add benchmark to CI/CD

.github/workflows/bench.yml

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+name: Benchmark CI
+
+on:
+  pull_request:
+    branches: [main]
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  benchmark:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout PR code
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          override: true
+
+      - name: Install critcmp
+        run: cargo install critcmp
+
+      - name: Run comparison script
+        run: |
+          ./scripts/ci_bench.sh
+
+      - name: Post or update benchmark comment
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const fs = require('fs');
+            const marker = '<!-- benchmark-comment -->';
+            const body = marker + '\n' + fs.readFileSync('benchmark-report.md', 'utf8');
+
+            // Get all comments on the PR
+            const { data: comments } = await github.rest.issues.listComments({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+            });
+
+            // Look for a comment containing our marker
+            const existing = comments.find(comment =>
+              comment.body.includes(marker)
+            );
+
+            if (existing) {
+              // Update the existing comment
+              await github.rest.issues.updateComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                comment_id: existing.id,
+                body,
+              });
+              console.log('Updated existing benchmark comment');
+            } else {
+              // Create a new comment
+              await github.rest.issues.createComment({
+                issue_number: context.issue.number,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                body,
+              });
+              console.log('Created new benchmark comment');
+            }

scripts/ci_bench.sh

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+#!/bin/bash
+set -euo pipefail
+
+# Configuration
+BENCH_NAME="serde_bench"
+WORKTREE_DIR="/tmp/bench-main"
+REPORT_FILE="benchmark-report.md"
+
+# Cleanup trap
+cleanup() {
+  echo "🧹 Cleaning up..."
+  git worktree remove --force "$WORKTREE_DIR" 2>/dev/null || true
+}
+trap cleanup EXIT
+
+# Fetch and prepare main branch in a clean worktree
+echo "📦 Checking out 'main' into temporary worktree..."
+git fetch origin main
+git worktree add --force "$WORKTREE_DIR" origin/main
+
+# Run benchmarks on main branch
+echo "🚀 Running benchmarks on 'main'..."
+(
+  cd "$WORKTREE_DIR"
+  cargo bench --bench "$BENCH_NAME" -- --save-baseline main
+)
+mkdir -p ./target/criterion
+cp -r "$WORKTREE_DIR/target/criterion" ./target/criterion
+
+# Run benchmarks on current branch
+echo "🚀 Running benchmarks on PR branch..."
+cargo bench --bench "$BENCH_NAME" -- --save-baseline pr
+
+# Compare with critcmp
+echo "📊 Comparing benchmarks..."
+
+cat <<EOF > "$REPORT_FILE"
+## 📊 Benchmark Comparison Report
+
+This pull request includes Criterion benchmarks comparing performance to the \`main\` branch.
+This comment will automatically update as the benchmarks are re-run on each commit.
+
+The table below shows **relative ratios** and **timing stats** for each benchmark group:
+
+\`\`\`
+$(critcmp main pr)
+\`\`\`
+
+✅ Benchmarks completed successfully.
+
+🧠 **Notes**:
+- These benchmarks are not a pass/fail gate and are informative only.
+- Use this as a signal to review performance-sensitive changes.
+- Results may be unreliable due to GHA runner hardware variance.
+- If results indicate a significant performance regression, run the benchmarks locally to confirm.
+
+_Reported by the benchmark CI bot_
+EOF
+
+echo "✅ Benchmark comparison saved to $REPORT_FILE"