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

[Stella Laurenzo via llvm-dev]

On Mon, Jan 4, 2021 at 11:04 AM Tom Stellard <tstellar at redhat.com> wrote:

> On 1/3/21 1:49 PM, Stella Laurenzo via llvm-dev wrote:
> > 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.
>
> Do you have a proposed list of components yet for LLVM?
>
> >   * 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.
> >
>
> It sounds like what you are proposing is BUILD_SHARED_LIBS=ON but with
> fewer total libraries, is this an accurate summary?
>
>
I think that is a reasonable summary for the level that most people care
about. It might be a bit pedantic, but what I'm aiming for is for us to be
able to define the shared library set to correspond with our notion of
component boundaries (follows public APIs), as that is what opens up the
ability to optimize them in the future (BUILD_SHARED_LIBS is just a 1:1
add_library call -> shared library approach and leaks a lot of private
boundaries). Also, it preserves the ability for executables to choose to
link statically or dynamically, which is important for some things (and
likely will remain so, especially when considering downstream).


> I would prefer for any large change like this that we do not add any net
> new configuration options (meaning if we add a new option we should
> remove an old one)to LLVM as we already have too many.  Would this be
> able to replace BUILD_SHARED_LIBS=ON?
>

Completely agree in the end state. I would like to converge on one
configuration option that enables shared linking and then remove the
others. I suspect that downstreams may want to customize things a bit more,
but we should avoid adding those options to the extent possible in favor of
seeing if we can make the default way workable before fragmenting.

Note that BUILD_SHARED_LIBS is a published way in the CMake ecosystem to
tell a project to build in shared library mode. If we get this all fixed,
we may still want to recognize when users set it and do the right thing
(i.e. make it more of an alias). This viewpoint would argue for removing
LLVM_BUILD_LLVM_DYLIB and just supporting BUILD_SHARED_LIBS (but with new
behavior). Either way, we should keep the variants to a minimum.


>
> - Tom
>
> > *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
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
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