Mac ARM wheels

.github/workflows/wheels.yml

@@ -64,16 +64,16 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
-#      - name: Install tools (macOS)
-#        if: contains(matrix.os, 'macos')
-#        # Install coreutils which includes `nproc` used by `make -j` in suitesparse.sh
-#        #
-#        # GitHub actions comes with libomp already installed, but for its native arch only. Must build universal one
-#        # manually so that both x86 and arm builds can be built.
-#        run: |
-#          brew install coreutils
-#          brew install libomp
-#          sh add_arm_to_libomp_dylib.sh
+      - name: Install tools (macOS)
+        if: contains(matrix.os, 'macos')
+        # Install coreutils which includes `nproc` used by `make -j` in suitesparse.sh
+        #
+        # GitHub actions comes with libomp already installed, but for its native arch only. Must build universal one
+        # manually so that both x86 and arm builds can be built.
+        run: |
+          brew install coreutils
+          brew install libomp
+          sh add_arm_to_libomp_dylib.sh
 
       - name: Build Wheels
         env:
@@ -92,8 +92,8 @@ jobs:
           # Uncomment to only build CPython wheels
 #          CIBW_BUILD: "cp*"
 
-          # macOS: build x86_64 and arm64
-          #CIBW_ARCHS_MACOS: "x86_64 arm64"
+          # macOS: build both x86_64 and arm64.
+          CIBW_ARCHS_MACOS: "x86_64 arm64"
 
           # No 32-bit builds
           CIBW_SKIP: "*-win32 *_i686 *musl*"

add_arm_to_libomp_dylib.sh

@@ -1,4 +1,23 @@
 #!/bin/sh
+# Construct a universal2 version of homebrew's libomp.
+#
+# Homebrew's libomp works well to patch Apple clang's missing OpenMP support. The problem is a combination of:
+# - Brew installs libomp built for x86 *or* ARM, matching the architecture of the machine it is running on.
+# - GitHub Actions only has x86 runners as of now. Check back in Q4 2023. https://github.com/github/roadmap/issues/528
+# - The linker will select the first found libomp, and if that version does not include the expected architecture then
+#   linking will fail.
+#
+# One solution is to build a universal2 version of libomp that includes both architectures. That's what this script
+# does. It adds the ARM version of libomp to the x86 version.
+#
+# This script assumes it is running on x86 with x86 libomp already installed.
+
+if [ "$(arch)" != "x86_64" ] && [ "$(arch)" != "i386" ]; then
+    echo "Not running on x86 as expected. Running on:"
+    arch
+    echo "If the above says arm64 then this hack is no longer necessary. Remove this script from the build."
+    exit 1;
+fi
 
 #mkdir x86lib
 mkdir armlib
@@ -11,8 +30,21 @@ brew fetch --force --bottle-tag=arm64_big_sur libomp
 #tar -xzf $(brew --cache --bottle-tag=x86_64_monterey libomp) --strip-components 2 -C x86lib
 tar -xzf $(brew --cache --bottle-tag=arm64_big_sur libomp) --strip-components 2 -C armlib
 
-# merge
+# ARM and x86 dylibs have different install names due to different brew install directories.
+# The x86 install name will be expected so make the ARM install name match.
+X86_INSTALL_NAME="$(otool -X -D $(brew --prefix libomp)/lib/libomp.dylib)"
+install_name_tool -id "${X86_INSTALL_NAME}" armlib/lib/libomp.dylib
+codesign --force -s - armlib/lib/libomp.dylib
+
+# merge the downloaded (arm) libomp with the already installed (x86) libomp to create a universal libomp
 lipo armlib/lib/libomp.dylib $(brew --prefix libomp)/lib/libomp.dylib -output libomp.dylib -create
+
+# print contents of universal library for reference
+otool -arch all -L libomp.dylib
+
+# replace the x86-only libomp with the newly-created universal one
 cp -f libomp.dylib $(brew --prefix libomp)/lib
+
+# clean up
 rm libomp.dylib
 rm -rf armlib

suitesparse.sh

@@ -25,9 +25,8 @@ if [ -n "${BREW_LIBOMP}" ]; then
     cmake_params+=(-DOpenMP_libomp_LIBRARY="omp")
     export LDFLAGS="-L$(brew --prefix libomp)/lib"
 
-    export CFLAGS="-arch x86_64"
-#    # build both x86 and ARM
-#    export CFLAGS="-arch x86_64 -arch arm64"
+    # build both x86 and ARM
+    export CFLAGS="-arch x86_64 -arch arm64"
 fi
 
 if [ -n "${CMAKE_GNUtoMS}" ]; then