CI: Add macOS LLVM smoke tests (#1256)

There is a small caveat: I only enabled these for the PR CI, not for the
`master` branch CI. If interested in the latter, feel free to request
that too.

---

To be used in #1219.

- run all IO tests on macOS with LLVM
- run with `--debug` flag
- takes some 3m30s (like JS tests)

Here's the log:

<img width="645" height="274" alt="Screenshot 2025-12-13 at 17 50 31"
src="https://github.com/user-attachments/assets/df402a2c-bbb3-400f-94d4-da9ec511418d"
/>

Author: jiribenes

.github/actions/setup-effekt/action.yml

@@ -15,7 +15,7 @@ inputs:
     required: false
     default: '18'
   install-dependencies:
-    description: 'Whether to install system dependencies (Linux only)'
+    description: 'Whether to install system dependencies (Linux & macOS only)'
     required: false
     default: 'false'
   install-valgrind:
@@ -41,6 +41,7 @@ runs:
       with:
         node-version: ${{ inputs.node-version }}
 
+
     # Only run the following on Linux-based runners
     - name: Update apt database
       if: ${{ inputs.install-dependencies == 'true' && runner.os == 'Linux' }}
@@ -66,3 +67,17 @@ runs:
       if: ${{ inputs.install-dependencies == 'true' && runner.os == 'Linux' }}
       run: sudo apt-get install -y libuv1-dev
       shell: bash
+
+
+    # Only run the following on macOS-based runners
+    - name: Install LLVM ${{ inputs.llvm-version }} (macOS)
+      if: ${{ inputs.install-dependencies == 'true' && runner.os == 'macOS' }}
+      run: |
+        brew install llvm@${{ inputs.llvm-version }}
+        echo "$(brew --prefix llvm@${{ inputs.llvm-version }})/bin" >> $GITHUB_PATH
+      shell: bash
+
+    - name: Install libuv (macOS)
+      if: ${{ inputs.install-dependencies == 'true' && runner.os == 'macOS' }}
+      run: brew install libuv
+      shell: bash 

.github/workflows/ci-pr.yml

@@ -96,6 +96,25 @@ jobs:
       - name: Run Windows smoke test
         run: sbt "effektJVM/testOnly effekt.JavaScriptTests -- --tests=.*examples[\\/]*pos[\\/]*sideeffects.*"
 
+  macos-llvm-tests:
+    name: "macOS LLVM Smoke Tests"
+    needs: build-and-compile
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v6
+        with: 
+          submodules: 'true'
+
+      - uses: ./.github/actions/setup-effekt
+        with:
+          install-dependencies: 'true'
+
+      - name: Compile project
+        run: sbt Test/compile
+
+      - name: Run macOS LLVM smoke tests (with debug)
+        run: EFFEKT_DEBUG=1 sbt "effektJVM/testOnly effekt.LLVMTests -- --tests=.*benchmarks[\\/]*input_output[\\/].*"
+
   js-tests:
     name: "JS Backend"
     needs: build-and-compile
@@ -146,5 +165,5 @@ jobs:
 
       - uses: ./.github/actions/restore-build-cache
 
-      - name: Run LLVM backend tests
+      - name: Run LLVM backend tests (with debug & valgrind)
         run: EFFEKT_VALGRIND=1 EFFEKT_DEBUG=1 sbt effektJVM/testBackendLLVM