NOTE:此示例代码可以在 https://github.com/dev-cafe/cmake-cookbook/tree/v1.0/chapter-02/recipe-05 中找到，包含一个C++示例。该示例在CMake 3.10版(或更高版本)中是有效的，并且已经在GNU/Linux、macOS和Windows上进行过测试。

本示例中，我们将讨论如何在CMake的帮助下检测主机处理器支持的指令集。这个功能是较新版本添加到CMake中的，需要CMake 3.10或更高版本。检测到的主机系统信息，可用于设置相应的编译器标志，或实现可选的源代码编译，或根据主机系统生成源代码。本示例中，我们的目标是检测主机系统信息，使用预处理器定义将其传递给C++源代码，并将信息打印到输出中。

准备工作

我们是C++源码(processor-info.cpp)如下所示：

  1. #include "config.h"
  3. #include <cstdlib>
  4. #include <iostream>
  6. int main()
  7. {
  8.   std::cout << "Number of logical cores: "
  9.             << NUMBER_OF_LOGICAL_CORES << std::endl;
  10.   std::cout << "Number of physical cores: "
  11.             << NUMBER_OF_PHYSICAL_CORES << std::endl;
  12.   std::cout << "Total virtual memory in megabytes: "
  13.             << TOTAL_VIRTUAL_MEMORY << std::endl;
  14.   std::cout << "Available virtual memory in megabytes: "
  15.             << AVAILABLE_VIRTUAL_MEMORY << std::endl;
  16.   std::cout << "Total physical memory in megabytes: "
  17.             << TOTAL_PHYSICAL_MEMORY << std::endl;
  18.   std::cout << "Available physical memory in megabytes: "
  19.             << AVAILABLE_PHYSICAL_MEMORY << std::endl;
  20.   std::cout << "Processor is 64Bit: "
  21.             << IS_64BIT << std::endl;
  22.   std::cout << "Processor has floating point unit: "
  23.             << HAS_FPU << std::endl;
  24.   std::cout << "Processor supports MMX instructions: "
  25.             << HAS_MMX << std::endl;
  26.   std::cout << "Processor supports Ext. MMX instructions: "
  27.             << HAS_MMX_PLUS << std::endl;
  28.   std::cout << "Processor supports SSE instructions: "
  29.             << HAS_SSE << std::endl;
  30.   std::cout << "Processor supports SSE2 instructions: "
  31.             << HAS_SSE2 << std::endl;
  32.   std::cout << "Processor supports SSE FP instructions: "
  33.             << HAS_SSE_FP << std::endl;
  34.   std::cout << "Processor supports SSE MMX instructions: "
  35.             << HAS_SSE_MMX << std::endl;
  36.   std::cout << "Processor supports 3DNow instructions: "
  37.             << HAS_AMD_3DNOW << std::endl;
  38.   std::cout << "Processor supports 3DNow+ instructions: "
  39.             << HAS_AMD_3DNOW_PLUS << std::endl;
  40.   std::cout << "IA64 processor emulating x86 : "
  41.             << HAS_IA64 << std::endl;
  42.   std::cout << "OS name: "
  43.             << OS_NAME << std::endl;
  44.   std::cout << "OS sub-type: "
  45.             << OS_RELEASE << std::endl;
  46.   std::cout << "OS build ID: "
  47.             << OS_VERSION << std::endl;
  48.   std::cout << "OS platform: "
  49.             << OS_PLATFORM << std::endl;
  50.   return EXIT_SUCCESS;
  51. }

其包含config.h头文件，我们将使用config.h.in生成这个文件。config.h.in如下:

  1. #pragma once
  3. #define NUMBER_OF_LOGICAL_CORES @_NUMBER_OF_LOGICAL_CORES@
  4. #define NUMBER_OF_PHYSICAL_CORES @_NUMBER_OF_PHYSICAL_CORES@
  5. #define TOTAL_VIRTUAL_MEMORY @_TOTAL_VIRTUAL_MEMORY@
  6. #define AVAILABLE_VIRTUAL_MEMORY @_AVAILABLE_VIRTUAL_MEMORY@
  7. #define TOTAL_PHYSICAL_MEMORY @_TOTAL_PHYSICAL_MEMORY@
  8. #define AVAILABLE_PHYSICAL_MEMORY @_AVAILABLE_PHYSICAL_MEMORY@
  9. #define IS_64BIT @_IS_64BIT@
  10. #define HAS_FPU @_HAS_FPU@
  11. #define HAS_MMX @_HAS_MMX@
  12. #define HAS_MMX_PLUS @_HAS_MMX_PLUS@
  13. #define HAS_SSE @_HAS_SSE@
  14. #define HAS_SSE2 @_HAS_SSE2@
  15. #define HAS_SSE_FP @_HAS_SSE_FP@
  16. #define HAS_SSE_MMX @_HAS_SSE_MMX@
  17. #define HAS_AMD_3DNOW @_HAS_AMD_3DNOW@
  18. #define HAS_AMD_3DNOW_PLUS @_HAS_AMD_3DNOW_PLUS@
  19. #define HAS_IA64 @_HAS_IA64@
  20. #define OS_NAME "@_OS_NAME@"
  21. #define OS_RELEASE "@_OS_RELEASE@"
  22. #define OS_VERSION "@_OS_VERSION@"
  23. #define OS_PLATFORM "@_OS_PLATFORM@"

