默认情况下在CMake中进行优化 [英] Optimize in CMake by default

问题描述

我有一个使用CMake作为其构建系统的C ++项目。我想要以下行为：

I have a C++ project which uses CMake as its build system. I'd like the following behavior:

如果cmake作为 cmake .. 调用，则 CMAKE_CXX_FLAGS 是 -O3 -Wall -Wextra

If cmake is invoked as cmake .., then CMAKE_CXX_FLAGS is -O3 -Wall -Wextra

如果调用cmake作为 cmake .. -DCMAKE_BUILD_TYPE = Debug ，然后 CMAKE_CXX_FLAGS 是 -g -Wall -Wextra

If cmake is invoked as cmake .. -DCMAKE_BUILD_TYPE=Debug, then CMAKE_CXX_FLAGS is -g -Wall -Wextra

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")

但这是一个很大的问题。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，如果我使用第二个调用，然后使用第一个调用，则 CMAKE_BUILD_TYPE 保持 Debug 的作用，尽管未明确指定，所以-获得调试版本，尽管我想要优化版本。

But this has a big problem. First of all, if the second invocation is used, then both -O3 and -g flags are passed to the compiler. Besides, if I use the second invocation and the first thereafter, CMAKE_BUILD_TYPE stays Debug although not explicitly ordered so - so I get a Debug build although I want an optimized build.

为什么？我该怎么做才能获得所需的行为？

Why? What can I do to get the desired behavior?

推荐答案

首先，建议：CMake的建议用法是始终指定 CMAKE_BUILD_TYPE 在命令行上显式显示（如果且仅当使用单配置生成器时）。您的用例偏离了此最佳实践，因此请将此答案视为您如何做，而不必视为您应该如何做。

First off: recommended usage of CMake is to always specify CMAKE_BUILD_TYPE explicitly 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."

要解决第一个问题，您应该可以在CMakeList的早期完成此操作：

To address the first issue, you should be able to do this early in your CMakeList:

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")

这将确保如果您完全不指定构建类型，它将确保默认为发布，因此将使用 CMAKE_CXX_FLAGS_RELEASE 。

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_RELEASE will be used.

第二个难度较大。从命令行传递的变量（例如 CMAKE_BUILD_TYPE = Debug ）由CMake 缓存，因此可在后续调用中重新使用（这是必需的，因为如果在构建之间修改其输入，CMake可以重新触发自身。）

The second one is harder to tackle. Variables passed from the command line (such as CMAKE_BUILD_TYPE=Debug) are cached by CMake and thus re-used in subsequent invocations (that is necessary, since CMake can re-trigger itself if you modify its inputs between builds).

唯一的解决方案是使用户再次使用<$ c显式切换构建类型。 $ c> cmake .. -DCMAKE_BUILD_TYPE =发布。

The only solution is to make the user switch the build type explicitly again, using cmake .. -DCMAKE_BUILD_TYPE=Release.

考虑为什么这样做是必要的：正如我说的，CMake可以重新触发如果自上一次CMake运行以来CMake的输入（ CMakeLists.txt 文件或其依赖项）已更改，则为构建的一部分。在这种情况下，它也将在没有命令行参数（例如 -DCMAKE_BUILD_TYPE =任何东西）的情况下运行，并且将依靠缓存提供与上次相同的值。这种情况与您手动运行 cmake .. 时没有区别，而没有其他参数。

Consider why this is necessary: as I said, CMake can re-trigger itself as part of a build if CMake's input (CMakeLists.txt files 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.

我可以提供一个解决方案如果未在命令行上明确指定，请始终将 CMAKE_BUILD_TYPE 重置为发布。但是，这也意味着以 Debug 生成的构建系统将以 Release 重新生成。 em>如果发生自动重新生成。我很确定那不是您想要的。

I could provide a hacky solution to always reset CMAKE_BUILD_TYPE to Release if not specified explicitly on the command line. However, it would also mean that a buildsystem generated as Debug would get re-generated as Release if automatic re-generation happened. I am pretty sure that's not what you want.

这篇关于默认情况下在CMake中进行优化的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！