Using pre-compiled headers with CMake

Question

I have seen a few (old) posts on the \'net about hacking together some support for pre-compiled headers in CMake. They all seem a bit all-over the place and everyone has the

Answer 1

CMake has just gained support for PCHs, it should be available in the upcoming 3.16 release, due 2019-10-01:

https://gitlab.kitware.com/cmake/cmake/merge_requests/3553

  target_precompile_headers(<target>
    <INTERFACE|PUBLIC|PRIVATE> [header1...]
    [<INTERFACE|PUBLIC|PRIVATE> [header2...] ...])

There is ongoing discussion on supporting sharing PCHs between targets: https://gitlab.kitware.com/cmake/cmake/issues/19659

There is some additional context (motivation, numbers) available at https://blog.qt.io/blog/2019/08/01/precompiled-headers-and-unity-jumbo-builds-in-upcoming-cmake/

Answer 2

Adapted from Dave, but more efficient (sets target properties, not for each file):

if (MSVC)
   set_target_properties(abc PROPERTIES COMPILE_FLAGS "/Yustd.h")
   set_source_files_properties(std.cpp PROPERTIES COMPILE_FLAGS "/Ycstd.h")
endif(MSVC)

Answer 3

CMake 3.16 introduced support for precompiled headers. There is a new CMake command target_precompile_headers which does everything you need under the hood. See its documentation for further details: https://cmake.org/cmake/help/latest/command/target_precompile_headers.html

Answer 4

There is a third party CMake module named 'Cotire' which automates the use of precompiled headers for CMake based build systems and also supports unity builds.

Answer 5

An example of usage precompiled header with cmake and Visual Studio 2015

"stdafx.h", "stdafx.cpp" - precompiled header name.

Put the following below in the root cmake file.

if (MSVC)
    # For precompiled header.
    # Set 
    # "Precompiled Header" to "Use (/Yu)"
    # "Precompiled Header File" to "stdafx.h"
    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Yustdafx.h /FIstdafx.h")
endif()

---

Put the following below in the project cmake file.

"src" - a folder with source files.

set_source_files_properties(src/stdafx.cpp
    PROPERTIES
    COMPILE_FLAGS "/Ycstdafx.h"
)

Answer 6

The cleanest way is to add the precompiled option as a global option. In the vcxproj file this will show up as <PrecompiledHeader>Use</PrecompiledHeader> and not do this for every individual file.

Then you need to add the Create option to the StdAfx.cpp. The following is how I use it:

MACRO(ADD_MSVC_PRECOMPILED_HEADER SourcesVar)
    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /YuStdAfx.h")
    set_source_files_properties(StdAfx.cpp
        PROPERTIES
        COMPILE_FLAGS "/YcStdAfx.h"
        )
    list(APPEND ${${SourcesVar}} StdAfx.cpp)
ENDMACRO(ADD_MSVC_PRECOMPILED_HEADER)

file(GLOB_RECURSE MYDLL_SRC
    "*.h"
    "*.cpp"
    "*.rc")

ADD_MSVC_PRECOMPILED_HEADER(MYDLL_SRC)
add_library(MyDll SHARED ${MYDLL_SRC})

This is tested and works for MSVC 2010 and will create a MyDll.pch file, I am not bothered what file name is used so I didn't make any effort to specify it.