commit	36df25b7bc4971f5a35da1f202fcd21033a066b9	[log] [tgz]
author	Guillaume Chatelet <gchatelet@google.com>	Tue Feb 13 14:19:25 2018 +0100
committer	Guillaume Chatelet <gchatelet@google.com>	Tue Feb 13 14:19:25 2018 +0100
tree	731abe6b4a27c97a60f2b1ef2d3f712e6c05a60f
parent	f7e0a6d066a8076512c1b82ad64c1a8ca1e27b57 [diff]

tree: 731abe6b4a27c97a60f2b1ef2d3f712e6c05a60f

README.md

cpu_features

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

Design Rationale
Code samples
Running sample code
What's supported
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.

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"

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"

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"

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"

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 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, sse4_1, sse4_2, ssse3

What's supported

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

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
Features revealed from CPU. features are retrieved by using the cpuid instruction.
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.

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.