[llvm-dev] Upcoming change with how libc++, libc++abi and libunwind are being built
Andrzej Warzynski via llvm-dev
llvm-dev at lists.llvm.org
Thu Oct 28 01:21:38 PDT 2021
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 ?
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
> 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
> 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.
> 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.
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