llvm dev - Jan 2021 - Build LLVM for Apple M1 (MacOS Big Sur)

I hope this is on topic for this list, since I expect that someone here must
already have done this successfully, while I am still foundering around.

What I want to achieve is to build LLVM (and, in particular, clang/clang++ with
OpenMP support) to run on and target the Apple M1 ARM processor running MacOS
Big Sur. (I have a new Mac Mini :-)).
Ideally, it would be nice to be able to build a dual target fat-binary in which
the X86_64 component compiled for X86_64 by default, and the Aarch64 component
compiled or Aarch64 by default (this is what the Apple compiler in Xcode does…),
however just being able to build an Aarch64 compiler that worked would be ahead
of what I have managed to achieve.

My best attempt is configured using this script from a build directory inside
the top-level llvm checkout.
The compilers found are those from Xcode.

# Xcode, Ninja
BUILD_SYSTEM=Ninja
BUILD_TAG=ninja

cmake ../llvm \
      -G$BUILD_SYSTEM -B ${BUILD_TAG}_build \
      -DCMAKE_OSX_ARCHITECTURES='arm64' \
      -DCMAKE_C_COMPILER=`which clang` \
      -DCMAKE_CXX_COMPILER=`which clang++` \
      -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_BUILD_WITH_INSTALL_RPATH=1 \
      -DCMAKE_INSTALL_PREFIX=$HOME/software/clang-12.0.0/arm64 \
      -DLLVM_LOCAL_RPATH=$HOME/software/clang-12.0.0/arm64/lib \
      -DLLVM_ENABLE_WERROR=FALSE \
      -DLLVM_TARGETS_TO_BUILD='AArch64' \
      -DLLVM_DEFAULT_TARGET_TRIPLE='aarch64-apple-darwin20.1.0' \
      -DDEFAULT_SYSROOT="$(xcrun --show-sdk-path)" \
     
-DLLVM_ENABLE_PROJECTS='clang;openmp;polly;clang-tools-extra;libcxx;libcxxabi'

Configured like that I can build a compiler, OpenMP runtime and so on. 

However…
1)  when linking it does not add -L$HOME/software/clang-12.0.0/arm64/lib to the
linker command, so it fails to link OpenMP codes which require libomp.dylib,
since it doesn't find it. (See below:  you can see that it did add the
local, LLVM, include directory, so was able to find <omp.h>).
2) If I fake that by explicitly adding the -L command, then it links OK, but
fails to find libomp.dylib at runtime, even though it does look in some “brew”
related places (despite not having anything like that in any envirables).
$ ./a.out
dyld: Library not loaded: @rpath/libomp.dylib
  Referenced from: /Users/jcownie/tmp/./a.out
  Reason: no suitable image found.  Did find:
	/usr/local/lib/libomp.dylib: mach-o, but wrong architecture
	/usr/local/Cellar/libomp/11.0.0/lib/libomp.dylib: mach-o, but wrong
architecture
Abort trap: 6

Any help gratefully received!

 (I have a StackOverflow question at
https://stackoverflow.com/questions/65293299/how-to-build-llvm-clang-clang-for-apple-m1
<https://stackoverflow.com/questions/65293299/how-to-build-llvm-clang-clang-for-apple-m1>
which contains similar information to that I have inlined here; if you would
rather answer there, that’s fine, but don't feel bound to. If i get an
answer here i’ll update that myself so others can see it.)

Thanks

-- Jim
James Cownie <jcownie at gmail.com>
Mob: +44 780 637 7146

Compilation, and linkage failure.
$ ~/software/clang-12.0.0/arm64/bin/clang -v -fopenmp hello_omp.c
clang version 12.0.0 (https://github.com/llvm/llvm-project.git
975b64b29375cdfb3672fedee4216c6512672fbf)
Target: aarch64-apple-darwin20.2.0
Thread model: posix
InstalledDir: /Users/jcownie/software/clang-12.0.0/arm64/bin
 "/Users/jcownie/software/clang-12.0.0/arm64/bin/clang-12" -cc1
-triple arm64-apple-macosx11.0.0 -Wundef-prefix=TARGET_OS_ -Werror=undef-prefix
-Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj
-mrelax-all --mrelax-relocations -disable-free -disable-llvm-verifier
-discard-value-names -main-file-name hello_omp.c -mrelocation-model pic
-pic-level 2 -mframe-pointer=non-leaf -fno-rounding-math -munwind-tables
-fcompatibility-qualified-id-block-type-checking
-fvisibility-inlines-hidden-static-local-var -target-cpu apple-a12
-target-feature +v8.3a -target-feature +fp-armv8 -target-feature +neon
-target-feature +crc -target-feature +crypto -target-feature +fullfp16
-target-feature +ras -target-feature +lse -target-feature +rdm -target-feature
+rcpc -target-feature +zcm -target-feature +zcz -target-feature +sha2
-target-feature +aes -target-abi darwinpcs -fallow-half-arguments-and-returns
-debugger-tuning=lldb -target-linker-version 609.8 -v -resource-dir
/Users/jcownie/software/clang-12.0.0/arm64/lib/clang/12.0.0 -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
-internal-isystem
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/local/include
-internal-isystem
/Users/jcownie/software/clang-12.0.0/arm64/lib/clang/12.0.0/include
-internal-externc-isystem
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
-fdebug-compilation-dir /Users/jcownie/tmp -ferror-limit 19 -fmessage-length=167
-fopenmp -fopenmp-cuda-parallel-target-regions -stack-protector 1 -fblocks
-fencode-extended-block-signature -fregister-global-dtors-with-atexit
-fgnuc-version=4.2.1 -fmax-type-align=16 -o
/var/folders/lt/nf3dtk8j16qfsl97d_vgv4dw000lts/T/hello_omp-ea548e.o -x c
hello_omp.c
clang -cc1 version 12.0.0 based upon LLVM 12.0.0git default target
aarch64-apple-darwin20.2.0
ignoring nonexistent directory
"/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/local/include"
ignoring nonexistent directory
"/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /Users/jcownie/software/clang-12.0.0/arm64/lib/clang/12.0.0/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks
(framework directory)
End of search list.
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld"
-demangle -lto_library
/Users/jcownie/software/clang-12.0.0/arm64/lib/libLTO.dylib -no_deduplicate
-dynamic -arch arm64 -platform_version macos 11.0.0 0.0.0 -syslibroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
-o a.out /var/folders/lt/nf3dtk8j16qfsl97d_vgv4dw000lts/T/hello_omp-ea548e.o
-lomp -lSystem
ld: library not found for -lomp
clang-12: error: linker command failed with exit code 1 (use -v to see
invocation)
$ 

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