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

Doug Schaefer via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 17 13:35:00 PDT 2018

Eclipse CDT does something similar. For each unique collection of compiler command line arguments (i.e. for files that build with the same options) we add -E -P -v -dD and call the real compiler to get the list of all include paths and symbols used for a given parse, including both user and built-in ones. Works with both clang and gcc.

For compiler specifics, we cheat and add macros that work us through the parse errors. You likely lose info but at least it rounds up to a pretty good parse.

I'm at the very beginning of trying to figure out how we'd do something similar with clangd. This feature will be a must for users with embedded systems toolchains which are mainly based on gcc.


-----Original Message-----
From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Milian Wolff via cfe-dev
Sent: Tuesday, April 17, 2018 4:03 PM
To: Clang Dev <cfe-dev at lists.llvm.org>
Subject: [cfe-dev] clangd/libclang: how to emulate other compilers?

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.

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

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?

Milian Wolff
mail at milianw.de

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