llvmPackages_11: darwin cross compilation and bootstrapping

pkgs/development/compilers/llvm/11/compiler-rt/default.nix

···
 let
 
   useLLVM = stdenv.hostPlatform.useLLVM or false;
+  isDarwin = stdenv.hostPlatform.isDarwin;
   bareMetal = stdenv.hostPlatform.parsed.kernel.name == "none";
   haveLibc = stdenv.cc.libc != null;
   inherit (stdenv.hostPlatform) isMusl;
···
   src = fetch "compiler-rt" "0x1j8ngf1zj63wlnns9vlibafq48qcm72p4jpaxkmkb4qw0grwfy";
 
   nativeBuildInputs = [ cmake python3 llvm.dev ];
-  buildInputs = lib.optional stdenv.hostPlatform.isDarwin libcxxabi;
 
   NIX_CFLAGS_COMPILE = [
     "-DSCUDO_DEFAULT_OPTIONS=DeleteSizeMismatch=0:DeallocationTypeMismatch=0"
···
     "-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON"
     "-DCMAKE_C_COMPILER_TARGET=${stdenv.hostPlatform.config}"
     "-DCMAKE_ASM_COMPILER_TARGET=${stdenv.hostPlatform.config}"
-  ] ++ lib.optionals (useLLVM || bareMetal || isMusl) [
+  ] ++ lib.optionals (useLLVM || isDarwin || bareMetal || isMusl) [
     "-DCOMPILER_RT_BUILD_SANITIZERS=OFF"
     "-DCOMPILER_RT_BUILD_XRAY=OFF"
     "-DCOMPILER_RT_BUILD_LIBFUZZER=OFF"
     "-DCOMPILER_RT_BUILD_PROFILE=OFF"
-  ] ++ lib.optionals ((useLLVM || bareMetal) && !haveLibc) [
+  ] ++ lib.optionals ((useLLVM || isDarwin || bareMetal) && !haveLibc) [
     "-DCMAKE_C_COMPILER_WORKS=ON"
     "-DCMAKE_CXX_COMPILER_WORKS=ON"
     "-DCOMPILER_RT_BAREMETAL_BUILD=ON"
     "-DCMAKE_SIZEOF_VOID_P=${toString (stdenv.hostPlatform.parsed.cpu.bits / 8)}"
-  ] ++ lib.optionals (useLLVM && !haveLibc) [
+  ] ++ lib.optionals ((useLLVM || isDarwin) && !haveLibc) [
     "-DCMAKE_C_FLAGS=-nodefaultlibs"
-  ] ++ lib.optionals (useLLVM) [
+  ] ++ lib.optionals (useLLVM || isDarwin) [
     "-DCOMPILER_RT_BUILD_BUILTINS=ON"
     #https://stackoverflow.com/questions/53633705/cmake-the-c-compiler-is-not-able-to-compile-a-simple-test-program
     "-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY"
···
   ]# ++ lib.optional stdenv.hostPlatform.isMusl ./sanitizers-nongnu.patch
     ++ lib.optional stdenv.hostPlatform.isAarch32 ./armv7l.patch;
 
+
+  preConfigure = lib.optionalString isDarwin ''
+    cmakeFlagsArray+=("-DCMAKE_LIPO=$(command -v ${stdenv.cc.targetPrefix}lipo)")
+  '';
+
   # TSAN requires XPC on Darwin, which we have no public/free source files for. We can depend on the Apple frameworks
   # to get it, but they're unfree. Since LLVM is rather central to the stdenv, we patch out TSAN support so that Hydra
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd
···
       --replace 'set(X86 i386)' 'set(X86 i386 i486 i586 i686)'
   '' + lib.optionalString stdenv.isDarwin ''
     substituteInPlace cmake/builtin-config-ix.cmake \
-      --replace 'set(ARM64 arm64 arm64e)' 'set(ARM64)'
+      --replace 'foreach(arch ''${ARM64})' 'foreach(arch)'
     substituteInPlace cmake/config-ix.cmake \
       --replace 'set(COMPILER_RT_HAS_TSAN TRUE)' 'set(COMPILER_RT_HAS_TSAN FALSE)'
   '' + lib.optionalString (useLLVM) ''

