[llvm-dev] [RFC] LLVM Directory Structure Changes (was Re: [PATCH] D20992: [CMake] Add LLVM runtimes directory)

Chris Bieneman via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 9 13:39:50 PDT 2016

Hey Ben,

Thank you for providing this feedback. I’m going to lay out some ideas that I have inline below.

> On Jun 9, 2016, at 11:26 AM, Craig, Ben via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> I'm great with moving the runtimes into their own directory and making cmake modules to standardize an interface between the LLVM build process and the runtime build process.  I would like to ask for more though.
> I find working with the runtimes a bit frustrating at times, and I think a lot of that frustration stems from the second-class nature of the runtime build process.  The "best" way to build just the runtimes today is to clone them in the llvm tree, pass a very long cmake line, then run a very specific make / ninja line (i.e. ninja cxx, ninja check-libcxxabi).  This is even the case if I don't want to use the freshly generated compiler.  Here are some specific nuisances that running in-tree has caused me:

Agree on all points. It might be useful to provide an alternate top-level CMake file for runtime projects. There is some complication because the runtimes do use CMake modules from LLVM, so you’ll need either an LLVM checkout or a built & installed LLVM. My general hope in formalizing a runtimes directory is to start treating runtimes as first-class members of the LLVM project. Admittedly I’m coming from a different perspective than you, so I’m tackling the "LLVM & Clang developer who also wants runtimes” side of the problem. I am, however, sympathetic to your side too.

> I routinely need to pull a new LLVM for new cmake files, even though I am not building LLVM.
Some of this may be avoidable through restructuring our CMake modules. I think we should discuss this separately from the changes I’m asking for, but my general idea is it might be reasonable to create a separate LLVM repository that stores common CMake modules. If we did that it would require that *everyone* building LLVM would need to have those modules. We may be able to hide that complexity using CMake’s ExternalProject stuff. I’ll take it as a line item to look into that.
> I don't get to use "standard" commands to build and test the components I work with.  I want to just run make / ninja with no arguments, but instead, I need to build specific targets.
This is the kind of thing I see as potentially solvable with a separate top-level CMake file for runtimes. That said, in general I’ve been moving toward adding more and more explicit targets into CMake. As a result of that my natural workflow is involving less and less running general commands and more and more running specific “ninja <some tool>” and “ninja check-llvm-<some test set>”.
> Choices made for LLVM's build end up affecting mine, and I don't have a good way to change them.  For example, I wanted to perform some libcxx builds without -Wl,-z,defs.  I had to do some wacky string filtering to remove it from the compile line after the fact, instead of preventing it from getting added in the first place.  There wasn't a flag available on the LLVM side to disable it at configure time.

LLVM’s flags impacting libcxx is fixed by my runtimes proposal. In fact, that’s part of the point. Bleeding options from LLVM & Clang builds into runtime libraries is not cool. It causes lots of problems on Darwin, so we’re sensitive to this.

> To get cmake to work, I have to set HAVE_CXX_ATOMICS_WITHOUT_LIB, even though I have no intention of building LLVM.  I then get to set LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB too, because reasons.
This is bad. I’m curious why you need to set those ever. Have you diagnosed this? For you to need to set that it means the host toolchain isn’t properly passing the CMake checks.
> Multi-libs require multiple independent build directories, with all the associated cmake overhead.
I strongly believe that for building multi-lib or more generally when building for multiple targets you want multiple build directories, and in particular the multiple-cmake invocations. I believe this because you want the checks to be relevant for the target, and the only way to do that is to run the checks once per target.

That said, I also strongly believe that for any user of our projects we should find a way to have a single simple CMake invocation that gets the end result that you want. I don’t believe these are mutually exclusive goals.

One of the development steps of the new runtime directory will be supporting specifying multiple targets to build the runtimes for, and having CMake construct the appropriate number of build directories and manage building them all through a single top-level configuration and build directory. If you’re skeptical about how doable this is I’d encourage you to look at this bot -> http://lab.llvm.org:8011/builders/clang-3stage-ubuntu.

