CMake 3.18中的新功能

野性酷女 2022-12-11 07:54 195阅读 0赞

本文翻译自[New features in CMake 3.18][]  
原文作者：Cristian Adam，Qt公司高级软件工程师  
翻译校审：Haipeng Yu

7月15日Kitware发布了CMake 3.18。其[发布说明][Link 1]包含了更新列表。

以下罗列那些可提升Qt开发者使用CMake体验的更新。

###  ###

### 支持搜集分析功能（Profiling support） ###

过去很难搜集CMake配置项目消耗的时间信息。现在有了支持搜集分析功能，可以很方便找出这些信息。

下面的命令行参数(可在”*Initial CMake参数*”项目设置中传递到Qt Creator 4.13)

--profiling-format=google-trace
    --profiling-output=qtcreator-cmake.json

将生成一个*qtcreator-cmake.json*的文件(大小为40 MiB，供Qt Creator使用)，可以通过*Chrome Trace Format Viewer*加载进Qt Creator。

看起来像这样:

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L1F0X0NoaW5h_size_16_color_FFFFFF_t_70][]

这个功能可在Kitware的[!2760][2760]中查看。

###    预编译头文件 ###

CMake 3.16支持预编译头文件，但之后加入CMake的功能都取消了对预编译头文件的支持。

CMake 3.18修复了以下用例：

*  iOS 多架构 ([\#20497][20497])
 *  基于MSVC的Ninja多功能配置 ([\#20711][20711])
 *  基于MSVC的Ninja多配置和可复用PCH支持 ([\#20721][20721])

Cmake 3.18将在生成源步骤（PCH，Unity）运行之后运行*AUTOMOC*步骤。之前的版本是反过来的。

这修复了[＃20119][20119]：需要Qt Creator让CMake配置项目两次作为变通方案。

缺点是如果您将*Q\_OBJECT*宏和*AUTOMOC*设置为ON，那么只有头文件的库在使用CMake 3.18时将无法编译。更多相关细节，请参阅[\#20980][20980]和[\#20968][20968]。

要解决这个问题，需要向这些库添加一个虚拟源文件:

file(GENERATE
         OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/empty_pch.cxx
         CONTENT "/*empty file*/")
    target_sources(CommonPCH_Lib PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/empty_pch.cxx)

###    Unity构建 ###

添加了*UNITY\_BUILD\_MODE*目标属性以告诉生成器使用哪种算法对包含的源文件进行分组。

CMake 3.16默认*BATCH*模式，新添加的CMake 3.18默认是*GROUP*模式。

add_library(example_library
                source1.cxx
                source2.cxx
                source3.cxx
                source4.cxx)
    
    set_target_properties(example_library PROPERTIES
                          UNITY_BUILD_MODE BATCH
                          UNITY_BUILD_BATCH_SIZE 2)

假设*source3.cxx*和*source4.cxx*之间存在命名冲突，并且他们在同一batch中，我们需要创造方法去解决这个问题。

相比仅具有*source4.cxx*的对象库,或更改源文件的顺序(*source4.cxx, source1.cxx, source2.cxx, source3.cxx*)，我们现在可以这样做：

add_library(example_library
                source1.cxx
                source2.cxx
                source3.cxx
                source4.cxx)
    
    set_target_properties(example_library PROPERTIES
                          UNITY_BUILD_MODE GROUP)
    
    set_source_files_properties(source1.cxx source2.cxx source3.cxx
                                PROPERTIES UNITY_GROUP "bucket1")
    set_source_files_properties(source4.cxx
                                PROPERTIES UNITY_GROUP "bucket2")

这个功能可在Kitware的[!4716][4716]中查看。

###    更少的编译器检查 ###

下面给出了C++的“Hello World”的CMakeLists.txt：

cmake_minimum_required(VERSION 3.17)
    
    project(hello)
    
    add_executable(hello hello.cpp)

当使用CMake 3.17进行配置时，我们会得到如下输出：

$ cmake -E time cmake -G Ninja -S . -B build
    -- The C compiler identification is MSVC 19.25.28614.0
    -- The CXX compiler identification is MSVC 19.25.28614.0
    -- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe
    -- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe - works
    -- Detecting C compiler ABI info
    -- Detecting C compiler ABI info - done
    -- Detecting C compile features
    -- Detecting C compile features - done
    -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe
    -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe - works
    -- Detecting CXX compiler ABI info
    -- Detecting CXX compiler ABI info - done
    -- Detecting CXX compile features
    -- Detecting CXX compile features - done
    -- Configuring done
    -- Generating done
    -- Build files have been written to: C:/Projects/c++/HelloWorld/build
    Elapsed time: 4 s. (time), 3.825 s. (clock)

使用CMake 3.18，我们会得到如下输出：

$ cmake -E time cmake -G Ninja -S . -B build
    -- The C compiler identification is MSVC 19.25.28614.0
    -- The CXX compiler identification is MSVC 19.25.28614.0
    -- Detecting C compiler ABI info
    -- Detecting C compiler ABI info - done
    -- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe - skipped
    -- Detecting C compile features
    -- Detecting C compile features - done
    -- Detecting CXX compiler ABI info
    -- Detecting CXX compiler ABI info - done
    -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe - skipped
    -- Detecting CXX compile features
    -- Detecting CXX compile features - done
    -- Configuring done
    -- Generating done
    -- Build files have been written to: C:/Projects/c++/HelloWorld/build
    Elapsed time: 2 s. (time), 2.029 s. (clock)

这是由于[!4789 enable\_language: Assume compiler works if ABI detection compiles！][4789 enable_language_ Assume compiler works if ABI detection compiles]

CMake现在只执行两个try\_compile，而不是四个，默认语言*C*和*CXX*各一次。

通过如下方法：

cmake_minimum_required(VERSION 3.17)
    
    project(hello LANGUAGES CXX)
    
    add_executable(hello hello.cpp)

我们只需有一个try\_compile：

$ cmake -E time cmake -G Ninja -S . -B build
    -- The CXX compiler identification is MSVC 19.25.28614.0
    -- Detecting CXX compiler ABI info
    -- Detecting CXX compiler ABI info - done
    -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.25.28610/bin/Hostx64/x64/cl.exe - skipped
    -- Detecting CXX compile features
    -- Detecting CXX compile features - done
    -- Configuring done
    -- Generating done
    -- Build files have been written to: C:/Projects/c++/HelloWorld/build
    Elapsed time: 1 s. (time), 1.222 s. (clock)

这对于使用Qt Creator的CMake编译Qt 6的情况会有所帮助。

###    Qt 6加速功能 ###

以下功能将使Qt6 CMake代码更加精简和快速:

*  创建*file(CONFIGURE)*子命令是为了复制*configure\_file()*功能，而不用利用磁盘上已存在的文件作为输入。而是将内容作为字符串传递。[\# 20388][20388]
 *  添加了*cmake\_language()*命令，用于对脚本或内置命令进行元操作。该命令以*CALL*其他命令的模式开始，并使用*EVAL CODE*就地评估CMake脚本。[\# 18392][18392]，[!4408][4408]
 *  具有SOURCE范围的*set\_property（）*命令获得了*DIRECTORY*和*TARGET\_DIRECTORY*选项，以在提供的目录范围中设置属性。[\#20128][20128]
 *  *file()*命令获得了*ARCHIVE\_CREATE*和*ARCHIVE\_EXTRACT*子命令，以便将*cmake(1) -E tar*功能公开给cmake脚本。[\# 20443][20443]

[New features in CMake 3.18]: https://www.qt.io/blog/new-features-in-cmake-3.18
[Link 1]: https://cmake.org/cmake/help/v3.18/release/3.18.html
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L1F0X0NoaW5h_size_16_color_FFFFFF_t_70]: /images/20221123/1b0fcb1473e7423ea804cb6a60c25bf5.png
[2760]: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/2760
[20497]: https://gitlab.kitware.com/cmake/cmake/-/issues/20497
[20711]: https://gitlab.kitware.com/cmake/cmake/-/issues/20711
[20721]: https://gitlab.kitware.com/cmake/cmake/-/issues/20721
[20119]: https://gitlab.kitware.com/cmake/cmake/-/issues/20119
[20980]: https://gitlab.kitware.com/cmake/cmake/-/issues/20980
[20968]: https://gitlab.kitware.com/cmake/cmake/-/issues/20968
[4716]: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4716
[4789 enable_language_ Assume compiler works if ABI detection compiles]: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4789
[20388]: https://gitlab.kitware.com/cmake/cmake/-/issues/20388
[18392]: https://gitlab.kitware.com/cmake/cmake/-/issues/18392
[4408]: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4408
[20128]: https://gitlab.kitware.com/cmake/cmake/-/issues/20128
[20443]: https://gitlab.kitware.com/cmake/cmake/-/issues/20443