[PATCH] D61804: Support building shared and static libraries simultaneously

[Chris Bieneman via Phabricator via cfe-commits]

beanz added a comment.

I question the general utility of this patch, and I don't really think this does anything we want the build system doing.

If you specify `STATIC` and `SHARED` to `llvm_add_library` it creates the shared library target as the one that everything links against, which is exactly what you don't want. There are two primary reasons why `BUILD_SHARED_LIBS` is not recommended for non-development builds both have to do with how shared object linking works:

(1) Is that clang when linked against shared libraries is *way* slower (I think last I measured was over 10%). This is caused by the need for resolving linkage at process launch every time a process is launched.

(2) Is that LLVM relies on a strange mess of static initializers used for static registration. The most problematic one is cl::opt. When you link a static archive only the object files that contain unresolved symbols get pulled in, when you link against a shared object, all of it gets pulled in. Because static archives selectively pull in object files (and their static initializers) we have had issues in the past where the static build of LLVM works, but the shared build produces runtime errors from cl::opt.

I'm curious to understand the problem you are trying to solve. From other posts on the list it really sounds to me like you want a variant of libClang that exports the entire Clang API surface rather than just the C API. I think there are better ways to accomplish that.

Exposing Clang's libraries for piecemeal consumption is a much bigger problem than just the build system not building libClang*.so. For example, Clang has no equivalent of llvm-config, so there is no external representation of the clang dependency graph that users can rely on.

This all aside, I also have issues with this implementation described inline below.



================
Comment at: clang/CMakeLists.txt:451
+option(CLANG_ENABLE_SHARED_LIBRARIES "Build libclang* as shared libraries." ON)
+option(CLANG_ENABLE_STATIC_LIBRARIES "Build libclang* as static libraries." ON)
+
----------------
These shouldn't both default to `On`, that is a change in behavior that would be a build-time regression for anyone not interested in building shared libraries. `STATIC` should default `On`, and `SHARED` default `Off`.

Also you need to check that one of the two options is enabled. If both are `Off` confusing things will happen.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D61804/new/

https://reviews.llvm.org/D61804