That bot does a full 3-stage clang build from a single CMake invocation:

cmake -C ../llvm.src/tools/clang/cmake/caches/3-stage.cmake -GNinja -DLLVM_TARGETS_TO_BUILD=all -DLLVM_BINUTILS_INCDIR=/opt/binutils/include ../llvm.src

> So why not run out of tree instead you may ask?
> No lit tests or lit utilities (FileCheck, not, etc…)
I don’t know if the runtimes you’re building are setup for this or not, but you can get out-of-tree tests working if you have an LLVM installation on the system or a build directory that you can point at. Compiler-RT does this. It isn’t ideal but it is workable.

We should have a better solution.
> Even more difficult to manage dependencies between libcxxabi and libcxx
Yep. That sucks.
> So some things I would like to see...
> Standalone runtime builds should use the "normal" build interfaces (bare make, make all, make check, make install.  s/make/ninja as desired).
I think this is doable, but I’m hesitant to rope it in with what I’m trying to do here. Nothing I want to do would prevent this or make it any harder than it already is.
> For in-tree builds, LLVM would use the new cmake ExternalProject feature.  This way LLVM's in-tree support could be implemented in terms of the runtime's "normal" build interfaces.  LLVM may also define a refinement of that interface to provide extra LLVM information on top.
> For example, maybe all llvm runtime projects get passed something like LLVM_RUNTIME_LIBCXXABI_PRESENT and LLVM_RUNTIME_LIBCXXABI_PATH, and those projects can act on that or not.
Yes, there will need to be a mechanism for communicating project dependencies between runtimes.
> Developers using standalone builds can use the same "LLVM build" interface as the in-tree builds use.
Yes. I’ve actually been having some hallway conversations about how to standardize the build interface a bit more cleanly. Some of this will depend on a per-project basis, but I think compiler-rt does some of this right today. 

Specifically if an LLVM build tree is available it builds as if it were in-tree and being distributed with that build. If the LLVM build tree isn’t available, it builds in a more standard *nix format that would be compatible with non-LLVM toolchains. I may be wrong, but I think that is probably the right behavior for all our runtimes.
> Break out testing infrastructure to a common repo, so that the runtimes can have access to the testing "banana" without dragging along the LLVM "gorilla”.
I’m hesitant to suggest more and more repos because I think there are some challenges and additional burdens with that. I do understand the benefit of what you’re asking for here, and I think it is worth considering. I think there is an argument for splitting out the LLVM testing infrastructure, as well as an argument for splitting out the LLVM build infrastructure.

In both cases I think those changes are larger than what I’m proposing, but worth considering.


