commit	439d371594c49d35b66493c8d2c33f2d16144f6d	[log] [tgz]
author	Guillaume Chatelet <gchatelet@google.com>	Thu Feb 01 10:03:09 2018 +0100
committer	Guillaume Chatelet <gchatelet@google.com>	Thu Feb 01 10:03:09 2018 +0100
tree	4d5c0ac7ab27b7730ea944822786de5850f29d89
parent	44d56a0a28ea6fee0cdeedb6823cca50d39e5209 [diff]

tree: 4d5c0ac7ab27b7730ea944822786de5850f29d89

README.md

cpu features

A cross platform C89 library to get cpu features at runtime.

Checking features at runtime.

#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.

Features are bit packed and some compilers generate poor code when combining them. A simple solution is to read them once and store the result in a global variable.

#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.

Checking compile time flags as well.

#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 is instructed to use AVX, 0 otherwise. This allows combining compile time and runtime knowledge.

Use x86 microarchitecture to reject poor hardware implementations.

#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.

On x86, the first incarnation of a feature in a microarchitecture may not be very efficient (e.g. AVX on Sandybridge). We provide a function to retrieve the underlying microarchitecture so clients can decide whether they want to use it or not.

What does it currently support

	x86	ARM	aarch64	mips	POWER
Features From cpu	yes	no*	no*	no yet	not yet
Features From Linux	no	yes	yes	yes	not yet
Micro Architecture Detection	yes	no	no	no	not yet
Windows support	yes	no	no	no	not yet

Features From Cpuid: features are retrieved by using the cpuid instruction. (*) Unfortunately this instruction is privileged for some architectures; in this case we fall back to Linux.
Features From Linux: we gather data from several sources depending on what's available:
- from glibc's getauxval
- by parsing /proc/self/auxv
- by parsing /proc/cpuinfo
Micro Architecture 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 some microarchitectures.

Design Rationale

Simple to use, API should be straightforward from the snippets above.
Unit tested.
Extensible. It should be easy to add missing features or architectures.
Compatible with old compilers and available on many architectures so it can be used widely. We target gnu89.
Works in sandboxed environment: some architectures rely on parsing files that may not be available in a sandboxed environment. It is useful when running integration tests in hermetic environments.
Thread safe, no allocation, no exception: suitable for implementation of fundamental libc functions like malloc, memcpy, memcmp...