How to Use CCache with CMake?

Question

I would like to do the following: If CCache is present in PATH, use \"ccache g++\" for compilation, else use g++. I tried writing a small my-cmake script containing

Answer 1

As of CMAKE 3.4 you can do:

-DCMAKE_CXX_COMPILER_LAUNCHER=ccache

Answer 2

From CMake 3.1, it is possible to use ccache with the Xcode generator and Ninja is supported from CMake 3.4 onwards. Ninja will honour RULE_LAUNCH_COMPILE just like the Unix Makefiles generator (so @Babcool's answer gets you there for Ninja too), but getting ccache working for the Xcode generator takes a little more work. The following article explains the method in detail, focussing on a general implementation which works for all three CMake generators and making no assumptions about setting up ccache symlinks or the underlying compiler used (it still lets CMake decide the compiler):

https://crascit.com/2016/04/09/using-ccache-with-cmake/

The general gist of the article is as follows. The start of your CMakeLists.txt file should be set up something like this:

cmake_minimum_required(VERSION 2.8)

find_program(CCACHE_PROGRAM ccache)
if(CCACHE_PROGRAM)
    # Support Unix Makefiles and Ninja
    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE "${CCACHE_PROGRAM}")
endif()

project(SomeProject)

get_property(RULE_LAUNCH_COMPILE GLOBAL PROPERTY RULE_LAUNCH_COMPILE)
if(RULE_LAUNCH_COMPILE AND CMAKE_GENERATOR STREQUAL "Xcode")
    # Set up wrapper scripts
    configure_file(launch-c.in   launch-c)
    configure_file(launch-cxx.in launch-cxx)
    execute_process(COMMAND chmod a+rx
                            "${CMAKE_BINARY_DIR}/launch-c"
                            "${CMAKE_BINARY_DIR}/launch-cxx")

    # Set Xcode project attributes to route compilation through our scripts
    set(CMAKE_XCODE_ATTRIBUTE_CC         "${CMAKE_BINARY_DIR}/launch-c")
    set(CMAKE_XCODE_ATTRIBUTE_CXX        "${CMAKE_BINARY_DIR}/launch-cxx")
    set(CMAKE_XCODE_ATTRIBUTE_LD         "${CMAKE_BINARY_DIR}/launch-c")
    set(CMAKE_XCODE_ATTRIBUTE_LDPLUSPLUS "${CMAKE_BINARY_DIR}/launch-cxx")
endif()

The two script template files launch-c.in and launch-cxx.in look like this (they should be in the same directory as the CMakeLists.txt file):

launch-c.in:

#!/bin/sh
export CCACHE_CPP2=true
exec "${RULE_LAUNCH_COMPILE}" "${CMAKE_C_COMPILER}" "$@"

launch-cxx.in:

#!/bin/sh
export CCACHE_CPP2=true
exec "${RULE_LAUNCH_COMPILE}" "${CMAKE_CXX_COMPILER}" "$@"

The above uses RULE_LAUNCH_COMPILE alone for Unix Makefiles and Ninja, but for the Xcode generator it relies on help from CMake's CMAKE_XCODE_ATTRIBUTE_... variables support. The setting of the CC and CXX user-defined Xcode attributes to control the compiler command and LD and LDPLUSPLUS for the linker command is not, as far as I can tell, a documented feature of Xcode projects, but it does seem to work. If anyone can confirm it is officially supported by Apple, I'll update the linked article and this answer accordingly.

Answer 3

It is now possible to specify ccache as a launcher for compile commands and link commands (since cmake 2.8.0). That works for Makefile and Ninja generator. To do this, just set the following properties :

find_program(CCACHE_FOUND ccache)
if(CCACHE_FOUND)
    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
    set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache) # Less useful to do it for linking, see edit2
endif(CCACHE_FOUND)

It is also possible to set these properties only for specific directories or targets.

For Ninja, this is possible since version 3.4. For XCode, Craig Scott gives a workaround in his answer.

Edit : Thanks to uprego and Lekensteyn's comment, I edited the answer to check if ccache is available before using it as launcher and for which generators is it possible to use a compile launcher.

Edit2: @Emilio Cobos recommended to avoid doing that for the linking part as ccache doesn't improve linking speed and can mess with other types of cache like sccache

Answer 4

In my opinion the best way is to symlink gcc,g++ to ccache, but if you would like to use within cmake, try this:

export CC="ccache gcc" CXX="ccache g++" cmake ...

Answer 5

I verified the following works (source: this link):

        CC="gcc" CXX="g++" cmake -D CMAKE_CXX_COMPILER="ccache" -D CMAKE_CXX_COMPILER_ARG1="g++" -D CMAKE_C_COMPILER="ccache" -D CMAKE_C_COMPILER_ARG1="gcc" $*

Update: I later realized that even this does not work. Strangely it works every alternate time (the other times cmake complains).

Answer 6

Let me add one important item that was not mentioned here before.

While bootstrapping a minimalistic build system from the ubunutu:18.04 docker image, I've found that order of installation makes a difference.

In my case ccache worked fine when calling gcc, but failed to catch invocations of the same compiler by the other names: cc and c++. To fully install ccache, you need to make sure all compilers are installed first, or add a call to update-ccache symlinks to be safe.

sudo apt-get install ccache build-essential # and everyhting ... sudo /usr/sbin/update-ccache-symlinks export PATH="/usr/lib/ccache/:$PATH"

... and then (due to updated symlinks) also calls to cc and c++ get caught!