Fix benchmark caching, and store (and show) baseline SHA

.github/workflows/bench.yml

@@ -45,8 +45,8 @@ jobs:
         uses: actions/cache/restore@v4
         with:
           path: ./target/criterion
-          key: criterion-${{ runner.os }}-${{ hashFiles('./target/criterion/**.json') }}
-          restore-keys: criterion-${{ runner.os }}-
+          key: criterion-${{ runner.name }}-${{ hashFiles('./target/criterion/**.json') }}
+          restore-keys: criterion-${{ runner.name }}-
 
       # Disable turboboost, hyperthreading and use performance governor.
       - name: Prepare machine
@@ -120,7 +120,11 @@ jobs:
         uses: actions/cache/save@v4
         with:
           path: ./target/criterion
-          key: ${{ steps.criterion-cache.outputs.cache-primary-key }}
+          key: criterion-${{ runner.name }}-${{ hashFiles('./target/criterion/**.json') }}
+
+      - name: Remember main-branch push URL
+        if: github.ref == 'refs/heads/main'
+        run: echo "${{ github.sha }}" > target/criterion/baseline-sha.txt
 
       - name: Export perf data
         id: export
@@ -138,15 +142,19 @@ jobs:
       - name: Format results as Markdown
         id: results
         run: |
-          echo '### Benchmark results' > results.md
+          {
+            echo "### Benchmark results"
+            echo
+            echo "Performance differences relative to $(cat target/criterion/baseline-sha.txt)."
+            echo
+          } > results.md
           grep -Ev 'ignored|running \d+ tests|%\)' results.txt |\
             sed -E  -e 's/(Performance has regressed.)/:broken_heart: **\1**/gi' \
                     -e 's/(Performance has improved.)/:green_heart: **\1**/gi' \
                     -e 's/^ +/   /gi' \
                     -e 's/^([a-z0-9].*)$/* **\1**/gi' \
                     -e 's/(change:[^%]*% )([^%]*%)(.*)/\1**\2**\3/gi' \
               >> results.md
-          echo '' >> results.md
 
       - name: Export PR comment data
         uses: ./.github/actions/pr-comment-data-export