline="neon neon_fp16 neon_vfpv4 asimd"

Build report

In most cases, the CPU build options do not produce any fatal errors that lead to hanging the build. Most of the errors that may appear in the build log serve as heavy warnings due to the lack of some expected CPU features by the compiler.

So we strongly recommend checking the final report log, to be aware of what kind of CPU features are enabled and what are not.

You can find the final report of CPU optimizations by tracing meson build log, and here is how it looks on x86_64/gcc:

Runtime Dispatch

Importing NumPy triggers a scan of the available CPU features from the set of dispatchable features. You can restrict this scan by setting the environment variable NPY_DISABLE_CPU_FEATURES to a comma-, tab-, or space-separated list of features to disable.

For instance, on x86_64 this will disable X86_V4

NPY_DISABLE_CPU_FEATURES="X86_V4"

This will raise an error if parsing fails or if the feature was not enabled through the cpu-dispatch build option. If the feature is supported by the build but not available on the current CPU, a warning will be emitted instead.

Tracking Dispatched Functions

You can discover which CPU targets are enabled for different optimized functions using the Python function numpy.lib.introspect.opt_func_info.

This function offers two optional arguments for filtering results:

• func_name - For refining function names

• signature - For specifying data types in the signatures

For example

 >> func_info = numpy.lib.introspect.opt_func_info(func_name='add|abs', signature='float64|complex64')
 >> print(json.dumps(func_info, indent=2))
 {
  "absolute": {
    "dd": {
      "current": "baseline(X86_V2)",
      "available": "baseline(X86_V2)"
    },
    "Ff": {
      "current": "X86_V3",
      "available": "X86_V3 baseline(X86_V2)"
    },
    "Dd": {
      "current": "X86_V3",
      "available": "X86_V3 baseline(X86_V2)"
    }
  },
  "add": {
    "ddd": {
      "current": "X86_V3",
      "available": "X86_V3 baseline(X86_V2)"
    },
    "FFF": {
      "current": "X86_V3",
      "available": "X86_V3 baseline(X86_V2)"
    }
  }
}