Obligatory troll: Maybe we should move to github and change the whole repo structure in the process?
> On 6/9/2016 12:20 PM, Chris Bieneman via llvm-dev wrote:
>> Moving to llvm-dev (I think this has gone a bit further than a patch review discussion)
>> In hindsight I probably should have explained more of my thinking on this with the patch, or done an RFC on llvm-dev to start with. I’l do that now, and answer the questions along the way. I sent a separate email discussing Justin’s patch review feedback.
>> In the build system today there is no strong distinction between ‘projects’ and ‘tools’. There are a few subtle differences, but I’m not sure any of them really matter. The differences are:
>> (1) The projects directory is always configured, tools can be disabled using LLVM_INCLUDE_TOOLS=Off (projects and tools can both be individually disabled too)
>> (2) Projects are configured before tools, so tools can rely on targets being created for projects (we don’t really use this, and anywhere we are is probably a bug)
>> (3) Some projects have special handling. For example test-suite isn’t actually treated as a project, it has special handling in LLVM/CMakeLists.txt:727, and Compiler-RT is handled by clang if you set LLVM_BUILD_EXTERNAL_COMPILER_RT=On.
>> With this in mind I was thinking about the general usability of our build system. The distinction between a project         and a tool is not very clear. At a high level I see three different use cases that are covered by our current projects & tools directories.
>> (1) Projects that are configured with LLVM
>> (2) Runtime projects that should be configured using the just-built tools
>> (3) The LLVM test-suite, which is really just external tests that should be configured and run with the just-built tools
>> My proposal is that we make the tools subdirectory the *only* place for projects that fall into category 1. I don’t think there is any technical reason to drop an in-tree project into projects over tools today, and I think we migrating people who are doing that away from it should be easy.
>> Second I want to add a “runtimes” directory to LLVM to cover case 2 (see D20992). The idea behind this is to use common code in LLVM to support building runtimes. This will allow the full LLVM toolchain to be visible during configuration. I will abstract this functionality into an installed CMake module so that Clang can use it for out-of-tree clang builds.
>> Lastly we need to give the test-suite a new home. I’m not super concerned with where we do that. It could be under tests, it could just be at the root of the LLVM directory. I don’t think it matters too much because it is a one-off. Thoughts welcome.
>> My proposed patch makes the runtimes directory work for Compiler-RT, but it doesn’t yet handle libcxxabi, libcxx and libunwind. There is some special case handling between libcxxabi and libcxx that will need to be handled to make the dependencies work between the two, and I still need to work that out.
>> If we want to go with this proposal I envision the transition being multi-staged:
>> (1) Adding the new functionality, getting it up and fully working for all runtime projects - this will involve changes to runtime projects
>> (2) Work with bot maintainers to migrate bots, and fix any issues that come up
>> (3) Add support for a new secondary location for the test-suite
>> (4) Set a date for removing the projects directory, post patches including updated documentation
>> (5) Remove the projects directory entirely
>> Thoughts?
>> -Chris
>>> On Jun 8, 2016, at 6:59 PM, Chandler Carruth <chandlerc at gmail.com <mailto:chandlerc at gmail.com>> wrote:
>>> On Wed, Jun 8, 2016 at 4:39 PM Justin Bogner via llvm-commits < <mailto:llvm-commits at lists.llvm.org>llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
>>> Chris Bieneman <beanz at apple.com <mailto:beanz at apple.com>> writes:
>>> > beanz created this revision.
>>> > beanz added reviewers: chandlerc, bogner.
>>> > beanz added a subscriber: llvm-commits.
>>> >
>>> > There are a few LLVM projects that produce runtime libraries. Ideally
>>> > runtime libraries should be built differently than other projects,
>>> > specifically they should be built using the just-built toolchain.
>>> >
>>> > There is support for building compiler-rt in this way from the clang
>>> > build. Moving this logic into the LLVM build is interesting because it
>>> > provides a simpler way to extend the just-built toolchain to include
>>> > LLD and the LLVM object file tools.
>>> >
>>> > Once this functionality is better fleshed out and tested we’ll want to
>>> > encapsulate it in a module that can be used for clang standalone
>>> > builds, and we’ll want to make it the default way to build compiler-rt.
>>> >
>>> > With this patch applied there is no immediate change in the build.
>>> > Moving compiler-rt out from llvm/projects into llvm/runtimes enables
>>> > the functionality.
>>> This seems reasonable, but I am a little worried about how transitioning
>>> to the new system will work. Will everyone have to move their
>>> compiler-rt checkout? Will we continue to support compiler-rt in either
