[llvm-dev] Upcoming change with how libc++, libc++abi and libunwind are being built

Thu Oct 28 06:34:28 PDT 2021

On Thu, Oct 28, 2021 at 4:22 AM Andrzej Warzynski <andrzej.warzynski at arm.com>
wrote:

> Hi Louis,
>
> This sounds like a huge simplification, thank you!
>
> With this change, will we still be able to use LLVM_ENABLE_PROJECTS to
> build libc++, libc++abi and libunwind? IIUC, you are disabling this
> option entirely, right? Will CMake warn us about the
> incorrect/deprecated usage? Should the LLVM documentation be updated [1]?
>

Hi,

The goal is to deprecate and then entirely remove building the runtimes
with LLVM_ENABLE_PROJECTS. Libc++'s own documentation has already been
updated, but I will update the documentation you linked to and add a
warning in the CMake -- thanks for the suggestion!

Cheers,
Louis

>
> Thank you,
> -Andrzej
>
> [1] https://llvm.org/docs/CMake.html
>
> On 11/10/2021 17:17, Louis Dionne via llvm-dev wrote:
> > Hi folks,
> >
> > This message is both a heads up and a request for comments on an
> > upcoming change we want to make to the way we build
> > libc++/libc++abi/libunwind.
> >
> > For a long time now, those three projects have supported various ways to
> > be built:
> >
> > 1. The Project build, where one would use <root>/llvm as the root of the
> > CMake invocation and pass LLVM_ENABLE_PROJECTS.
> > 2. The Standalone build, where one would perform one CMake invocation
> > per project being built, and fill in various CMake variables to tell
> > each project where to find bits of the other projects.
> > 3. The "Runtimes" or "Bootstrapping" build, where one would use
> > <root>/llvm as the root of the CMake invocation and pass
> > LLVM_ENABLE_RUNTIMES. This would result in Clang being built, and then
> > the runtimes being built with that fresh Clang.
> > 4. The "Unified Standalone" build, where one would use
> > <root>/libcxx/utils/ci/runtimes as the root of the CMake invocation and
> > pass LLVM_ENABLE_PROJECTS. This was introduced to work around the fact
> > that the Project build doesn't work on several embedded platforms, and I
> > think it is only used at Apple.
> >
> > Through the years, this has gotten rather confusing and difficult to
> > support. We currently support all of those on a best-effort basis and we
> > do have build bots checking each of them, however the complexity is
> > often getting in the way of providing proper support and improving the
> > build system of those projects. We've been trying to migrate to a
> > simpler way to build the runtimes that works for all use cases (notably
> > embedded platforms) for a long time, and we recently got to a point
> > where this is possible (thanks mainly to @phosek and @mstorsjo). To
> > summarize what we're aiming for, we'll now have two ways of building
> > libc++/lib++abi/libunwind:
> >
> > 1. The "Default" build, where the CMake invocation is rooted at
> > <root>/runtimes and one uses LLVM_ENABLE_RUNTIMES.
> > 2. The "Bootstrapping" build, where the CMake invocation is rooted at
> > <root>/llvm and one uses LLVM_ENABLE_RUNTIMES (this is the current
> > "Bootstrapping" build unchanged).
> >
> > If you have been using the "Project", "Standalone" or "Unified
> > Standalone" builds, you should be moving to the new "Default" build.
> > This should be really easy to achieve, basically you can just change
> > LLVM_ENABLE_PROJECTS to LLVM_ENABLE_RUNTIMES, perform a single
> > invocation of CMake instead of multiple invocations, and pass all the
> > LIBCXX_FOO, LIBCXXABI_FOO and LIBUNWIND_FOO options to that single CMake
> > invocation. See the release note in the PR linked below for more details.
> >
> > We are aiming to deprecate all the old ways to build as soon as
> > possible, and to keep supporting them until after the LLVM 14 release,
> > at which point we would like to remove support for those and get started
> > with making improvements. The review at https://reviews.llvm.org/D111356
> > <https://reviews.llvm.org/D111356> enshrines this deprecation in the
> > documentation. For technical discussion on this proposed change, please
> > comment on that review to keep everything in one place.
> >
> > Cheers,
> > Louis
> >
> > Questions you might be asking yourself:
> >
> > Q: Why doesn't the Project build work for embedded platforms?
> > A: A lot of CMake code in <root>/llvm expects that we're building for a
> > target with the usual capabilities for a build host, but that isn't the
> > case for several embedded targets. While it doesn't make sense to build
> > LLVM or Clang for those embedded targets (which would never be used as
> > build hosts), it makes sense to build the runtimes for those targets if
> > you are cross-compiling.
> >
> > Q: Currently, we use the Standalone build and are not required to have
> > all of the Monorepo available. Does this change our ability to do that?
> > A: Not in the mid term. Building any of the runtimes already requires
> > all the runtimes to be co-located. The only thing you could get away
> > with previously was not having the part in <root>/llvm. With the
> > "Default" build, this will be possible as soon as we move a few CMake
> > helpers from <root>/llvm/cmake to <root>/cmake. Then, you'll only have
> > to have <root>/runtimes, <root>/cmake and the projects you are building.
> >
> > Q: What's the difference between the "Runtimes build" and the
> > "Bootstrapping build"?
> > A: None. We're trying to move away from the term "Runtimes build"
> > because it's waaaay too confusing, and doesn't describe the fact that
> > we're really building the runtimes with a just-built Clang.
> >
> > Q: What's up with naming the new default build the "Default build"?
> > A: The name sucks, but we had to pick one and the "Runtimes build" was
> > already taken for the build that does a bootstrap. Suggestions are
> > welcome -- basically what this performs is a normal build of the
> > runtimes using your system compiler, like you used to do with
> > LLVM_ENABLE_PROJECTS (but it's better).
> >
> > Q: What are the benefits of this migration beyond simplifying the code
> > for maintainers?
> > A: Reducing the number of ways we can build the runtimes means that all
> > the workflows for building the runtimes will become simpler for users
> > (especially vendors). Furthermore, the new "Default" build is as simple
> > as the old Project build, but it is faster to configure and works on
> > embedded platforms. In the near future, it should also be possible to
> > perform the equivalent of an old "Project build" without having all of
> > the monorepo available.
> >
> > Q: Does this affect other "runtimes" projects like compiler-rt?
> > A: Not for the time being. We will probably attempt to move those over
> > in the future as well, however for the time being I can only confirm
> > that this works fine for libc++, libc++abi and libunwind, so that's
> > where my deprecation efforts are focused.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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