[libc] Enable llvmlibc clang-tidy checks

[PaulkaToast]

Summary

Add clang-tidy for llvm-libc source directory.

Example of check in action:

/workspace/llvm-project/libc/src/string/strcpy.cpp:10:1: error: system include stdio.h not allowed [llvmlibc-restrict-system-libc-headers,-warnings-as-errors]
#include <stdio.h>
^~~~~~~~~~~~~~~~~~

[abrachet]

libc/src/.clang-tidy

7

Everything in asm/* should be safe, I imagine.

libc/src/math/round_redirector.cpp

10

Is it possible for value to be something like *redirector* so we don't need to manually add the nolint manually like this? Not that its a big deal as this is the only one.

[PaulkaToast]

libc/src/.clang-tidy

7

That's what I thought but it has stuff like https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/string_32.h

which might be unsafe to include?

libc/src/math/round_redirector.cpp

10

I think it might be better to be on the safe side here and avoid a blanket "allow all includes in redirectors" sort of mechanism. Just to avoid accidentally making a mistake, thoughts?

[abrachet]

libc/src/.clang-tidy

7

That's not in /usr/include/asm on my machine, and is protected by an #ifdef __KERNEL__ it looks like.

I have no strong preference, we're only using asm/unistd.h I suppose so it makes no big difference either way.

libc/src/math/round_redirector.cpp

10

That's probably better, you're right.

[sivachandra]

This is OK as a start. But, can we follow up with a study on running clang-tidy as part of our build rules? Like, what are the pros and cons? For the pros, I can think of:

1. We will not require NOLINT... annotations for redirectors.
2. Generated headers can be checked.
3. We force checks as part of our development workflow.

For cons, our builds will now depend on clang and clang-tools-extra. May be there are ways to reduce the "burden".

libc/src/.clang-tidy

+Checks: '-*,llvmlibc-*'
+HeaderFilterRegex: '.*'
+WarningsAsErrors: 'llvmlibc-*'
+CheckOptions:
+  - key:             llvmlibc-restrict-system-libc-headers.Includes
+    value:           '-*, linux/*, asm/unistd.h'

libc/src/math/round_redirector.cpp

 //===----------------  Implementation of round redirector -----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+// Include okay for this redirector.
+// NOLINTNEXTLINE(llvmlibc-restrict-system-libc-headers)
 #include <math.h>
 
 namespace __llvm_libc {
 
 double __round_redirector(double x) {
   return ::round(x);
 }
 
 } // namespace __llvm_libc