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How to Use CCache with CMake?

Posted by: admin November 29, 2017 Leave a comment

Questions:

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

    CC="ccache gcc" CXX="ccache g++" cmake $*

but it does not seem to work (running make still does not use ccache; I checked this using CMAKE_VERBOSE_MAKEFILE on).

Update:

As per this link I tried changing my script to

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

but cmake bails out complaining that a test failed on using the compiler ccache (which can be expected).

Answers:

I personally have /usr/lib/ccache in my $PATH. This directory contains loads of symlinks for every possible name the compiler could be called from (like gcc and gcc-4.3), all pointing to ccache.

And I didn’t even create the symlinks. That directory comes pre-filled when I install ccache on Debian.

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Answers:

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)
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.

Questions:
Answers:

As of CMAKE 3.4 you can do:

-DCMAKE_CXX_COMPILER_LAUNCHER=ccache

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I didn’t like to set a symlink from g++ to ccache. And CXX="ccache g++" didn’t work for me as some cmake test case wanted to have just the compiler program without attributes.

So I used a small bash script instead:

#!/bin/bash
ccache g++ "[email protected]"

and saved it as an executable in /usr/bin/ccache-g++.

Then C configured cmake to use /usr/bin/ccache-g++ as C++ compiler.
This way it passes the cmake test cases and I feel more comfortable than having symlinks that I might forget about in 2 or 3 weeks and then maybe wonder if something doesn’t work…

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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).

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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")
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}" "[email protected]"

launch-cxx.in:

#!/bin/sh
export CCACHE_CPP2=true
exec "${RULE_LAUNCH_COMPILE}" "${CMAKE_CXX_COMPILER}" "[email protected]"

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 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.

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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 ...