Add support for a clang-tidy linter to CMake. Add a files utility function to P4CUtils.cmake.

[fruffy]

Add the ability to run clang-tidy across P4C files via CMake. This is not enabled yet for CI because clang-tidy is quite slow still.

However, one can still use it to run clang-tidy across one's files in bulk.

Supersedes #4253 if merged before it.

[vlstill]

Generally looks good.

[vlstill]

I've just now realized that you did not enable clang-tidy for any files. I think it would make more sense to enable it for the same set of files as cpplint and just run it from time to time. Otherwise there is not much point in having the infrastructure for it if it has no files to run on.

[vlstill]

Hmm, I understand that clang-tidy is slow, but having the target does not mean it will have to run. I think it would be best to add the same set as is used of cpplint and just let the programmers decide if they run it or not.

add_clang_tidy_files(${P4C_SOURCE_DIR} "${P4C_LINT_LIST}")

[fruffy]

The problem is that, until we have figured out this slowness issue, we can only apply clang-tidy selectively. If we added all the cpplint files the amount we would have to check would be overwhelming, when you just want to check a local set of files in your target.

I have been spot-checking files by adding add_clang_tidy_files(${P4C_SOURCE_DIR} "${P4C_LINT_LIST}") for a smaller subset of code I am interested in. I have not yet contributed these automated fixes because I am still tweaking rules etc.

[vlstill]

Hmm, this is also made worse by using the xargs which prevents running clang-tidy in parallel. This can be side-stepped by the following construction:

• Use empty add_custom_target to add the actual target clang-tidy.
• Use add_custom_command(TARGET clang-tidy POST_BUILD COMMAND ... for each file

But I still wonder if there is a better way of doing this then just manually adding files one wants to check (I would not expect clang-tidy to get significantly faster any time soon...). How long does it take to run clang-tidy over the all the CPPLINT-linted files?

[fruffy]

The xargs is necessary because of a shell quirk. At some point the number of files as arguments grew too large.
There is a parallel runner but not sure it helps much when it takes ~10-30 second to analyze a single file.

I think the underlying problem is that clang-tidy checks system-files by default. Once we find a workaround for that it should be significantly faster.

Currently, checking all the CPPLINT files would a day at least.

[fruffy]

@vlstill This PR is ready. #4326 shows how it can be used. While we can not use clang-tidy at scale we can at least apply it in small increments. I would like to merge this PR before it starts to become too dated.

[fruffy]

@asl This might help in applying clang-tidy in broader strokes. If you have any advice on how to improve the performance of the analyser please let me know.

[asl]

@fruffy By "performance" you mean that it is currently slow?

[fruffy]

Yes, clang-tidy requires many seconds to analyse a single file. This makes it a non-option for CI.

[vlstill]

Let's get this in. In the end, it will be useful even if we can't run in in the P4C CI for now.