aosp12/external/cpu_features

cpu_features

A cross-platform C library to retrieve CPU features (such as available instructions) at runtime.

Table of Contents

• Design Rationale
• Code samples
• Running sample code
• What's supported
• Android NDK's drop in replacement
• License
• Build with cmake

Design Rationale

• Simple to use. See the snippets below for examples.
• Extensible. Easy to add missing features or architectures.
• Compatible with old compilers and available on many architectures so it can be used widely. To ensure that cpu_features works on as many platforms as possible, we implemented it in a highly portable version of C: C99.
• Sandbox-compatible. The library uses a variety of strategies to cope with sandboxed environments or when cpuid is unavailable. This is useful when running integration tests in hermetic environments.
• Thread safe, no memory allocation, and raises no exceptions. cpu_features is suitable for implementing fundamental libc functions like malloc, memcpy, and memcmp.
• Unit tested.

Code samples

Note: For C++ code, the library functions are defined in the CpuFeatures namespace.

Checking features at runtime

Here's a simple example that executes a codepath if the CPU supports both the AES and the SSE4.2 instruction sets:

#include "cpuinfo_x86.h"

// For C++, add `using namespace CpuFeatures;`
static const X86Features features = GetX86Info().features;

void Compute(void) {
  if (features.aes && features.sse4_2) {
    // Run optimized code.
  } else {
    // Run standard code.
  }
}

Caching for faster evaluation of complex checks

If you wish, you can read all the features at once into a global variable, and then query for the specific features you care about. Below, we store all the ARM features and then check whether AES and NEON are supported.

#include <stdbool.h>
#include "cpuinfo_arm.h"

// For C++, add `using namespace CpuFeatures;`
static const ArmFeatures features = GetArmInfo().features;
static const bool has_aes_and_neon = features.aes && features.neon;

// use has_aes_and_neon.

This is a good approach to take if you're checking for combinations of features when using a compiler that is slow to extract individual bits from bit-packed structures.

Checking compile time flags

The following code determines whether the compiler was told to use the AVX instruction set (e.g., g++ -mavx) and sets has_avx accordingly.

#include <stdbool.h>
#include "cpuinfo_x86.h"

// For C++, add `using namespace CpuFeatures;`
static const X86Features features = GetX86Info().features;
static const bool has_avx = CPU_FEATURES_COMPILED_X86_AVX || features.avx;

// use has_avx.

CPU_FEATURES_COMPILED_X86_AVX is set to 1 if the compiler was instructed to use AVX and 0 otherwise, combining compile time and runtime knowledge.

Rejecting poor hardware implementations based on microarchitecture

On x86, the first incarnation of a feature in a microarchitecture might not be the most efficient (e.g. AVX on Sandy Bridge). We provide a function to retrieve the underlying microarchitecture so you can decide whether to use it.

Below, has_fast_avx is set to 1 if the CPU supports the AVX instruction set—but only if it's not Sandy Bridge.

#include <stdbool.h>
#include "cpuinfo_x86.h"

// For C++, add `using namespace CpuFeatures;`
static const X86Info info = GetX86Info();
static const X86Microarchitecture uarch = GetX86Microarchitecture(&info);
static const bool has_fast_avx = info.features.avx && uarch != INTEL_SNB;

// use has_fast_avx.

This feature is currently available only for x86 microarchitectures.

Running sample code

Building cpu_features (check quickstart below) brings a small executable to test the library.

 % ./build/list_cpu_features
arch            : x86
brand           :        Intel(R) Xeon(R) CPU E5-1650 0 @ 3.20GHz
family          :   6 (0x06)
model           :  45 (0x2D)
stepping        :   7 (0x07)
uarch           : INTEL_SNB
flags           : aes,avx,cx16,smx,sse4_1,sse4_2,ssse3

% ./build/list_cpu_features --json
{"arch":"x86","brand":"       Intel(R) Xeon(R) CPU E5-1650 0 @ 3.20GHz","family":6,"model":45,"stepping":7,"uarch":"INTEL_SNB","flags":["aes","avx","cx16","smx","sse4_1","sse4_2","ssse3"]}

What's supported

	x86³	ARM	AArch64	MIPS⁴	POWER
Android	yes²	yes¹	yes¹	yes¹	N/A
iOS	N/A	not yet	not yet	N/A	N/A
Linux	yes²	yes¹	yes¹	yes¹	yes¹
MacOs	yes²	N/A	not yet	N/A	no
Windows	yes²	not yet	not yet	N/A	N/A

1. Features revealed from Linux. We gather data from several sources depending on availability:
   • from glibc's getauxval
   • by parsing /proc/self/auxv
   • by parsing /proc/cpuinfo
2. Features revealed from CPU. features are retrieved by using the cpuid instruction.
3. Microarchitecture detection. On x86 some features are not always implemented efficiently in hardware (e.g. AVX on Sandybridge). Exposing the microarchitecture allows the client to reject particular microarchitectures.
4. All flavors of Mips are supported, little and big endian as well as 32/64 bits.

Android NDK's drop in replacement

cpu_features is now officially supporting Android and offers a drop in replacement of for the NDK's cpu-features.h , see ndk_compat folder for details.

License

The cpu_features library is licensed under the terms of the Apache license. See LICENSE for more information.

Build with CMake

Please check the CMake build instructions.

Quickstart with Ninja

• build list_cpu_features

    cmake -B/tmp/cpu_features -H. -GNinja -DCMAKE_BUILD_TYPE=Release
    ninja -C/tmp/cpu_features
    /tmp/cpu_features/list_cpu_features --json

• run tests

    cmake -B/tmp/cpu_features -H. -GNinja -DBUILD_TESTING=ON
    ninja -C/tmp/cpu_features
    ninja -C/tmp/cpu_features test