目录

CPU Features

CPU features are hardware extensions provided by CPUs. You can call instructions and set special registers to use the CPU features, such as VFPv3-D32, NEON, IDIV, and AES on the ARMv7-A architecture. Many CPU features are optional, and different CPU vendors provide different CPU features.

How can you use CPU features in HarmonyOS native library development? How can you handle CPU feature-related code? This section provides methods to help you develop apps that maintain compatibility while leveraging CPU feature capabilities.

Currently, HarmonyOS does not provide APIs for obtaining CPU features. You can import library functions of the existing open source community as described below. Alternatively, you can directly read /proc/cpuinfo or call libc getauxval(AT_HWCAP) to obtain the CPU features of a device.

How to Use

  1. Introduce the open-source library to the HarmonyOS C++ project. Download the cpu_features library, decompress it to the cpp directory of the project, and change the decompressed file name to cpu_features. The following shows a sample application of the DevEco Studio C++ template.

    • The directory structure after the decompression is as follows:
      MyApplication
  |-- entry
   |-- src
        |-- main
             |-- cpp
                 |--cpu_features
                 |--CMakeLists.txt
                 |--hello.cpp
    • Add the path to the cpu_features library to the CMakeLists.txt file.
      add_subdirectory(cpu_features) # Add a subdirectory to the project.
target_link_libraries(entry PUBLIC cpu_features) # Add the library files that need to be linked.

  2. Add the statement for determining the support for the CPU features to the code. The following uses the ARM and AArch64 architectures as an example:

    // ...
// Include the header file for CPU architecture target detection.
#include "cpu_features_macros.h"
// In the ARM architecture, this macro is automatically defined in the preceding header file based on the target.
#if defined (CPU_FEATURES_ARCH_ARM)
#include "cpuinfo_arm.h" // Include the cpuinfo header file of the ARM architecture.
#elif defined (CPU_FEATURES_ARCH_AARCH64)
#include "cpuinfo_aarch64.h" // Include the cpuinfo header file of the AArch64 architecture.
#endif
// Add `using namespace cpu_features;` in the C++ code.
// Obtain CPU feature information based on the architecture in use.
#if defined (CPU_FEATURES_ARCH_ARM)
static const ArmFeatures features = GetArmInfo().features;
#elif defined (CPU_FEATURES_ARCH_AARCH64)
static const Aarch64Features features = GetAarch64Info().features;
#endif
void Compute(void) {
  // Determine whether the CPU features are supported based on the features field.
  if (features.aes) {
    // ...
  } else {
    // ...
  }
}

    The cpu_features library reads the corresponding CPU feature information from the kernel through /proc/cpuinfo or getauxval. The information may fail to be read due to the use of the application sandbox. Pay attention to error handling.