[cfe-dev] clangd/libclang: how to emulate other compilers?

Manuel Klimek via cfe-dev cfe-dev at lists.llvm.org
Thu Apr 19 03:42:59 PDT 2018

On Wed, Apr 18, 2018 at 4:22 PM Doug Schaefer via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> *From:* cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] *On Behalf Of *Manuel
> Klimek via cfe-dev
> *Sent:* Wednesday, April 18, 2018 6:26 AM
> *To:* Milian Wolff <mail at milianw.de>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] clangd/libclang: how to emulate other compilers?
> On Tue, Apr 17, 2018 at 8:02 PM Milian Wolff via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> Hey all,
> how does clangd or other users of the libclang handle situations where you
> want to parse code that is dependent on a certain other compiler or
> compiler
> environment? The most common scenario being embedded projects that rely on
> the
> compiler-builtin defines and include paths to find the sysroot include
> paths
> and such.
> I'm not sure I understand what you mean - do you mean the compiler has
> builtins that clang doesn't provide and relies on their existence?
> Yes.
> Generally, you'll want to use the builtin defines and includes from clang
> (at the point at which you compiled libclang), but the standard library and
> so forth that the system is using. Clang should be able to find that given
> the right flags.
> Well, no. The plan for a lot of us is to use clangd with projects that use
> gcc as the compiler. We need to be able to reach out to gcc to ask it what
> the built-ins are. We’ll then need to convince clang to parse in the same
> manner. Given all the variants of compilers that we need to be able to
> support in our various IDE’s, it’s already something we’ve gotten quite use
> to.

If you want to use intrinsics where they differ between compilers, you'll
still need to use clang's builtin headers, otherwise clang will incorrectly
parse, well, basically anything.

The problem is that there is no layering; builtin headers depend on each
other and compiler internals somewhat randomly.

We'd like to better understand examples of differences in language
extensions, as clang tries really hard to emulate all the different
dialects out there :)

For KDevelop, which is using libclang, we have tried to build a sort of
> emulation layer that originally yielded good results. The approach is as
> followed:
> 1) We use the actual compiler that is used to compile a given project,
> e.g.
> gcc, arm-none-eabi-gcc, ...
> 2) We take this compiler and query it for its builtin defines:
> /usr/bin/gcc -xc++ -std=c++11 -dM -E - < /dev/null
> 3) And also query the include paths:
> /usr/bin/gcc -xc++ -std=c++11 -v -E - < /dev/null
> 4) Then for the libclang calls to clang_parseTranslationUnit2 we pass `-
> nostdinc -nostdinc++` followed by the defines and includes we got from 2)
> and
> 3).
> Now, for simply things this actually worked quite well. But once you
> include a
> file that heavily relies on the compiler, such as all the SIMD intrinsic
> headers, you are easily drowning in parse errors. And once you have too
> many
> parse errors, clang will just give up. We have tried to workaround this
> via
> compatibility headers such as [1], but it keeps breaking.
> More recently, we now also got bug reports where the user system has
> clang3
> and they use that to to compile the code, but then download a KDevelop
> AppImage built against libclang v5 (e.g. via AppImage). Once again this
> easily
> yields tons of parse errors when encountering system headers that are
> using
> intrinsics specific to clang v3.
> I am now thinking about removing the emulation layer described above. But
> then
> it will be essentially impossible to work on a lot of embedded projects
> which
> rely on the cross compiler defines and include paths...
> So, once again - how do other users of libclang handle this scenario? What
> is
> the plan for clangd in this regard?
> Thanks
> --
> Milian Wolff
> mail at milianw.de
> http://milianw.de_______________________________________________
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