Unverified Commit ca59a201 authored by Rahul Butani's avatar Rahul Butani
Browse files

llvmPackages_15.libcxx: use clang 15 instead of the stdenv's compiler 

libc++ has switched to using `__attribute__((using_if_exists))` to handle
incomplete libc implementations; see: https://github.com/llvm/llvm-project/commit/a9c9183ca42629fa83cdda297d1d30c7bc1d7c91

These essentially require a modern C++ compiler (clang gained support in
LLVM 13: https://github.com/llvm/llvm-project/commit/369c64839946d89cf5697550b6feeea031b2f270,
gcc appears to not have support yet: https://gcc.gnu.org/bugzilla//show_bug.cgi?id=105584).

Previously this was not an issue for us (despite the transition happening
around LLVM 13) but something about the changes to the libc++/libc++-abi
build has made it so that on platforms with incomplete libc impls (i.e.
Darwin is missing `quick_exit`/`at_quick_exit`) we error during the `libcxx-abi`
build when the stdenv's (older, not supporting `using_if_exists`) compiler
tries to import libc symbols that aren't present.

The libc++ docs suggest we use a modern compiler to build libc++ anyways
(https://releases.llvm.org/15.0.0/projects/libcxx/docs/index.html#platform-and-compiler-support)
so this commit uses stdenv's containing the package set's clang to build
libcxx/libcxx-abi.

This is similar to how libc++ bootstrapping builds (https://releases.llvm.org/15.0.0/projects/libcxx/docs/BuildingLibcxx.html#bootstrapping-build)
work.
parent 4fabcf49

pkgs/development/compilers/llvm/15/default.nix

+23 −11
Original line number Diff line number Diff line
@@ -247,27 +247,39 @@ let 


    libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;



    libcxx = callPackage ./libcxx {

      inherit llvm_meta;

      stdenv = if stdenv.hostPlatform.useLLVM or false

               then overrideCC stdenv buildLlvmTools.clangNoLibcxx

               else stdenv;

    };



    libcxxabi = let

      stdenv_ = if stdenv.hostPlatform.useLLVM or false

               then overrideCC stdenv buildLlvmTools.clangNoLibcxx

               else stdenv;

      # CMake will "require" a compiler capable of compiling C++ programs

      # cxx-header's build does not actually use one so it doesn't really matter

      # what stdenv we use here, as long as CMake is happy.

      cxx-headers = callPackage ./libcxx {

        inherit llvm_meta;

        stdenv = stdenv_;

        headersOnly = true;

      };



      # `libcxxabi` *doesn't* need a compiler with a working C++ stdlib but it

      # *does* need a relatively modern C++ compiler (see:

      # https://releases.llvm.org/15.0.0/projects/libcxx/docs/index.html#platform-and-compiler-support).

      #

      # So, we use the clang from this LLVM package set, like libc++

      # "boostrapping builds" do:

      # https://releases.llvm.org/15.0.0/projects/libcxx/docs/BuildingLibcxx.html#bootstrapping-build

      #

      # We cannot use `clangNoLibcxx` because that contains `compiler-rt` which,

      # on macOS, depends on `libcxxabi`, thus forming a cycle.

      stdenv_ = overrideCC stdenv buildLlvmTools.clangNoCompilerRtWithLibc;

    in callPackage ./libcxxabi {

      stdenv = stdenv_;

      inherit llvm_meta cxx-headers;

    };



    # Like `libcxxabi` above, `libcxx` requires a fairly modern C++ compiler,

    # so: we use the clang from this LLVM package set instead of the regular

    # stdenv's compiler.

    libcxx = callPackage ./libcxx {

      inherit llvm_meta;

      stdenv = overrideCC stdenv buildLlvmTools.clangNoLibcxx;

    };



    libunwind = callPackage ./libunwind {

      inherit llvm_meta;

      stdenv = overrideCC stdenv buildLlvmTools.clangNoLibcxx;

pkgs/development/compilers/llvm/15/libcxxabi/default.nix

+7 −0
Original line number Diff line number Diff line
@@ -58,6 +58,13 @@ stdenv.mkDerivation rec { 
  cmakeFlags = [

    "-DLLVM_ENABLE_RUNTIMES=libcxxabi"

    "-DLIBCXXABI_LIBCXX_INCLUDES=${cxx-headers}/include/c++/v1"



    # `libcxxabi`'s build does not need a toolchain with a c++ stdlib attached

    # (we specify the headers it should use explicitly above).

    #

    # CMake however checks for this anyways; this flag tells it not to. See:

    # https://github.com/llvm/llvm-project/blob/4bd3f3759259548e159aeba5c76efb9a0864e6fa/llvm/runtimes/CMakeLists.txt#L243

    "-DCMAKE_CXX_COMPILER_WORKS=ON"

  ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [

    "-DLLVM_ENABLE_LIBCXX=ON"

    "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"