[PATCH] D125683: [runtimes] Replace LIBCXX_ENABLE_STATIC_ABI_LIBRARY & friends by a new LIBCXX_CXX_ABI choice

Mon Sep 26 10:14:50 PDT 2022

nikic added a comment.

In D125683#3815348 <https://reviews.llvm.org/D125683#3815348>, @ldionne wrote:

> In D125683#3811492 <https://reviews.llvm.org/D125683#3811492>, @nikic wrote:
>
>> I have some concerns about this change:
>>
>> - It makes it fundamentally impossible to link against a static system-libcxxabi (or any other `LIBCXX_CXX_ABI` option). This is currently already broken (https://github.com/llvm/llvm-project/issues/57759), but used to work prior to LLVM 15.
>> - It makes it fundamentally impossible to use a linker script for the shared library while linking the abi library into the static library. I think this currently doesn't work either, but would be trivial to support. (I was going to work on this when I found this revision.)
>>
>> I think both of these points highlight that `LIBCXX_CXX_ABI=libcxxabi-objects` is not the correct representation for this functionality.
>
> I don't think these limitations are an inherent property of this approach. What I'm trying to do with this change is eliminate these boolean flags that control how we build the library to instead use a more open-ended selection mechanism that is more flexible, more expressive, and which reduces the need for `if(....)` within the build.
>
> I think the fundamental problem we have is that the ABI library for the static libc++ and for the shared libc++ are selected in one go. If we could instead specify the choice of the ABI library for each independently (for example `-DLIBCXX_CXX_SHARED_ABI=libcxxabi -DLIBCXX_CXX_STATIC_ABI=system-libcxxabi`), would that solve your problem? Also, this would actually increase the flexibility of the current build by a significant margin without much complexity on the libc++ CMake side.

This would indeed address the second point, but not the first. For that, one would have to add additional variations like `system-libcxxabi-objects`, which would end up increasing the possible abi choices by a factor of two (unless static linking doesn't make sense for some of them).

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https://reviews.llvm.org/D125683