如何实施

我们将使用CMake为平台填充config.h中的定义，并将示例源文件编译为可执行文件:

  1. 首先，我们定义了CMake最低版本、项目名称和项目语言:
    1. cmake_minimum_required(VERSION 3.10 FATAL_ERROR)
    2. project(recipe-05 CXX)
  1. 然后，定义目标可执行文件及其源文件，并包括目录: ```cmake add_executable(processor-info “”)

target_sources(processor-info PRIVATE processor-info.cpp )

target_include_directories(processor-info PRIVATE ${PROJECT_BINARY_DIR} )



3. 
继续查询主机系统的信息，获取一些关键字:
```cmake
foreach(key
  IN ITEMS
    NUMBER_OF_LOGICAL_CORES
    NUMBER_OF_PHYSICAL_CORES
    TOTAL_VIRTUAL_MEMORY
    AVAILABLE_VIRTUAL_MEMORY
    TOTAL_PHYSICAL_MEMORY
    AVAILABLE_PHYSICAL_MEMORY
    IS_64BIT
    HAS_FPU
    HAS_MMX
    HAS_MMX_PLUS
    HAS_SSE
    HAS_SSE2
    HAS_SSE_FP
    HAS_SSE_MMX
    HAS_AMD_3DNOW
    HAS_AMD_3DNOW_PLUS
    HAS_IA64
    OS_NAME
    OS_RELEASE
    OS_VERSION
    OS_PLATFORM
  )
  cmake_host_system_information(RESULT _${key} QUERY ${key})
endforeach()
  1. 定义了相应的变量后，配置config.h:
    configure_file(config.h.in config.h @ONLY)
  1. 现在准备好配置、构建和测试项目: ```shell $ mkdir -p build $ cd build $ cmake .. $ cmake —build . $ ./processor-info

Number of logical cores: 4 Number of physical cores: 2 Total virtual memory in megabytes: 15258 Available virtual memory in megabytes: 14678 Total physical memory in megabytes: 7858 Available physical memory in megabytes: 4072 Processor is 64Bit: 1 Processor has floating point unit: 1 Processor supports MMX instructions: 1 Processor supports Ext. MMX instructions: 0 Processor supports SSE instructions: 1 Processor supports SSE2 instructions: 1 Processor supports SSE FP instructions: 0 Processor supports SSE MMX instructions: 0 Processor supports 3DNow instructions: 0 Processor supports 3DNow+ instructions: 0 IA64 processor emulating x86 : 0 OS name: Linux OS sub-type: 4.16.7-1-ARCH OS build ID: #1 SMP PREEMPT Wed May 2 21:12:36 UTC 2018 OS platform: x86_64 ```

  1. 输出会随着处理器的不同而变化。

工作原理

CMakeLists.txt中的foreach循环会查询多个键值，并定义相应的变量。此示例的核心函数是cmake_host_system_information，它查询运行CMake的主机系统的系统信息。本例中，我们对每个键使用了一个函数调用。然后，使用这些变量来配置config.h.in中的占位符，输入并生成config.h。此配置使用configure_file命令完成。最后，config.h包含在processor-info.cpp中。编译后，它将把值打印到屏幕上。我们将在第5章(配置时和构建时操作)和第6章(生成源代码)中重新讨论这种方法。

更多信息

对于更细粒度的处理器指令集检测，请考虑以下模块: https://github.com/VcDevel/Vc/blob/master/cmake/OptimizeForArchitecture.cmake 。有时候，构建代码的主机可能与运行代码的主机不一样。在计算集群中，登录节点的体系结构可能与计算节点上的体系结构不同。解决此问题的一种方法是，将配置和编译作为计算步骤，提交并部署到相应计算节点上。