ci: compare PR benchmarks with base

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

.github/workflows/pr-benchmarks.yml

@@ -21,12 +21,15 @@ jobs:
     runs-on: ubuntu-24.04
     permissions:
       contents: read # checkout only
+    env:
+      HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+      PR_NUMBER: ${{ github.event.pull_request.number }}
     steps:
-      - name: Checkout PR head
+      - name: Checkout PR base
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
-          repository: ${{ github.event.pull_request.head.repo.full_name }}
-          ref: ${{ github.event.pull_request.head.sha }}
+          repository: ${{ github.repository }}
+          ref: ${{ github.event.pull_request.base.sha }}
           persist-credentials: false
           fetch-depth: 0
 
@@ -45,17 +48,33 @@ jobs:
             ${{ runner.os }}-pr-bench-maven-
             ${{ runner.os }}-maven-
 
-      - name: Run JMH benchmarks
+      - name: Run base JMH benchmarks
+        run: |
+          mise run benchmark:ci-json
+          mv benchmark-results.json /tmp/base-benchmark-results.json
+
+      - name: Checkout PR head
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: ${{ github.event.pull_request.head.sha }}
+          persist-credentials: false
+          fetch-depth: 0
+
+      - name: Run head JMH benchmarks
         run: mise run benchmark:ci-json
 
       - name: Generate benchmark summary
         run: |
           mise run benchmark:generate-summary \
             --input benchmark-results.json \
+            --baseline /tmp/base-benchmark-results.json \
             --output-dir benchmark-results \
-            --commit-sha "${{ github.event.pull_request.head.sha }}"
+            --commit-sha "${{ github.event.pull_request.head.sha }}" \
+            --baseline-sha "${{ github.event.pull_request.base.sha }}"
         env:
           GITHUB_REPOSITORY: ${{ github.event.pull_request.head.repo.full_name }}
+          GITHUB_BASE_REPOSITORY: ${{ github.repository }}
 
       - name: Prepare PR comment summary
         run: |
@@ -64,13 +83,13 @@ jobs:
       - name: Upload PR benchmark results
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: pr-benchmark-results-pr-${{ github.event.pull_request.number }}-${{ github.event.pull_request.head.sha }}
+          name: pr-benchmark-results-pr-${{ env.PR_NUMBER }}-${{ env.HEAD_SHA }}
           path: benchmark-results
           retention-days: 5
 
       - name: Upload PR benchmark comment
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: pr-benchmark-comment-pr-${{ github.event.pull_request.number }}-${{ github.event.pull_request.head.sha }}
+          name: pr-benchmark-comment-pr-${{ env.PR_NUMBER }}-${{ env.HEAD_SHA }}
           path: pr-benchmark-comment.md
           retention-days: 5