[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 11:23:18 PDT 2016

> On Jun 9, 2016, at 10:49 AM, Justin Bogner <mail at justinbogner.com> wrote:
> Chris Bieneman <beanz at apple.com <mailto:beanz at apple.com>> writes:
>> 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
>> 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.
> This all seems pretty sensible. Should we also use the opportunity to
> split compiler-rt's builtins and profiling/sanitizer/etc runtimes, since
> we'll be moving things around anyway?

About that… So this is a complicated issue, but we should discuss it. Building compiler-rt as a monolithic chunk under the runtimes model is actually problematic because it would have circular dependencies.

For example:

libclang_rt.asan.aarch64 depends on libcxx.aarch64, but libcxx.aarch64 depends on libclang_rt.builtins.aarch64.

That means to satisfy the proper build dependencies you need to build clang, then builtins, then libcxx, then the sanitizer libraries.

Since Compiler-RT’s build does support configuring the builtins directory separately from the sanitizers, we could support this with my runtimes proposal without changing anything else in Compiler-RT through the application of some project-specific hacks. Not idea, but it means this doesn’t need to block my proposal.

We should consider other alternatives, because it would nice to not have hacks. As an added benefit, if we separated the builtins and sanitizers into separate libraries we would be able to have the sanitizer libraries licensed the same way as LLVM (with the attribution clause), which would limit the amount of code that has its wonky modified license.


> Some place like test/external or test/integration would probably make
> sense. It could potentially also be used for other optional tests like
> debuginfo-tests, which are currently somewhat awkwardly checked out into
> clang's tests.
>> 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
> Sure, but we might as well update the documentation earlier (in step 1)
> - as soon as compiler-rt can live in runtimes it makes sense to tell
> people to put it there, even if we still have legacy logic to make it
> continue to work out of projects as well.
>> (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
>>> <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org> <mailto: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> <mailto: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 <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()
>>>> +
>>>>     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
>>>> +               -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}
>>>>                ${PASSTHROUGH_VARIABLES}
>>>> @@ -152,7 +165,7 @@
>>>>     ExternalProject_Add_Step(${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
>>>> -      NO_INSTALL)
>>>> +      NO_INSTALL
>>>> +      ALWAYS_CLEAN)
>>>>   endif()
>>>>   add_subdirectory(test)
>>>>   add_subdirectory(unittests)
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