[cfe-dev] RFC: How should Clang/LLVM runtime libraries be installed and found during link steps?

[Jean-Daniel Dupas]

Le 23 nov. 2011 à 09:18, Ruben Van Boxem a écrit :

> 2011/11/23 Chandler Carruth <chandlerc at google.com>
> It has come up when reviewing Kostya's patch to add the necessary support fort linking in the Address Sanitizer runtime library that we need a proper scheme and plan for deploying runtime libraries along with Clang.
> 
> I've CC'ed llvmdev on this for compiler-rt developers' input.
> 
> The key issues I see when locating runtime libraries are the following:
> 
> - These libraries should be shipped with Clang, not installed separately on the system.
> - They are likely to be somewhat tied to specific versions of Clang/LLVM.
> - To support cross-compiling, we should be able to install multiple copies of these libraries in a single Clang installation, and use the appropriate one when targeting a particular platform.
> - The above cross-compiling concern extends to ABI compatibility -- many of the ABIs are already fixed in this space.
> - These are different from builtin header files which can use the preprocessor to internally differentiate their contents based on the target platform.
> 
> My proposed solution:
> 
> - Base the path on the shared "resource directory" concept which already exists in Clang.
>   - Builtin header files are at <ResourceDir>/include already.
> - Append a "base" triple directory name.
> - Append a "lib" directory name for runtime libraries
> - Place the runtime libraries as "libcompiler_rt_<sublib>.a" for each sub-library component "<sublib>".
> 
> Example for "x/bin/clang" installation:
>   x/bin/../lib/clang/3.1/x86_64-linux-gnu/lib/libcompiler_rt_asan.a
> 
> What is a "base" triple? It is the simplest form we can reduce a triple to while guaranteeing compatibility. The concept is best explained by an example. All of the following triples reduce to the base triple of "x86_64-linux-gnu":
>   x86_64-linux-gnu
>   x86_64-pc-linux-gnu
>   x86_64-unknown-linux-gnu
>   x86_64-redhat-linux
>   x86_64-redhat-linux6E
>   ...
> 
> A different ABI could be expressed much like ARM's is with the last element: "*-gnueabi". This would *not* reduce to the triple ending in '-gnu'. Due to the adhoc and poorly spec'ed nature of existing triples, this will largely be a fixed mapping much like already exists in the Clang driver, but consolidated into LLVM's core triple logic.
> 
> This does kind of push the GNU triple language on LLVM/Clang internally, although the jury was still out on the true specification for cross-compiling platform names. Although internal, it really will keep spreading like a "disease" to more exposed parts of Clang.

And it does not play well with universal binaries, which are actually used for darwin compiler_rt libraries. Not that I think this is a major issue, but it probably should be considered too.

-- Jean-Daniel




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