diff --git a/.github/actions/setup/directories/action.yml b/.github/actions/setup/directories/action.yml
new file mode 100644
index 0000000..8e382d9
--- /dev/null
+++ b/.github/actions/setup/directories/action.yml
@@ -0,0 +1,176 @@
+name: Setup directories etc.
+description: >-
+  Set up the source code and build directories (plus some
+  environmental tweaks)
+
+inputs:
+  srcdir:
+    required: false
+    default: ${{ github.workspace }}
+    description: >-
+      Directory to (re-)checkout source codes.  This will be created
+      if absent.  If there is no `configure` file that is also
+      generated inside.
+
+  builddir:
+    required: false
+    default: ${{ github.workspace }}
+    description: >-
+      Where binaries and other generated contents go.  This will be
+      created if absent.
+
+  makeup:
+    required: false
+    type: boolean
+    # Note that `default: false` evaluates to a string constant
+    # `'false'`, which is a truthy value :sigh:
+    # https://github.com/actions/runner/issues/2238
+    default: ''
+    description: >-
+      If set to true, additionally runs `make up`.
+
+  checkout:
+    required: false
+    type: boolean
+    default: true
+    description: >-
+      If set to '' (false), skip running actions/checkout. This is useful when
+      you don't want to overwrite a GitHub token that is already set up.
+
+  dummy-files:
+    required: false
+    type: boolean
+    default: ''
+    description: >-
+      If set to true, creates dummy files in build dir.
+
+  fetch-depth:
+    required: false
+    default: '1'
+    description: The depth of commit history fetched from the remote repository
+
+  clean:
+    required: false
+    type: boolean
+    default: ''
+    description: >-
+      If set to true, clean build directory.
+
+outputs: {} # nothing?
+
+runs:
+  using: composite
+
+  steps:
+    # Note that `shell: bash` works on both Windows and Linux, but not
+    # `shell: sh`.  This is because GitHub hosted Windows runners have
+    # their bash manually installed.
+    - shell: bash
+      run: |
+        mkdir -p ${{ inputs.srcdir }}
+        mkdir -p ${{ inputs.builddir }}
+
+    # Did you know that actions/checkout works without git(1)?  We are
+    # checking that here.
+    - id: which
+      shell: bash
+      run: |
+        echo "git=`command -v git`" >> "$GITHUB_OUTPUT"
+        echo "sudo=`command -v sudo`" >> "$GITHUB_OUTPUT"
+        echo "autoreconf=`command -v autoreconf`" >> "$GITHUB_OUTPUT"
+
+    - if: steps.which.outputs.git
+      shell: bash
+      run: |
+        git config --global core.autocrlf false
+        git config --global core.eol lf
+        git config --global advice.detachedHead 0
+        git config --global init.defaultBranch garbage
+
+    - if: inputs.checkout
+      uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      with:
+        path: ${{ inputs.srcdir }}
+        fetch-depth: ${{ inputs.fetch-depth }}
+
+    - uses: actions/cache@6849a6489940f00c2f30c0fb92c6274307ccb58a # v4.1.2
+      with:
+        path: ${{ inputs.srcdir }}/.downloaded-cache
+        key: downloaded-cache
+
+    - if: steps.which.outputs.autoreconf
+      shell: bash
+      working-directory: ${{ inputs.srcdir }}
+      run: ./autogen.sh --install
+
+    # This is for MinGW.
+    - if: runner.os == 'Windows'
+      shell: bash
+      run: echo "GNUMAKEFLAGS=-j$((2 * NUMBER_OF_PROCESSORS))" >> $GITHUB_ENV
+
+    - if: runner.os == 'Linux'
+      shell: bash
+      run: echo "GNUMAKEFLAGS=-sj$((1 + $(nproc --all)))" >> "$GITHUB_ENV"
+
+    # macOS' GNU make is so old that they doesn't understand `GNUMAKEFLAGS`.
+    - if: runner.os == 'macOS'
+      shell: bash
+      run: echo "MAKEFLAGS=-j$((1 + $(sysctl -n hw.activecpu)))" >> "$GITHUB_ENV"
+
+    - if: inputs.makeup
+      shell: bash
+      working-directory: ${{ inputs.srcdir }}
+      run: |
+        touch config.status
+        touch .rbconfig.time
+        sed -f tool/prereq.status template/Makefile.in > Makefile
+        sed -f tool/prereq.status template/GNUmakefile.in > GNUmakefile
+        make up
+
+    # Cleanup, runs even on failure
+    - if: always() && inputs.makeup
+      shell: bash
+      working-directory: ${{ inputs.srcdir }}
+      run: rm -f config.status Makefile rbconfig.rb .rbconfig.time
+
+    - if: steps.which.outputs.sudo
+      shell: bash
+      run: |
+        sudo chmod -R go-w /usr/share
+        chmod -v go-w $HOME $HOME/.config || :
+        SAVE_IFS="$IFS" IFS=:; set $PATH; dirs=() IFS="$SAVE_IFS"
+        for d do [ ! -d "$d" ] || dirs+=("$d"); done
+        sudo chmod -v go-w "${dirs[@]}" || :
+
+    - if: inputs.dummy-files == 'true'
+      shell: bash
+      id: dummy-files
+      working-directory: ${{ inputs.builddir }}
+      run: |
+        : Create dummy files in build dir
+        set {{a..z},{A..Z},{0..9},foo,bar,test,zzz}.rb
+        for file; do \
+          echo > $file "raise 'do not load $file'"; \
+        done
+        # drop {a..z}.rb if case-insensitive filesystem
+        grep -F A.rb a.rb > /dev/null && set "${@:27}"
+        echo clean="cd ${{ inputs.builddir }} && rm $*" >> $GITHUB_OUTPUT
+
+    - if: inputs.clean == 'true'
+      shell: bash
+      id: clean
+      run: |
+        echo distclean='make -C ${{ inputs.builddir }} distclean' >> $GITHUB_OUTPUT
+        echo remained-files='find ${{ inputs.builddir }} -ls' >> $GITHUB_OUTPUT
+        [ "${{ inputs.builddir }}" = "${{ inputs.srcdir }}" ] ||
+        echo final='rmdir ${{ inputs.builddir }}' >> $GITHUB_OUTPUT
+
+    - name: clean
+      uses: gacts/run-and-post-run@4683764dd706df847f57b9bed39d08164bcd2690 # v1.4.1
+      with:
+        working-directory:
+        post: |
+          ${{ steps.dummy-files.outputs.clean }}
+          ${{ steps.clean.outputs.distclean }}
+          ${{ steps.clean.outputs.remained-files }}
+          ${{ steps.clean.outputs.final }}
diff --git a/.github/actions/setup/macos/action.yml b/.github/actions/setup/macos/action.yml
new file mode 100644
index 0000000..fd7b5e7
--- /dev/null
+++ b/.github/actions/setup/macos/action.yml
@@ -0,0 +1,29 @@
+name: Setup macOS environment
+description: >-
+  Installs necessary packages via Homebrew.
+
+inputs: {} # nothing?
+
+outputs: {} # nothing?
+
+runs:
+  using: composite
+
+  steps:
+    - name: brew
+      shell: bash
+      run: |
+        brew install --quiet gmp libffi openssl@3 zlib autoconf automake libtool
+
+    - name: Set ENV
+      shell: bash
+      run: |
+        dir_config() {
+          local args=() lib var="$1"; shift
+          for lib in "$@"; do
+            args+="--with-${lib%@*}-dir=$(brew --prefix $lib)"
+          done
+          echo "$var=${args[*]}" >> $GITHUB_ENV
+        }
+        dir_config ruby_configure_args gmp
+        dir_config CONFIGURE_ARGS openssl@3
diff --git a/.github/actions/setup/ubuntu/action.yml b/.github/actions/setup/ubuntu/action.yml
new file mode 100644
index 0000000..a9e5b41
--- /dev/null
+++ b/.github/actions/setup/ubuntu/action.yml
@@ -0,0 +1,53 @@
+name: Setup ubuntu environment
+description: >-
+  At the beginning there was no way but to copy & paste `apt-get`
+  everywhere.  But now that we have composite actions, it seems better
+  merge them into one.
+
+inputs:
+  arch:
+    required: false
+    default: ''
+    description: >-
+      Architecture.  Because we run this on a GitHub-hosted runner
+      acceptable value for this input is very limited.
+
+outputs:
+  arch:
+    value: ${{ steps.uname.outputs.uname }}
+    description: >-
+      Actual architecture.  This could be different from the one
+      passed to the `inputs.arch`.  For instance giving `i386` to this
+      action yields `i686`.
