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

[Joel Rosdahl]

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