[llvm-dev] Clarifying the supported ways to build libc++, libc++abi and libunwind

Shoaib Meenai via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 8 13:21:01 PDT 2020

(CCing Chris and Petr, who’ve done the most work on the runtimes build)

At least for me on Linux, using LLVM_ENABLE_PROJECTS is actually the unusual way of building libc++; I use LLVM_ENABLE_RUNTIMES. The reason is, my host compiler is often gcc, but I want to build, test, and ship libc++ with the clang I just built.

The runtimes build is when you use LLVM_ENABLE_RUNTIMES. It sets up the build of all runtimes (compiler-rt, libc++, libc++abi, libunwind, etc.) as a CMake ExternalProject which depends on the build of clang and other toolchain tools. In other words, if I run the following:

cmake -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi' path/to/my/llvm-project/llvm
ninja cxx

The build system will automatically build clang and other toolchain tools (e.g. llvm-ar), run the ExternalProject configuration with e.g. CMAKE_C_COMPILER and CMAKE_CXX_COMPILER set to the just-built clang, and then build libc++ with that configuration (so with the just-built clang). It’s a pretty convenient workflow for my setup. It also takes care of e.g. automatically rebuilding libc++ if you make changes to clang and then run `ninja cxx` again.

As for why the runtimes build use the “standalone build” setup, it’s because there’s a separate CMake configuration happening for the runtimes in this setup (which is necessary in order to be able to configure them to use the just-built toolchain), so e.g. clang isn’t available as an in-tree target. See https://reviews.llvm.org/D62410 for more details. Your top-level CMakeLists.txt in the runtimes build is llvm/runtimes/CMakeLists.txt and not libcxx/CMakeLists.txt (as it would be in a fully standalone build), but it’s also not llvm/CMakeLists.txt (as it would be with LLVM_ENABLE_PROJECTS).

At the CMake round table at the dev meeting last October, we’d discussed the runtimes builds, and Chris had advanced that there should be two supported ways to build the runtimes:

  1.  If you’re building as part of the LLVM build and using LLVM_ENABLE_RUNTIMES
  2.  If you’re building fully standalone (as in you’re pointing CMake to libc++’s source directory, so that your top-level CMakeLists is libcxx/CMakeLists.txt)

I agree with that position. In particular, I think LLVM_ENABLE_PROJECTS is definitely the wrong thing to use for compiler-rt, which is strongly tied to your just-built compiler. I think it’s arguably the wrong thing to use for libc++/libc++abi/libunwind as well, where you either want to use your just-built Clang (which LLVM_ENABLE_RUNTIMES was made for), or a different compiler (in which case you’d do a fully standalone build), but silently using your host compiler for them is probably not what you want. (The LLVM_ENABLE_PROJECTS workflow for libc++/libc++abi/libunwind probably works out better on macOS, where your host compiler is a recent Clang anyway so the difference between it and a just-built Clang aren’t as marked, but I’ve had issues even on macOS in the past where using the host compiler via LLVM_ENABLE_PROJECTS gave me weird libc++ test errors, and LLVM_ENABLE_RUNTIMES just worked.)

What do you think?

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Louis Dionne via llvm-dev <llvm-dev at lists.llvm.org>
Reply-To: Louis Dionne <ldionne at apple.com>
Date: Wednesday, April 8, 2020 at 8:46 AM
To: Libc++ Dev <libcxx-dev at lists.llvm.org>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Clarifying the supported ways to build libc++, libc++abi and libunwind

[Cross-post to llvm-dev to make sure everybody relevant sees this]


I'm currently trying to simplify the libc++/libc++abi/libunwind build systems and testing setup. In doing so, I am encountering issues related to "unusual" ways of building them. By unusual, I just mean "not the usual monorepo build with LLVM_ENABLE_PROJECTS". I would like to pin down what the set of supported use cases for building the runtime libraries are. In particular, the world I would like to live in is one where the only way to build libc++/libc++abi/libunwind is:

    $ mkdir build
    $ cd build
    $ cmake <monorepo-root>/llvm -DLLVM_ENABLE_PROJECTS=libcxx;libcxxabi;libunwind <options>
    $ ninja -C build install-{cxx,cxxabi,unwind}

The "runtimes" build would be built on top of this -- it would be just a driver for building these libraries using documented options against the just-built Clang. I think it already does so in essence, however if I'm not mistaken it uses the "Standalone build" and it definitely sets some magic and undocumented CMake variables (like HAVE_LIBCXXABI) that we have to be really careful not to break.

So, to better understand what people use today, I have some questions. I know the answer to some of those, but I want to see what others have to say:

1. What is a "Standalone build"? What does it enable that a normal monorepo build can't?
2. What is the "Runtimes" build? How does it work, what is it used for, and what does it expect from libc++/libc++abi/libunwind?
3. Are there other "hidden" ways to build the runtime libraries?


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