Hi folks, happy new year!
- See comments at the top of LLVMComponents.cmake in my fork.
- Draft phab: https://reviews.llvm.org/D94000
As I’ve been working on NPCOMP trying to come up with a release flow for MLIR derived Python projects (see py-mlir-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. 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:
- Well, first, I tried looking the other way for a few months and hoping someone else would fix it
- 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.
- 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
- 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.1. 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 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.