Add steps for comparing with 'main' branch (#842)

Author: natecook1000

.github/workflows/pull_request.yml

@@ -26,6 +26,7 @@ jobs:
       BUILD_CMD: swift build -c release
       BENCH_CMD: .build/release/RegexBenchmark
       BASELINE_FILE: benchmark-baseline
+      MAIN_FILE: benchmark-main
       COMPARE_FILE: benchmark-pr
       COMPARE_OUT_FILE: benchmark-results.txt
     steps:
@@ -46,6 +47,26 @@ jobs:
           set -euo pipefail
           eval "$BENCH_CMD --save $RUNNER_TEMP/$BASELINE_FILE"
           test -s "$RUNNER_TEMP/$BASELINE_FILE" || { echo "Baseline not created at $BASELINE_FILE"; exit 1; }
+      - name: Check out main branch
+        if: ${{ github.event.pull_request.base.ref != 'main' }}
+        uses: actions/checkout@v4
+        with:
+          ref: main
+          path: main-branch
+          fetch-depth: 0
+      - name: Build main branch
+        if: ${{ github.event.pull_request.base.ref != 'main' }}
+        working-directory: main-branch
+        run: |
+          set -euo pipefail
+          eval "$BUILD_CMD"
+      - name: Run main benchmark
+        if: ${{ github.event.pull_request.base.ref != 'main' }}
+        working-directory: main-branch
+        run: |
+          set -euo pipefail
+          eval "$BENCH_CMD --save $RUNNER_TEMP/$MAIN_FILE"
+          test -s "$RUNNER_TEMP/$MAIN_FILE" || { echo "Baseline (main) not created at $MAIN_FILE"; exit 1; }
       - name: Check out PR branch
         uses: actions/checkout@v4
         with:
@@ -64,11 +85,17 @@ jobs:
           eval "$BENCH_CMD --save $RUNNER_TEMP/$COMPARE_FILE"
           test -s "$RUNNER_TEMP/$COMPARE_FILE" || { echo "Comparison not created at $COMPARE_FILE"; exit 1; }
           eval "$BENCH_CMD --compare $RUNNER_TEMP/$BASELINE_FILE" | tee "$RUNNER_TEMP/$COMPARE_OUT_FILE"
-      - name: 📊 Compare benchmarks
+      - name: 📊 Compare benchmarks with base
         working-directory: pr
         run: |
           set -euo pipefail
           eval "$BENCH_CMD --load $RUNNER_TEMP/$COMPARE_FILE --compare $RUNNER_TEMP/$BASELINE_FILE --compare-compile-time $RUNNER_TEMP/$BASELINE_FILE" | tee "$RUNNER_TEMP/$COMPARE_OUT_FILE"
+      - name: 📊 Compare benchmarks with `main`
+        if: ${{ github.event.pull_request.base.ref != 'main' }}
+        working-directory: pr
+        run: |
+          set -euo pipefail
+          eval "$BENCH_CMD --load $RUNNER_TEMP/$COMPARE_FILE --compare $RUNNER_TEMP/$MAIN_FILE --compare-compile-time $RUNNER_TEMP/$MAIN_FILE"
       - name: Upload benchmark artifacts
         uses: actions/upload-artifact@v4
         with: