ci: add comment with binary size difference

This makes reviewing PRs a lot easier because I don't have to run this
myself :)

This only uses the drivers repo so far, which is a good starting point
but doesn't include binary size changes for WebAssembly for example. A
future change could add some real-world programs to get a better idea of
the real-world impact.

To be clear: the intention is not to just look at the number at the
bottom. It is important to look at the actual size difference to see the
overall pattern (like, the difference may be due to a few outlier).

.github/workflows/sizediff.yml

@@ -0,0 +1,92 @@
+name: Binary size difference
+
+on:
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  sizediff:
+    runs-on: ubuntu-22.04
+    steps:
+      # Prepare, install tools
+      - name: Add GOBIN to $PATH
+        run: |
+          echo "$HOME/go/bin" >> $GITHUB_PATH
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0 # fetch all history (no sparse checkout)
+          submodules: true
+      - name: Install apt dependencies
+        run: |
+          echo 'deb https://apt.llvm.org/jammy/ llvm-toolchain-jammy-15 main' | sudo tee /etc/apt/sources.list.d/llvm.list
+          wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+          sudo apt-get update
+          sudo apt-get install --no-install-recommends -y \
+              llvm-15-dev \
+              clang-15 \
+              libclang-15-dev \
+              lld-15
+      - name: Restore LLVM source cache
+        uses: actions/cache@v3
+        id: cache-llvm-source
+        with:
+          key: llvm-source-15-sizediff-v1
+          path: |
+            llvm-project/compiler-rt
+      - name: Download LLVM source
+        if: steps.cache-llvm-source.outputs.cache-hit != 'true'
+        run: make llvm-source
+      - name: Cache Go
+        uses: actions/cache@v3
+        with:
+          key: go-cache-linux-sizediff-v1-${{ hashFiles('go.mod') }}
+          path: |
+            ~/.cache/go-build
+            ~/go/pkg/mod
+      - run: make gen-device -j4
+      - name: Download drivers repo
+        run: git clone https://github.com/tinygo-org/drivers.git
+      - name: Save HEAD
+        run: git branch github-actions-saved-HEAD HEAD
+
+      # Compute sizes for the dev branch
+      - name: Checkout dev branch
+        run: git checkout --no-recurse-submodules `git merge-base HEAD origin/dev`
+      - name: Build tinygo binary for the dev branch
+        run: go install
+      - name: Determine binary sizes on the dev branch
+        run: (cd drivers; make smoke-test XTENSA=0 | tee sizes-dev.txt)
+
+      # Compute sizes for the PR branch
+      - name: Checkout PR branch
+        run: git checkout --no-recurse-submodules github-actions-saved-HEAD
+      - name: Build tinygo binary for the PR branch
+        run: go install
+      - name: Determine binary sizes on the PR branch
+        run: (cd drivers; make smoke-test XTENSA=0 | tee sizes-pr.txt)
+
+      # Create comment
+      # TODO: add a summary, something like:
+      #  - overall size difference (percent)
+      #  - number of binaries that grew / shrank / remained the same
+      #  - don't show the full diff when no binaries changed
+      - name: Calculate size diff
+        run: ./tools/sizediff drivers/sizes-dev.txt drivers/sizes-pr.txt | tee sizediff.txt
+      - name: Create comment
+        run: |
+          echo "Size difference with the dev branch:" > comment.txt
+          echo "<details><summary>Binary size difference</summary>" >> comment.txt
+          echo "<pre>" >> comment.txt
+          cat sizediff.txt >> comment.txt
+          echo "</pre></details>" >> comment.txt
+      - name: Comment contents
+        run: cat comment.txt
+      - name: Add comment
+        uses: thollander/actions-comment-pull-request@v2
+        with:
+          filePath: comment.txt
+          comment_tag: sizediff