README.md - external/github.com/google/cpu_features.git - Git at Google

 # cpu_features [![Build Status](https://travis-ci.org/google/cpu_features.svg?branch=master)](https://travis-ci.org/google/cpu_features) [![Build status](https://ci.appveyor.com/api/projects/status/46d1owsj7n8dsylq/branch/master?svg=true)](https://ci.appveyor.com/project/gchatelet/cpu-features/branch/master)

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

 ## Table of Contents

 - [Design Rationale](#rationale)
 - [Code samples](#codesample)
 - [Running sample code](#usagesample)
 - [Mocking the library](#mock)
 - [What's supported](#support)
 - [Android NDK's drop in replacement](#ndk)
 - [License](#license)
 - [Build with cmake](#cmake)

 <a name="rationale"></a>
 ## 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.**

 <a name="codesample"></a>
 ### 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:

 ```c
 #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.

 ```c
 #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.

 ```c
 #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&mdash;but only if it's not Sandy Bridge.

 ```c
 #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.

 <a name="usagesample"></a>
 ### Running sample code

 Building `cpu_features` brings a small executable to test the library.

 ```shell
  % ./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
 ```

 ```shell
 % ./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"]}
 ```

 <a name="mock"></a>
 ### Mocking the library

 When testing code depending on `cpu_features` it may be interesting to disable
 support for a particular extension (e.g. test `sse4` support on a `Skylake`
 machine).

 It is easily done by setting up an interceptor callback.

 ```C++
 TEST(X86Test, TestOnlySSE4) {
   RegisterX86InfoInterceptor([](X86Info* info) {
     info->features = X86Features{};
     info->features.sse4 = true;
   });
   // 1. Call code that uses GetX86Info(),
   // 2. Check the expected values,
   // 3. Don't forget to cancel.
   RegisterX86InfoInterceptor(NULL);
 }
 ```

 <a name="support"></a>
 ## 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](https://www.gnu.org/software/libc/manual/html_node/Auxiliary-Vector.html)
     +   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.

 <a name="ndk"></a>
 ## Android NDK's drop in replacement

 [cpu_features](https://github.com/google/cpu_features) is now officially
 supporting Android and offers a drop in replacement of for the NDK's [cpu-features.h](https://android.googlesource.com/platform/ndk/+/master/sources/android/cpufeatures/cpu-features.h)
 , see [ndk_compat](ndk_compat) folder for details.

 <a name="license"></a>
 ## License

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

 <a name="cmake"></a>
 ## Build with CMake

 Please check the [CMake build instructions](cmake/README.md).
	# cpu_features [![Build Status](https://travis-ci.org/google/cpu_features.svg?branch=master)](https://travis-ci.org/google/cpu_features) [![Build status](https://ci.appveyor.com/api/projects/status/46d1owsj7n8dsylq/branch/master?svg=true)](https://ci.appveyor.com/project/gchatelet/cpu-features/branch/master)

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

	## Table of Contents

	- [Design Rationale](#rationale)
	- [Code samples](#codesample)
	- [Running sample code](#usagesample)
	- [Mocking the library](#mock)
	- [What's supported](#support)
	- [Android NDK's drop in replacement](#ndk)
	- [License](#license)
	- [Build with cmake](#cmake)

	<a name="rationale"></a>
	## 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.

	<a name="codesample"></a>
	### 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:

	```c
	#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.

	```c
	#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.

	```c
	#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.

	```c
	#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.

	<a name="usagesample"></a>
	### Running sample code

	Building `cpu_features` brings a small executable to test the library.

	```shell
	% ./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
	```

	```shell
	% ./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"]}
	```

	<a name="mock"></a>
	### Mocking the library

	When testing code depending on `cpu_features` it may be interesting to disable
	support for a particular extension (e.g. test `sse4` support on a `Skylake`
	machine).

	It is easily done by setting up an interceptor callback.

	```C++
	TEST(X86Test, TestOnlySSE4) {
	RegisterX86InfoInterceptor([](X86Info* info) {
	info->features = X86Features{};
	info->features.sse4 = true;
	});
	// 1. Call code that uses GetX86Info(),
	// 2. Check the expected values,
	// 3. Don't forget to cancel.
	RegisterX86InfoInterceptor(NULL);
	}
	```

	<a name="support"></a>
	## 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](https://www.gnu.org/software/libc/manual/html_node/Auxiliary-Vector.html)
	+ 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.

	<a name="ndk"></a>
	## Android NDK's drop in replacement

	[cpu_features](https://github.com/google/cpu_features) is now officially
	supporting Android and offers a drop in replacement of for the NDK's [cpu-features.h](https://android.googlesource.com/platform/ndk/+/master/sources/android/cpufeatures/cpu-features.h)
	, see [ndk_compat](ndk_compat) folder for details.

	<a name="license"></a>
	## License

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

	<a name="cmake"></a>
	## Build with CMake

	Please check the [CMake build instructions](cmake/README.md).