+
+runs:
+  using: composite
+
+  steps:
+    - name: set SETARCH
+      shell: bash
+      run: echo "SETARCH=${setarch}" >> "$GITHUB_ENV"
+      env:
+        setarch: ${{ inputs.arch && format('setarch {0} --', inputs.arch) }}
+
+    - id: uname
+      name: uname
+      shell: bash
+      run: |
+        echo uname=`${SETARCH} uname -m` >> "$GITHUB_OUTPUT"
+        echo dpkg=`${SETARCH} uname -m | sed s/686/386/` >> "$GITHUB_OUTPUT"
+
+    - name: apt-get
+      shell: bash
+      env:
+        arch: ${{ inputs.arch && format(':{0}', steps.uname.outputs.dpkg) || '' }}
+      run: |
+        set -x
+        ${arch:+sudo dpkg --add-architecture ${arch#:}}
+        sudo apt-get update -qq || :
+        sudo apt-get install --no-install-recommends -qq -y -o=Dpkg::Use-Pty=0 \
+        ${arch:+cross}build-essential${arch/:/-} \
+        libssl-dev${arch} libyaml-dev${arch} libreadline6-dev${arch} \
+        zlib1g-dev${arch} libncurses5-dev${arch} libffi-dev${arch} \
+        autoconf ruby
+        sudo apt-get install -qq -y pkg-config${arch} || :
diff --git a/.github/workflows/modgc.yml b/.github/workflows/modgc.yml
new file mode 100644
index 0000000..1c7ea64
--- /dev/null
+++ b/.github/workflows/modgc.yml
@@ -0,0 +1,97 @@
+name: ModGC
+on:
+  push:
+    paths-ignore:
+      - '**.md'
+  pull_request:
+    # Do not use paths-ignore for required status checks
+    # https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks#handling-skipped-but-required-checks
+  merge_group:
+
+concurrency:
+  group: ${{ github.workflow }} / ${{ startsWith(github.event_name, 'pull') && github.ref_name || github.sha }}
+  cancel-in-progress: ${{ startsWith(github.event_name, 'pull') }}
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    strategy:
+      fail-fast: false
+      matrix:
+        gc:
+          - name: mmtk
+            mmtk_plan: MarkSweep
+            mmtk_build: release
+          - name: mmtk
+            mmtk_plan: MarkSweep
+            mmtk_build: debug
+            timeout: 60
+        os: [macos-latest, ubuntu-latest]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
+
+      - name: Install libraries (macOS)
+        uses: ./.github/actions/setup/macos
+        if: ${{ contains(matrix.os, 'macos') }}
+
+      - name: Install libraries (Ubuntu)
+        uses: ./.github/actions/setup/ubuntu
+        if: ${{ contains(matrix.os, 'ubuntu') }}
+
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+      - name: Set MMTk environment variables
+        run: |
+          if [[ ${{ matrix.gc.mmtk_build }} == debug ]]; then
+            echo 'RUST_LOG=' >> $GITHUB_ENV
+            # Debug builds run much slower so we should increase the timeout
+            echo 'RUBY_TEST_TIMEOUT_SCALE=10' >> $GITHUB_ENV
+            # SYNTAX_SUGGEST_TIMEOUT defaults to 1 second
+            echo 'SYNTAX_SUGGEST_TIMEOUT=60' >> $GITHUB_ENV
+          fi
+          echo 'MMTK_PLAN=${{ matrix.gc.mmtk_plan }}' >> $GITHUB_ENV
+
+        if: ${{ matrix.gc.name == 'mmtk' }}
+      - uses: ruby/setup-ruby@a6e6f86333f0a2523ece813039b8b4be04560854 # v1.190.0
+        with:
+          ruby-version: '3.3'
+          bundler: none
+        if: ${{ contains(matrix.os, 'ubuntu') }}
+
+      - name: make sure that kern.coredump=1
+        run: |
+          sysctl -n kern.coredump
+          sudo sysctl -w kern.coredump=1
+          sudo chmod -R +rwx /cores/
+        if: ${{ contains(matrix.os, 'macos') }}
+
+      - name: Install Bundler dependencies
+        run: bundle install
+
+      - name: build Ruby & MMTk shared GC
+        run: rake compile
+
+      - name: Verify MMTk bindgen
+        run: |
+          cd gc/mmtk
+          cargo install --force cbindgen
+          cbindgen --config cbindgen.toml --output ../mmtk.h
+          if read -n1 -d '' < <(git diff ../mmtk.h); then
+            git diff ../mmtk.h
+            exit 1
+          fi
+
+      - name: make check
+        run: RUST_LOG= make -s check
+        timeout-minutes: 40
+        working-directory: ./res/ruby
+        env:
+          RUBY_TESTOPTS: '-q --tty=no'
+          RUBY_GC_LIBRARY: mmtk
+          TEST_BUNDLED_GEMS_ALLOW_FAILURES: 'typeprof'
+          PRECHECK_BUNDLED_GEMS: 'no'
+