[llvm-dev] [RFC] Modernize CMake LLVM "Components"/libLLVM Facility

[Stella Laurenzo via llvm-dev]

Hi folks, happy new year!

*Proposal:*

   - See comments at the top of LLVMComponents.cmake
   <https://github.com/stellaraccident/llvm-project/blob/newcomponents/llvm/cmake/modules/LLVMComponents.cmake>
   in my fork
   <https://github.com/stellaraccident/llvm-project/tree/newcomponents>.
   - Draft phab: https://reviews.llvm.org/D94000


*Background:*
As I've been working on NPCOMP <https://github.com/llvm/mlir-npcomp> trying
to come up with a release flow for MLIR derived Python projects (see
py-mlir-release <https://github.com/stellaraccident/mlir-py-release>), I've
repeatedly run into issues with how the LLVM build system generates shared
libraries. While the problems have been varied, I pattern match most of
them to a certain "pragmatic" nature to how components/libLLVM/libMLIR have
come to be: in my experience, you can fix most individual dynamic linkage
issues with another work-around, but the need for this tends to be rooted
in a lack of definition and structure to the libraries themselves, causing
various kinds of problems and scenarios that don't arise if developed to
stricter standards. (This isn't a knock on anyone -- I know how these
things tend to grow. My main observation is that I think we have outgrown
the ad-hoc nature of shared libraries in the LLVM build now).

I think I'm hitting this because reasonable Python projects and releases
pre-supposes a robust dynamic linkage story. Also, I use Windows and am
very aware that LLVM basically does not support dynamic linking on Windows
-- and cannot without more structure (and in my experience, this structure
would also benefit the robustness of dynamic linking on the others).

Several of us got together to discuss this in November
<https://llvm.discourse.group/t/meeting-notes-mlir-build-install-and-shared-libraries/2257>.
We generally agreed that BUILD_SHARED_LIBS was closer to what we wanted vs
libLLVM/libMLIR, but the result is really only factored for development
(i.e. not every add_library should result in a shared object -- the shared
library surface should mirror public interface boundaries and add_library
mirrors private boundaries). The primary difference between the two is:

   - BUILD_SHARED_LIBS preserves the invariant that every translation unit
   will be "homed" in one library at link time (either .so/.dll or .a) and the
   system will never try to link together shared and static dependencies of
   the same thing (which is what libLLVM/libMLIR do today). It turns out that
   this is merely a good idea on most platforms but is the core requirement on
   native Windows (leaving out mingw, which uses some clever and dirty tricks
   to try to blend the worlds).
   - LLVM_BUILD_LLVM_DYLIB treats libLLVM.so as a "bucket" to throw things
   that might benefit from shared linkage, but end binaries end up also
   needing to link against the static libraries in case if what you want isn't
   in libLLVM.so. When this is done just right, it can work (on Unix) but it
   is very fragile and prone to multiple definition and other linkage issues
   that can be extremely hard to track down.

*What I did:*

   1. Well, first, I tried looking the other way for a few months and
   hoping someone else would fix it :)
   2. When I started trying to generalize some of the shared library
   handling for MLIR and NPCOMP, I noted that the LLVM_LINK_COMPONENTS (as in
   named groups of things) are in the right direction of having a structure to
   the libraries, and I found that I could actually rebase all of what the
   LLVM_LINK_COMPONENTS was trying to do on the same facility, relegating the
   existing LLVM_LINK_COMPONENTS to a name normalization layer on top of a
   more generic "LLVM Components" facility that enforces stricter layering and
   more control than the old libLLVM.so facility did.
   3. I rewrote it twice to progressively more modern CMake and was able to
   eliminate all of the ad-hoc dependency tracking in favor of
   straight-forward use of INTERFACE libraries and $<TARGET_PROPERTY>
   generator expressions for selecting static or dynamic component trees based
   on global flags and the presence (or absence) of per-executable
   LLVM_LINK_STATIC properties
      1. Note that since this is rooted only in CMake features and not LLVM
      macros, out of tree, non-LLVM projects should be able to depend on LLVM
      components in their own targets.
   4. I hacked up AddLLVM/LLVM-Build/LLVM-Config to (mostly) use the new
   facility (leaving out a few things that can be fixed but aren't conceptual
   issues), applied a bunch of fixes to the tree that were revealed by
   stricter checks and got all related tests passing for LLVM and MLIR (on X86
   -- some mechanical changes need to be made to other targets) for both
   dynamic and static builds.

*What I'd like to do:*

   - Get some consensus that we'd like to improve things in this area and
   that the approach I'm taking makes sense. I can do a lot of the work, but I
   don't want to waste my time, and this stuff is fragile if we keep it in an
   intermediate state for too long (I'm already paying this price downstream).
   - Land LLVMComponents.cmake
   <https://github.com/stellaraccident/llvm-project/blob/newcomponents/llvm/cmake/modules/LLVMComponents.cmake>
   as the basis of the new facility.
   - Finish implementing the "Redirection" feature that would allow us to
   emulate an aggregate libLLVM as it is today.
   - Start pre-staging the various stricter constraints to the build tree
   that will be needed to swap AddLLVM to use the new facility.
   - Rewrite component-related AddLLVM/LLVM-Build/LLVM-Config bits in a
   more principled way to use the new facility (or remove features entirely
   that are no longer needed) -- what I did in the above patch was just a
   minimal amount of working around for a POC.
   - Agree on whether we should try to have the two co-exist for a time or
   do a more clean break with the old.
   - Start applying the facility to downstream projects like MLIR and
   NPCOMP.

*What I would need:*

   - Help, testing and expertise. I am reasonably confident in my
   understanding of how to make shared libraries work and how to use CMake,
   but the legacy in LLVM here is deep -- I likely pattern matched some old
   features as no longer needed when they actually are (I am not clear at all
   on how much of LLVM-Config is still relevant).
   - Pointers to who the stakeholders are that I should be coordinating
   with.

Comments?

Thanks!
- Stella
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