Measure performance on PRs

.github/workflows/pull_request.yml

@@ -20,3 +20,35 @@
     with:
       license_header_check_project_name: "Swift.org"
       api_breakage_check_allowlist_path: "api-breakages.txt"
+  performance_test:
+    runs-on: ubuntu-latest
+    container:
+      image: swift:latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Download performance comparison script
+        run: |
+          apt-get update && apt-get install -y curl
+          curl -s https://raw.githubusercontent.com/ahoppen/swift-format/refs/heads/main/.github/workflows/scripts/compare-performance-measurements.swift > /tmp/compare-performance-measurements.swift
+      - name: Measure performance with PR changes
+        id: pr_performance
+        run: |
+          OUTPUT=$(echo "Random: $(random)")
+          echo "output=$OUTPUT" >> "$GITHUB_OUTPUT"
+      - name: Measure baseline performance
+        id: baseline_performance
+        run: |
+          OUTPUT=$(echo "Random: $(random)")
+          echo "output=$OUTPUT" >> "$GITHUB_OUTPUT"
+      - name: Post comment if performance changed
+        run: |
+          if ! OUTPUT=$(swift /tmp/compare-performance-measurements.swift "${{ steps.baseline_performance.outputs.output }}" "${{ steps.pr_performance.outputs.output }}" 0.5); then
+            gh pr comment --body "$OUTPUT"
+          else
+            echo "No significant performance change detected"
+            echo "$OUTPUT"
+          fi
+

.github/workflows/scripts/compare-performance-measurements.swift

@@ -0,0 +1,90 @@
+import Foundation
+
+func printToStderr(_ value: String) {
+  fputs(value, stderr)
+}
+
+func extractMeasurements(output: String) -> [String: Double] {
+  var measurements: [String: Double] = [:]
+  for line in output.split(separator: "\n") {
+    guard let colonPosition = line.lastIndex(of: ":") else {
+      printToStderr("Ignoring following measurement line because it doesn't contain a colon: \(line)")
+      continue
+    }
+    let beforeColon = String(line[..<colonPosition]).trimmingCharacters(in: .whitespacesAndNewlines)
+    let afterColon = String(line[line.index(after: colonPosition)...]).trimmingCharacters(
+      in: .whitespacesAndNewlines
+    )
+    guard let value = Double(afterColon) else {
+      printToStderr("Ignoring following measurement line because the value can't be parsed as a Double: \(line)")
+      continue
+    }
+    measurements[beforeColon] = value
+  }
+  return measurements
+}
+
+extension Double {
+  func round(toDecimalDigits decimalDigits: Int) -> Double {
+    return (self * pow(10, Double(decimalDigits))).rounded() / pow(10, Double(decimalDigits))
+  }
+}
+
+func run(
+  baselinePerformanceOutput: String,
+  changedPerformanceOutput: String,
+  sensitivityPercentage: Double
+) -> (output: String, hasDetectedSignificantChange: Bool) {
+  let baselineMeasurements = extractMeasurements(output: baselinePerformanceOutput)
+  let changedMeasurements = extractMeasurements(output: changedPerformanceOutput)
+
+  var hasDetectedSignificantChange = false
+  var output = ""
+  for (measurementName, baselineValue) in baselineMeasurements.sorted(by: { $0.key < $1.key }) {
+    guard let changedValue = changedMeasurements[measurementName] else {
+      output += "🛑 \(measurementName) not present after changes\n"
+      continue
+    }
+    let differencePercentage = (changedValue - baselineValue) / baselineValue * 100
+    let rawMeasurementsText = "(baseline: \(baselineValue), after changes: \(changedValue))"
+    if differencePercentage < -sensitivityPercentage {
+      output +=
+        "🎉 \(measurementName) improved by \(-differencePercentage.round(toDecimalDigits: 3))% \(rawMeasurementsText)\n"
+      hasDetectedSignificantChange = true
+    } else if differencePercentage > sensitivityPercentage {
+      output +=
+        "⚠️ \(measurementName) regressed by \(differencePercentage.round(toDecimalDigits: 3))% \(rawMeasurementsText)\n"
+      hasDetectedSignificantChange = true
+    } else {
+      output +=
+        "➡️ \(measurementName) did not change significantly with \(differencePercentage.round(toDecimalDigits: 3))% \(rawMeasurementsText)\n"
+    }
+  }
+  return (output, hasDetectedSignificantChange)
+}
+
+guard CommandLine.arguments.count > 2 else {
+  print("Expected at least two parameters: The baseline performance output and the changed performance output")
+  exit(1)
+}
+
+let baselinePerformanceOutput = CommandLine.arguments[1]
+let changedPerformanceOutput = CommandLine.arguments[2]
+
+let sensitivityPercentage =
+  if CommandLine.arguments.count > 3, let percentage = Double(CommandLine.arguments[3]) {
+    percentage
+  } else {
+    1.0 /* percent */
+  }
+
+let (output, hasDetectedSignificantChange) = run(
+  baselinePerformanceOutput: baselinePerformanceOutput,
+  changedPerformanceOutput: changedPerformanceOutput,
+  sensitivityPercentage: sensitivityPercentage
+)
+
+print(output)
+if hasDetectedSignificantChange {
+  exit(1)
+}