>>> place? Both of these are workable, but neither is great. Thoughts?
>>> I share your concerns, but I also kind of like the direction this is going.
>>> But there is a higher-level meta-point: do we want to keep the 'projects' directory *at all*.
>>> Every single resident of it I can think of except for the test-suite is either dead (dragonegg) or a runtime library.
>>> I think we should either have all the build-integrated projects in a single 'projects' directory (including LLD and Clang), or we should have none of them and use more domain relevant organization (today "tools", you're adding "runtimes", maybe we move the test-suite to go under one of the test directories).
>>> I think we should have a consistent plan here before moving stuff. But once we have it, I think we shouldn't be afraid of re-organizing stuff to make more sense, and just work to get folks to update their checkouts.
>>> -Chandler
>>> > This code has a few improvements over the method provided by
>>> > LLVM_BUILD_EXTERNAL_COMPILER_RT. Specifically the sub-ninja command is
>>> > always invoked, so changes to compiler-rt source files will get built
>>> > properly, so this patch can be used for iterative development with
>>> > just-built tools.
>>> >
>>> >  <http://reviews.llvm.org/D20992>http://reviews.llvm.org/D20992 <http://reviews.llvm.org/D20992>
>>> >
>>> > Files:
>>> >   CMakeLists.txt
>>> >   cmake/modules/LLVMExternalProjectUtils.cmake
>>> >   runtimes/CMakeLists.txt
>>> >
>>> > Index: runtimes/CMakeLists.txt
>>> > ===================================================================
>>> > --- /dev/null
>>> > +++ runtimes/CMakeLists.txt
>>> > @@ -0,0 +1,16 @@
>>> > +include(LLVMExternalProjectUtils)
>>> > +
>>> > +# Discover the projects that use CMake in the subdirectories.
>>> > +# Note that explicit cmake invocation is required every time a new project is
>>> > +# added or removed.
>>> > +
>>> > +add_custom_target(runtimes)
>>> > +
>>> > +file(GLOB entries *)
>>> > +foreach(entry ${entries})
>>> > +  if(IS_DIRECTORY ${entry} AND EXISTS ${entry}/CMakeLists.txt)
>>> > +    get_filename_component(projName ${entry} NAME)
>>> > +    llvm_ExternalProject_Add(${projName} ${entry} USE_TOOLCHAIN)
>>> > +    add_dependencies(runtimes ${projName})
>>> > +  endif()
>>> > +endforeach(entry)
>>> > Index: cmake/modules/LLVMExternalProjectUtils.cmake
>>> > ===================================================================
>>> > --- cmake/modules/LLVMExternalProjectUtils.cmake
>>> > +++ cmake/modules/LLVMExternalProjectUtils.cmake
>>> > @@ -29,7 +29,8 @@
>>> >  #     Extra targets in the subproject to generate targets for
>>> >  #   )
>>> >  function(llvm_ExternalProject_Add name source_dir)
>>> > -  cmake_parse_arguments(ARG "USE_TOOLCHAIN;EXCLUDE_FROM_ALL;NO_INSTALL"
>>> > +  cmake_parse_arguments(ARG
>>> >      "SOURCE_DIR"
>>> >    canonicalize_tool_name(${name} nameCanon)
>>> > @@ -52,6 +53,10 @@
>>> >      endif()
>>> >    endforeach()
>>> >
>>> > +  if(ARG_ALWAYS_CLEAN)
>>> > +    set(always_clean clean)
>>> > +  endif()
>>> > +
>>> >    if(FOUND_CLANG GREATER -1)
>>> >      set(CLANG_IN_TOOLCHAIN On)
>>> > @@ -135,6 +140,14 @@
>>> >      CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
>>> >                 ${compiler_args}
>>> > +               -DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config
>>> > +               -DPACKAGE_VERSION=${PACKAGE_VERSION}
>>> > +               -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
>>> How did you decide which variables need to be passed through like this?
>>> The set seems somewhat arbitrary, but I may be missing something
>>> obvious.
>>> >                 ${ARG_CMAKE_ARGS}
>>> >                 ${PASSTHROUGH_VARIABLES}
>>> >      INSTALL_COMMAND ""
>>> > @@ -152,7 +165,7 @@
>>> >      ExternalProject_Add_Step(${name} force-rebuild
>>> >        COMMAND ${run_build}
>>> >        COMMENT "Forcing rebuild of ${name}"
>>> > -      DEPENDEES configure clean
>>> > +      DEPENDEES configure ${always_clean}
>>> I'm not sure I understand what this does. If I had to guess I'd say that
>>> when ALWAYS_CLEAN is passed the rebuild of the external project always
>>> invokes clean first, but if it's not passed we'll just invoke the
>>> external build and allow it to be incremental if appropriate. Is that
>>> right?
>>> >        ${cmake_3_4_USES_TERMINAL} )
>>> >    endif()
>>> > Index: CMakeLists.txt
>>> > ===================================================================
>>> > --- CMakeLists.txt
>>> > +++ CMakeLists.txt
>>> > @@ -720,6 +720,8 @@
>>> >    add_subdirectory(tools)
>>> >  endif()
>>> >
>>> > +add_subdirectory(runtimes)
>>> > +
>>> >    add_subdirectory(examples)
>>> >  endif()
>>> > @@ -730,7 +732,8 @@
>>> >      llvm_ExternalProject_Add(test-suite ${LLVM_MAIN_SRC_DIR}/projects/test-suite
>>> >        USE_TOOLCHAIN
>>> >        EXCLUDE_FROM_ALL
>>> > -      NO_INSTALL)
>>> > +      NO_INSTALL
>>> > +      ALWAYS_CLEAN)
>>> >    endif()
>>> >    add_subdirectory(test)
>>> >    add_subdirectory(unittests)
>>> >
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