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Date:   Tue, 18 Apr 2023 21:03:21 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     rcu@vger.kernel.org, nathan@kernel.org, trix@redhat.com,
        llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
        paulmck@kernel.org
Subject: Re: clangd cannot handle tree_nocb.h
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Hi Nick,

On Tue, Apr 18, 2023 at 01:46:40PM -0700, Nick Desaulniers wrote:
> On Tue, Apr 18, 2023 at 1:36 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Fri, Apr 14, 2023 at 03:47:51PM -0700, Nick Desaulniers wrote:
> > > On Thu, Apr 13, 2023 at 5:53 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > >
> > > > Hello!
> > > >
> > > > I have been trying to get clangd working properly with tree_nocb.h. clangd
> > > > trips quite badly when trying to build tree_nocb.h to generate ASTs.
> > >
> > > Hi Joel,
> > > Thanks for the report.  What are you using clangd for? I'll bet
> > > something interesting.
> >
> > Thanks for the response and sorry for the late reply. I am at the OSPM
> > conference. I use vim and vscode with clangd. In vim, YCM uses it to
> > highlight compiler errors live while editing, I am pretty happy with it so
> > far and has been a huge time saver. Enough that now I want to use it for
> > everything...
> 
> Cool! I use vim, can you share more info about your set up for this?
> I'll have to try it.

This is how I installed YCM:

# Install YouCompleteMe for vim
# cd ~/.vim/bundle
# git clone https://github.com/Valloric/YouCompleteMe.git
# cd YouCompleteMe/
# git submodule update --init --recursive
# python3 install.py --clang-completer

Then install and run bear in the kernel sources to generate
compile_compands.json:
bear -- make -j99 CC=clang

However, there's also the script:
scripts/clang-tools/gen_compile_commands.py

This generates the .json from an existing build. Thank God because we can
probably make this generate better .json files which may make clangd better.

You don't need YCM to reproduce the issue though if you just use vscode with
the clangd plugin.

> > I first came across clangd when developing Chrome userspace code which is C++
> > :). In Chrome, ninja builds can be made to output compile_commands.json.
> > However, now I noticed the support in the kernel and was like, wow I need to
> > try it. Further, YCM seems to work much better with it than without :)
> >
> > > I've never used it myself, so I don't know where to even begin with
> > > how to reproduce the issue.
> >
> > Ah ok. :). When I ran get_maintainer on the script, your name popped up and
> > someone also suggested that you're the goto person for clang on the kernel
> > (which I kind of already knew ;)
> 
> You've cc'ed the right set of folks.  We might not have the expertise
> related to clangd specifically, but we can point you in the right
> direction.

Sure, thanks! And thanks for CC'ing the right folks.

> > > It might be worth filing a bug upstream at
> > > https://github.com/llvm/llvm-project/issues
> > > or internally under the component
> > > Language Platforms > C++ > Clang > Tools > Clangd
> > > with detailed steps to reproduce (and what the observed error actually
> > > is). Feel free to cc me, though I don't know the first thing about
> > > clangd.
> >
> > Ok I will consider doing this if needed. One thing I do observe is lack of
> > good support for header files and it is a known clangd issue [1].
> >
> > However, the fixes I was proposing can purely be done in the kernel itself
> > since all it'd require is generating compile_compands.json with the -D<macro>
> > and editing files to keep clangd happy. I guess one question is, how welcome
> > would such changes to header files be since they're for tooling and isn't
> > code that will be compiled outside of clangd.
> 
> Specifically your patch sites some log print that doesn't look
> indicative of a failure:
> https://github.com/llvm/llvm-project/blob/53430bfd5c9d0074dd6de06dccea366e1d40bce4/clang-tools-extra/clangd/TUScheduler.cpp#L903-L906
> so something else is going on here. Just trying to make sure we root cause this.
> 

Right, so clangd's log does not show failure, the failure is when it displays
in the code editor that your code has compiler errors when in fact it does
not.

So if you open up tree_nocb.h in vscode, for example, you'll see squiggles
saying 'undefined reference to rcu_data', etc. That makes clangd stop working
at the error. Sorry to not make the failure mode clear earlier.

thanks,

 - Joel