How to build LLVM (clang,clang++) for Apple M1?

You can set -DDEFAULT_SYSROOT=/path/to/MacOSX11.1.sdk at build time or do export SDKROOT=/path/to/MacOSX11.1.sdk at runtime.

You need to compile with clang -arch arm64 -arch x86_64 to get a fat binary out of clang. You need to do this for Apple clang as well.

UPDATED 8 Feb 2021 Homebrew now supports the M1 based Arm machines, so using that is a better answer than the one below. The info below is potentially still useful if you want to do this on your own, but using brew is likely to be much simpler.

Pre-brew answer

I haven't found a clean solution, but in case it helps anyone else, I do have a horrible hack.

The full recipe, then is configure with this script, then build and install.

# Xcode, Ninja
BUILD_SYSTEM=Ninja
BUILD_TAG=ninja
INSTALLDIR=$HOME/software/clang-12.0.0/arm64

cmake ../llvm \
      -G$BUILD_SYSTEM -B ${BUILD_TAG}_build \
      -DCMAKE_OSX_ARCHITECTURES='arm64' \
      -DCMAKE_C_COMPILER=`which clang` \
      -DCMAKE_CXX_COMPILER=`which clang++` \
      -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_INSTALL_PREFIX=$INSTALLDIR \
      -DLLVM_LOCAL_RPATH=$INSTALLDIR/lib \
      -DLLVM_ENABLE_WERROR=FALSE \
      -DLLVM_TARGETS_TO_BUILD='AArch64' \
      -DLLVM_DEFAULT_TARGET_TRIPLE='aarch64-apple-darwin20.1.0' \
      -DDEFAULT_SYSROOT="$(xcrun --show-sdk-path)" \
      -DLLVM_ENABLE_PROJECTS='clang;openmp;polly;clang-tools-extra;libcxx;libcxxabi' \
#      -DLLVM_ENABLE_PROJECTS='clang;openmp;polly' 

That gives a compiler that finds the right headers, but won't link successfully if OpenMP is used because it doesn't pass on any useful -L path or add a necessary rpath.

To overcome that I created a small shell script that sits in my ~/bin, at the front of my $PATH, which adds those extra linker flags.

#
# A truly awful hack, but it seems necessary.
# Install this with execute permissions as clang and clang++ in
# a directory early in your path, so that it is executed when clang or
# clang++ is needed.
#
# For brew...
INSTALLDIR=/usr/local/opt/llvm
# For a local build.
INSTALLDIR=${HOME}/software/clang-12.0.0/arm64/

# Find out the name of this file, and then invoke the same file in the
# compiler installation, adding the necessary linker directives
CMD=`echo $0 | sed "s/\/.*\///"`
${INSTALLDIR}/bin/${CMD} -L${INSTALLDIR}/lib -Wl,-rpath,${INSTALLDIR}/lib $*

I am not recommending this particularly; there should clearly be a better way to make it work, but it'll do for now, and lets me get back to using the compiler rather than building it!

I was able to build with -DDEFAULT_SYSROOT="$(xcrun --show-sdk-path)" -DCMAKE_INSTALL_PREFIX=/Users/foo/lokal/ and install into the lokal/bin lokal/lib path. Once that is done you can use LD_LIBRARY_PATH=/Users/foo/lokal/lib and all the libraries should be found without mucking with anything else rpath related.

Replying to an old thread, since with MacOS 15 LD_LIBRARY_PATH is no longer an option. I achieved to build LLVM with OpenMP support and make clang generate executables with the right rpath, with this patch to LLVM 18:

cd llvm-project
git apply <<EOF
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 2b916f000336..d3141bfc4007 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -1106,2 +1106,4 @@ void tools::addOpenMPRuntimeLibraryPath(const ToolChain &TC,
   CmdArgs.push_back(Args.MakeArgString("-L" + DefaultLibPath));
+  CmdArgs.push_back(Args.MakeArgString("-rpath"));
+  CmdArgs.push_back(Args.MakeArgString(DefaultLibPath));
 }
EOF