ci: Add Windows ARM64 build

.github/workflows/ci.yaml

@@ -14,7 +14,13 @@ jobs:
       matrix:
         python-version: ["3.12", "3.13", "3.14", "3.14t"]
         # macos-15-large is an intel runner, macos-14 is an arm64 runner
-        platform: [ubuntu-latest, ubuntu-22.04-arm, windows-latest, macos-15-large, macos-14]
+        platform: [ubuntu-latest, ubuntu-22.04-arm, windows-latest, windows-11-arm, macos-15-large, macos-14]
+        exclude:
+          # Free-threaded win_arm64 is skipped because CPython's _Py_ThreadId()
+          # tries calling __getReg() that CLANGARM64 clang does not provide.
+          # https://github.com/python/cpython/issues/157212
+          - python-version: "3.14t"
+            platform: windows-11-arm
 
     defaults:
       run:
@@ -33,9 +39,30 @@ jobs:
           python-version: ${{ matrix.python-version }}
           cache: 'pip'
 
+      - name: Set up MSYS2
+        if: runner.os == 'Windows'
+        id: msys2
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: ${{ runner.arch == 'ARM64' && 'CLANGARM64' || 'CLANG64' }}
+          pacboy: clang:p
+
+      # Prepended to PATH so clang wins over the x86_64 MinGW gcc in the runner image
+      - name: Select the MSYS2 clang toolchain
+        if: runner.os == 'Windows'
+        env:
+          MSYS2_LOCATION: ${{ steps.msys2.outputs.msys2-location }}
+          MSYS2_PREFIX: ${{ runner.arch == 'ARM64' && 'clangarm64' || 'clang64' }}
+        run: |
+          echo "$MSYS2_LOCATION/$MSYS2_PREFIX/bin" >> $GITHUB_PATH
+          echo "CC=clang" >> $GITHUB_ENV
+
       - name: Install numcodecs
+        # zfpy does not have a win_arm64 wheel
         if: ${{ !endsWith(matrix.python-version, 't') }}
-        run: python -m pip install -v ".[test,test_extras,msgpack,google_crc32c,crc32c,pcodec,zfpy]"
+        env:
+          ZFPY_EXTRA: ${{ matrix.platform != 'windows-11-arm' && ',zfpy' || '' }}
+        run: python -m pip install -v ".[test,test_extras,msgpack,google_crc32c,crc32c,pcodec$ZFPY_EXTRA]"
 
       - name: Install numcodecs (free-threaded)
         # pcodec, zfpy and google_crc32c do not ship free-threaded wheels yet

.github/workflows/wheel.yaml

@@ -20,13 +20,16 @@ jobs:
       fail-fast: false
       matrix:
         # macos-15-large is an intel runner, macos-14 is an arm64 runner
-        os: [ubuntu-latest, ubuntu-22.04-arm, windows-latest, macos-15-large, macos-14]
+        os: [ubuntu-latest, ubuntu-22.04-arm, windows-latest, windows-11-arm, macos-15-large, macos-14]
     env:
       # on free-threaded builds also check that no extension re-enables the GIL
       CIBW_TEST_COMMAND: python -c "import sys, sysconfig, numcodecs; assert not (sysconfig.get_config_var('Py_GIL_DISABLED') and sys._is_gil_enabled())"
       CIBW_BUILD: "cp312-* cp313-* cp314-* cp314t-*"
       CIBW_ENABLE: cpython-freethreading
-      CIBW_SKIP: "*-musllinux_* *win32 *_i686 *_s390x"
+      # Free-threaded win_arm64 is skipped because CPython's _Py_ThreadId()
+      # tries calling __getReg() that CLANGARM64 clang does not provide.
+      # https://github.com/python/cpython/issues/157212
+      CIBW_SKIP: "*-musllinux_* *win32 *_i686 *_s390x cp314t-win_arm64"
       # note: cibuildwheel config-settings are set in pyproject.toml
 
     steps:
@@ -35,6 +38,25 @@ jobs:
           submodules: true
           fetch-depth: 0  # required for version resolution for nightly wheels
 
+      - name: Set up MSYS2
+        if: runner.os == 'Windows'
+        id: msys2
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: ${{ runner.arch == 'ARM64' && 'CLANGARM64' || 'CLANG64' }}
+          pacboy: clang:p
+
+      # Prepended to PATH so clang wins over the x86_64 MinGW gcc in the runner image
+      - name: Select the MSYS2 clang toolchain
+        if: runner.os == 'Windows'
+        shell: bash
+        env:
+          MSYS2_LOCATION: ${{ steps.msys2.outputs.msys2-location }}
+          MSYS2_PREFIX: ${{ runner.arch == 'ARM64' && 'clangarm64' || 'clang64' }}
+        run: |
+          echo "$MSYS2_LOCATION/$MSYS2_PREFIX/bin" >> $GITHUB_PATH
+          echo "CC=clang" >> $GITHUB_ENV
+
       - uses: pypa/cibuildwheel@v3.4.1
 
       - uses: actions/upload-artifact@v7

pyproject.toml

@@ -158,10 +158,19 @@ filterwarnings = [
 # AVX2 disabled for portable wheels (not all x86_64 CPUs have it)
 config-settings = "setup-args=-Davx2=disabled"
 
+[tool.cibuildwheel.windows]
+# Vendor any MSYS2 clang runtime DLLs the extensions end up linking against
+before-build = "pip install delvewheel"
+repair-wheel-command = "delvewheel repair -w {dest_dir} {wheel}"
+
 [[tool.cibuildwheel.overrides]]
 select = "*-macosx_arm64"
 config-settings = {setup-args = ["-Davx2=disabled", "-Dsse2=disabled"]}
 
+[[tool.cibuildwheel.overrides]]
+select = "*-win_arm64"
+config-settings = {setup-args = ["-Davx2=disabled", "-Dsse2=disabled"]}
+
 [tool.ruff]
 line-length = 100
 extend-exclude = ["c-blosc"]