Apple Silicon M1 Pro 上的 MPI 和 OpenMP

Question

So, someone has to initiate such a question regarding apple silicon. Long story short, I need to compile and run a C++ code that runs hybrid with both MPI and OpenMP on a MBP M1 pro (which uses arm-based architecture). The code runs perfectly fine on a typical Intel x86 architecture. Also, for completion sake, the Mac OS is Monterey. Additionally, I do not compile/run via an IDE (eg XCode), I do so via a Makefile, directly from the terminal.

I haven't found a clear answer to such a question. I tried setting things up myself, but this is way above my comfort zone. Moreover, I trust answering such a question would help future developers that are using these Macs for work:)

So far, I understood that the installed Clang compiler that comes with Xcode does not support OpenMP ( -fopenmp ) or recognizes MPI ( mpirun ). So, I installed "brew" and the " llvm " version which should support the multi-threading instructions via OpenMP. As for the MPI library, I installed the " libomp " library through " brew ". These all should run natively on Apple Silicon.

However, now the environment is all over the place, since the system has 2 Clang compilers:

• one preinstalled with Xcode (the default)
• one installed via brew, situated in: /opt/homebrew/opt/llvm

I do not know how to proceed. The simple solution might be to change the path of the compiler to that in the /opt/homebrew/ directory, however I suspect that might mess a lot of things in the system.

I would (and many others) be very thankful if you could help, in case someone has prior experience with something like this before.

Answer 1

To use Homebrew's Clang you will need set up your PATH and compiler flags accordingly.

Actually, Homebrew gives you the respective advice and hints already once you install llvm (or try brew info llvm ):

To use the bundled libc++ please add the following LDFLAGS:
  LDFLAGS="-L/opt/homebrew/opt/llvm/lib -Wl,-rpath,/opt/homebrew/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /opt/homebrew,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH, run:
  echo 'export PATH="/opt/homebrew/opt/llvm/bin:$PATH"' >> ~/.zshrc

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/opt/homebrew/opt/llvm/lib"
  export CPPFLAGS="-I/opt/homebrew/opt/llvm/include"

You will need to forward those flags into your build setup, ie, if you have build files from CMake/Make/whatsoever you will need to make sure that the respective flags are used or forwarded.