pkgs/development/compilers/llvm/11/default.nix

···
 
     compiler-rt-libc = callPackage ./compiler-rt {
       inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
+      stdenv = if (stdenv.hostPlatform.useLLVM or false) || stdenv.hostPlatform.isDarwin
                then overrideCC stdenv buildLlvmTools.clangNoCompilerRtWithLibc
                else stdenv;
     };
 
     compiler-rt-no-libc = callPackage ./compiler-rt {
       inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
+      stdenv = if (stdenv.hostPlatform.useLLVM or false) || stdenv.hostPlatform.isDarwin
                then overrideCC stdenv buildLlvmTools.clangNoCompilerRt
                else stdenv;
     };
 
     # N.B. condition is safe because without useLLVM both are the same.
-    compiler-rt = if stdenv.hostPlatform.isAndroid
+    compiler-rt = if stdenv.hostPlatform.isAndroid || (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64)
       then libraries.compiler-rt-libc
       else libraries.compiler-rt-no-libc;
 
···
 
     libcxx = callPackage ./libcxx {
       inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
+      stdenv = if (stdenv.hostPlatform.useLLVM or false) || stdenv.hostPlatform.isDarwin
                then overrideCC stdenv buildLlvmTools.clangNoLibcxx
                else stdenv;
     };
 
     libcxxabi = callPackage ./libcxxabi {
       inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
+      stdenv = if (stdenv.hostPlatform.useLLVM or false) || stdenv.hostPlatform.isDarwin
                then overrideCC stdenv buildLlvmTools.clangNoLibcxx
                else stdenv;
     };
 
     libunwind = callPackage ./libunwind {
       inherit llvm_meta;
-      stdenv = if stdenv.hostPlatform.useLLVM or false
+      stdenv = if (stdenv.hostPlatform.useLLVM or false) || stdenv.hostPlatform.isDarwin
                then overrideCC stdenv buildLlvmTools.clangNoLibcxx
                else stdenv;
     };

pkgs/development/compilers/llvm/11/libcxx/default.nix

···
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
-    ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF";
+    ] ++ lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF"
+
+    # TODO: this is a bit of a hack to cross compile to Apple Silicon.  libcxx
+    # starting with 11 enables CMAKE_BUILD_WITH_INSTALL_NAME_DIR which requires
+    # platform setup for rpaths. In cmake, this is enabled when macos is newer
+    # than 10.5. However CMAKE_SYSTEM_VERSION is set to empty (TODO: why?)
+    # which prevents the conditional configuration, and configure fails.  The
+    # value here corresponds to `uname -r`. If stdenv.hostPlatform.release is
+    # not null, then this property will be set via mkDerivation (TODO: how can
+    # we set this?).
+    ++ lib.optional (
+      stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64 &&
+      stdenv.hostPlatform != stdenv.buildPlatform
+    ) "-DCMAKE_SYSTEM_VERSION=20.1.0";
 
   passthru = {
     isLLVM = true;

pkgs/development/compilers/llvm/11/libcxxabi/default.nix

···
         # the magic combination of necessary CMake variables
         # if you fancy a try, take a look at
         # https://gitlab.kitware.com/cmake/community/-/wikis/doc/cmake/RPATH-handling
-        install_name_tool -id $out/$file $file
+        ${stdenv.cc.targetPrefix}install_name_tool -id $out/$file $file
       done
       make install
       install -d 755 $out/include

pkgs/top-level/all-packages.nix

···
     inherit (stdenv.targetPlatform) libc;
   in     if libc == "msvcrt" then targetPackages.windows.mingw_w64_headers or windows.mingw_w64_headers
     else if libc == "nblibc" then targetPackages.netbsdCross.headers or netbsdCross.headers
+    else if libc == "libSystem" && stdenv.targetPlatform.isAarch64 then targetPackages.darwin.LibsystemCross or darwin.LibsystemCross
     else null;
 
   # We can choose: