C++ 默认在 CMake 中优化
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Optimize in CMake by default
提问by marmistrz
I have a C++ project which uses CMake as its build system. I'd like the following behavior:
我有一个 C++ 项目，它使用 CMake 作为其构建系统。我想要以下行为：
If cmake is invoked as cmake .., then CMAKE_CXX_FLAGSis -O3 -Wall -Wextra
如果 cmake 被调用为cmake ..，则CMAKE_CXX_FLAGS是-O3 -Wall -Wextra
If cmake is invoked as cmake .. -DCMAKE_BUILD_TYPE=Debug, then CMAKE_CXX_FLAGSis -g -Wall -Wextra
如果 cmake 被调用为cmake .. -DCMAKE_BUILD_TYPE=Debug，则CMAKE_CXX_FLAGS是-g -Wall -Wextra
I tried the following
我尝试了以下
message(STATUS "Build type: ${CMAKE_BUILD_TYPE}")
set(CMAKE_CXX_FLAGS "-O3 -Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g -Wall -Wextra")
But this has a big problem. First of all, if the second invocation is used, then both -O3and -gflags are passed to the compiler. Besides, if I use the second invocation and the first thereafter, CMAKE_BUILD_TYPEstays Debugalthough not explicitly ordered so - so I get a Debug build although I want an optimized build.
但这有一个很大的问题。首先，如果使用第二次调用，则-O3和-g标志都将传递给编译器。此外，如果我使用第二次调用和此后的第一次调用，尽管没有明确排序，但CMAKE_BUILD_TYPE仍然保持不变Debug- 所以尽管我想要一个优化的构建，但我得到了一个调试构建。
Why? What can I do to get the desired behavior?
为什么？我该怎么做才能获得所需的行为？
回答by Angew is no longer proud of SO
First off: recommended usage of CMake is to always specify CMAKE_BUILD_TYPEexplicitly on the command line (if and only if using a single-configuration generator). Your use case deviates from this best practice, so treat this answer as "how you can do it," not necessarily as "how you should do it."
首先：推荐的 CMake 用法是始终CMAKE_BUILD_TYPE在命令行上明确指定（当且仅当使用单配置生成器时）。您的用例偏离了此最佳实践，因此将此答案视为“您可以如何做”，而不一定是“您应该如何做”。
To address the first issue, you should be able to do this early in your CMakeList:
要解决第一个问题，您应该能够在 CMakeList 的早期执行此操作：
if(NOT CMAKE_BUILD_TYPE)
set(CMAKE_BUILD_TYPE Release)
endif()
set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")
This will make sure that if you do not specify a build type at all, it will default to "Release" and thus CMAKE_CXX_FLAGS_RELEASEwill be used.
这将确保如果您根本不指定构建类型，它将默认为“发布”并因此CMAKE_CXX_FLAGS_RELEASE将被使用。
The second one is harder to tackle. Variables passed from the command line (such as CMAKE_BUILD_TYPE=Debug) are cachedby CMake and thus re-used in subsequent invocations (that is necessary, since CMake can re-trigger itself if you modify its inputs between builds).
第二个更难解决。从命令行传递的变量（例如CMAKE_BUILD_TYPE=Debug）由 CMake缓存，因此在后续调用中重新使用（这是必要的，因为如果您在构建之间修改其输入，CMake 可以重新触发自身）。
The only solution is to make the user switch the build type explicitly again, using cmake .. -DCMAKE_BUILD_TYPE=Release.
唯一的解决方案是让用户再次显式切换构建类型，使用cmake .. -DCMAKE_BUILD_TYPE=Release.
Consider why this is necessary: as I said, CMake can re-trigger itself as part of a build if CMake's input (CMakeLists.txtfiles or their dependencies) has changed since last CMake ran. In such case, it will also be run without command-line arguments such as -DCMAKE_BUILD_TYPE=whatever, and will rely on the cache to supply the same value as last time. This scenario is indistinguishable from you manually running cmake ..without additional arguments.
考虑为什么这是必要的：正如我所说，如果 CMake 的输入（CMakeLists.txt文件或其依赖项）自上次 CMake 运行以来发生了变化，CMake 可以重新触发自身作为构建的一部分。在这种情况下，它也将在没有命令行参数的情况下运行，例如-DCMAKE_BUILD_TYPE=whatever, 并将依赖缓存提供与上次相同的值。这种情况与您在cmake ..没有其他参数的情况下手动运行没有区别。
I could provide a hacky solution to always reset CMAKE_BUILD_TYPEto Releaseif not specified explicitly on the command line. However, it would also mean that a buildsystem generated as Debugwould get re-generated as Releaseif automatic re-generation happened. I am pretty sure that's not what you want.
如果未在命令行中明确指定，我可以提供一个 hacky 解决方案，以始终重置CMAKE_BUILD_TYPE为Release。但是，这也意味着生成的构建系统Debug会重新生成，Release就像自动重新生成发生一样。我很确定这不是你想要的。
回答by roalz
For CXX flags specific for Release target, you should set
CMAKE_CXX_FLAGS_RELEASE
instead of
CMAKE_CXX_FLAGS
对于特定于发布目标的 CXX 标志，您应该设置
CMAKE_CXX_FLAGS_RELEASE
而不是
CMAKE_CXX_FLAGS
In your case you can use:
在您的情况下，您可以使用：
set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O3")
设置（CMAKE_CXX_FLAGS“-Wall -Wextra”）
设置（CMAKE_CXX_FLAGS_DEBUG“-g”）
设置（CMAKE_CXX_FLAGS_RELEASE“-O3”）
A more modern CMake approach (which I suggest, if you are using CMake version 2.8.12 or newer), is well described in this StackOverflow answerand involves the use of target_compile_options.
更现代的 CMake 方法（我建议，如果您使用的是 2.8.12 或更高版本的 CMake），在这个 StackOverflow 答案中有很好的描述，并且涉及到target_compile_options的使用。
回答by Mike T
The default optimization level for various release modes is O3, which often isn't the best choice. Within CMakeLists.txtfile, these can be modified to O2:
各种发布模式的默认优化级别是 O3，这通常不是最佳选择。在CMakeLists.txt文件中，这些可以修改为 O2：
# Modify compile flags to change optimization level from O3 to O2
string(REGEX REPLACE "([\/\-]O)3" "\12"
CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
string(REGEX REPLACE "([\/\-]O)3" "\12"
CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
string(REGEX REPLACE "([\/\-]O)3" "\12"
CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL}")
string(REGEX REPLACE "([\/\-]O)3" "\12"
CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
These regular expressions will be modified (e.g.):
这些正则表达式将被修改（例如）：
-O3to-O2usually for Linux-based compilers/O3to/O2usually for Windows-based compilers
-O3以-O2通常基于Linux编译器/O3以/O2通常为基于Windows的编译器
