feat: use ccache for very quick recompiles if installed on build system

[dksmiffs]

Implements #46.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.11%. Comparing base (a025929) to head (00ac86d).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #47   +/-   ##
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  Coverage   32.11%   32.11%           
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  Files         290      290           
  Lines       27470    27470           
  Branches     1125     1125           
=======================================
  Hits         8823     8823           
  Misses      18647    18647           

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[craigscott-crascit]

Don't use RULE_LAUNCH_COMPILE or RULE_LAUNCH_LINK for these. Since CMake 3.4, the right way to do this is to use the CMAKE_<LANG>_COMPILER_LAUNCHER variables instead. And ccache doesn't cache linker commands, so there's no need to use it as a launcher for the linker.

See updated explanation here: https://crascit.com/2016/04/09/using-ccache-with-cmake/#h-improved-functionality-from-cmake-3-4

[craigscott-crascit]

• Minimize output on the configure log at the default log level. Only log abnormal or unexpected things at STATUS or higher.
• The result of find_program() is the location of the program, not a boolean.

Suggested change

	find_program(CCACHE_FOUND ccache)
	if(CCACHE_FOUND AND ENABLE_CCACHE)
	set(CMAKE_CXX_COMPILER_LAUNCHER ccache)
	message(STATUS "Using ccache for faster recompilation")
	else()
	message(STATUS "ccache not used for compilation")
	endif()
	if(ENABLE_CCACHE)
	find_program(CCACHE_EXECUTABLE ccache)
	if(CCACHE_EXECUTABLE)
	set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE_EXECUTABLE})
	if(CMAKE_VERSION VERSION_GREATER_EQUAL 15)
	message(VERBOSE "Enabled using ccache")
	endif()
	else()
	message(WARNING "Using ccache requested, but could not find ccache executable")
	endif()
	endif()

[dksmiffs]

@craigscott-crascit I'm unclear about your suggestion in lines 60-62 above. Why is the VERBOSE message printed only for CMake version >= 3.15?

[craigscott-crascit]

The VERBOSE keyword is only supported since CMake 3.15. The first line of the top level CMakeLists.txt in this project lists CMake 3.14 as its minimum supported version. Te if() I added was to handle the case where the developer is using CMake 3.14, to prevent making a call to message() that would treat VERBOSE as part of the message instead of as the message log level.

[karljj1]

I have not been keeping up to date with the cmake changes so ill defer to others

[carlocorradini]

@dksmiffs
Thank you for the PR, but do we really need ccache?
KDIS is not a small project... but I don't believe it is so large that it requires faster compilation via ccache.
Do not mistake my viewpoint, but (personally), I do not believe there is a need to include ccache directly in the main CMakeLists.txt.
On the other hand, I would prefer (please let me know what you think) to include a standalone README.md (or documentation or BUILDING.md) that explains how to speed up compilation time using ccache.
Furthermore, AFAIK, even though ccache now supports Windows (i.e. MSVC), it cannot be automatically included in CMakeLists.txt but must be added via CMake arguments.
What are your thoughts?

[dksmiffs]

@karljj1 understood. When I learned about ccache last year, I incorrectly thought it was part of the CMake ecosystem (like CTest), but it's a separate tool. Without ccache, KDIS recompiles are just too slow, in my opinion, which makes major refactoring work too cumbersome.

@carlocorradini no, I wouldn't argue you need ccache. I definitely prefer using it in my refactoring work so far on KDIS. I already set it up as optional, and I'd be happy to set it to OFF by default, if that's what you'd prefer. Sure thing on adding words in a README.md, I'd be happy to do that. Regarding your Windows concern - wouldn't defaulting ENABLE_CCACHE to OFF address that? KDIS developers wouldn't be forced to use it.

EDIT: No, I don't like my own suggestion above. Defaulting to OFF would imply that it's optional to turn it ON in Windows. If that doesn't work inside the CMakeLists.txt as you suggest, then mine would be a hacky solution. Yes, I have set up ccache to work via CMake arguments in the past. I like your suggestion.