[ccache] Speeding up builds with clang-tidy, clang-analyzer and iwyu

[Christian Ledergerber]

I have created the following issue for cmake to launch the discussion:
https://gitlab.kitware.com/cmake/cmake/-/issues/21092

On 04.08.20 19:46, Joel Rosdahl wrote:
> On Sun, 2 Aug 2020 at 19:49, Christian Ledergerber via ccache
> <ccache at lists.samba.org> wrote:
>> In the mean-time I have been thinking some more and what I am afraid of
>> is that the result will not be correct if the flags to clang-tidy change.
>>
>> A typical command run by make looks like this:
>>
>> cd /home/dev/erx-soft/cmake-build-debug/modules/utils && ccache
>> /home/dev/clion-2020.1.3/bin/cmake/linux/bin/cmake -E __run_co_compile [...]
> If I interpret the example command correctly ccache is used as a
> prefix to cmake? That won't have any effect; ccache can't cache the
> result of cmake so it will just fall back to executing cmake with the
> given arguments (and increase some statistics counter).
>
>> --launcher=ccache
> I guess this means that cmake will execute "ccache $compiler ..." later.
>
>> [...] In other words: it seems like ccache could also hash the flags to
>> clang-tidy. The question: does it?
> No, since ccache doesn't see any clang-tidy arguments when cmake
> executes "ccache $compiler ..." as mentioned above.
>
>> Now that I am looking at the above command line: Will this require to
>> also modify the cmake binary to generate other command lines?
> I have no idea, but it sounds likely.
>
> Here's an idea to consider: You could write the clang-tidy arguments
> to a file and set CCACHE_EXTRAFILES to the file. Or just set it to
> preexisting .clang-tidy files? Then the ccache results will be
> invalidated when the clang-tidy arguments change. That in combination
> with the hypothetical mode where ccache would exit with failure on a
> cache miss would maybe enough for your initial idea.
>
> -- Joel