[cfe-dev] RFC: Place libs in Clang-dedicated directories (affects openmp, libcxx, libunwind, compiler-rt)

[Ilya Biryukov via cfe-dev]

Using the resource-dir in the header search paths would break tools that
use compilation database, e.g. clang-tidy and clangd.
They override the resource-dir as it's very common for them to be built
from a different revision than the used compiler. They rely on the fact
that the same standard library can be found with the overridden resouce-dir.

>From this point of view, what LLVM_ENABLE_PER_TARGET_RUNTIME_DIR breaks the
tools and the proposed alternative fixes this problem.

On Mon, Feb 25, 2019 at 11:17 AM Petr Hosek via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Wed, Feb 20, 2019 at 6:14 PM Joel E. Denny <jdenny.ornl at gmail.com>
> wrote:
>
>> My alternative to LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is the preceding
>> bullets.  In other words, you wouldn't need to specify
>> LLVM_ENABLE_PER_TARGET_RUNTIME_DIR because it would effectively be
>> always on (except the directories might be different than now if the
>> version locking issue is important, as noted above).  Is that what
>> you're asking?
>>
>
> That would be my preference. I always hoped that
> LLVM_ENABLE_PER_TARGET_RUNTIME_DIR would eventually become the default. It
> would be nice to finish the Darwin support so we can completely deprecate
> the old layout, but I don't know how far along beanz is in his effort. We
> should also update openmp to stop using the custom Android-specific runtime
> layout.
>
> There's also the unresolved question of where should libc++ headers and
> libraries go. Currently, in LLVM_ENABLE_PER_TARGET_RUNTIME_DIR we use the
> resource dir, but some people expressed the opinion that we shouldn't be
> using these for libc++ et al. since they're not version-locked to Clang.
> This is different from what GCC does (e.g. GCC would use
> $prefix/lib/gcc/x86_64-linux-gnu/8/libstdc++.a) and it's one of the reasons
> why I used the resource dir for libc++ et al. when implementing
> LLVM_ENABLE_PER_TARGET_RUNTIME_DIR.
>
> So concretely, today LLVM_ENABLE_PER_TARGET_RUNTIME_DIR uses the following
> layout:
>
> headers: $prefix/lib/clang/$version/include(/$triple)(/c++/v1)
> libraries: $prefix/lib/clang/$version/$triple/lib/$name.$ext
>
> The alternative that doesn't use resource dir for libc++ would be the
> following:
>
> compiler-rt:
>   headers: $prefix/lib/clang/$version/include
>   libraries: $prefix/lib/clang/$version/$triple/lib/$name.$ext
>
> libc++, libc++abi, libunwind:
>   headers: $prefix/include/c++/v1
>   libraries: $prefix/lib/$triple/$name.$ext
>
> Making this change should be trivial, it's the matter of changing three
> CMakeLists.txt files (libunwind
> <https://github.com/llvm/llvm-project/blob/master/libunwind/CMakeLists.txt#L189>,
> libc++abi
> <https://github.com/llvm/llvm-project/blob/master/libcxxabi/CMakeLists.txt#L179>
> and libc++
> <https://github.com/llvm/llvm-project/blob/master/libcxx/CMakeLists.txt#L419>)
> and Clang driver in one place
> <https://github.com/llvm/llvm-project/blob/master/clang/lib/Driver/ToolChain.cpp#L79>.
> However, if we're going to make that change, I'd like to get a broader
> consensus. It'd be also useful to get feedback from libc++ maintainers on
> this.
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>


-- 
Regards,
Ilya Biryukov
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