diff -r 000000000000 -r 6474c204b198 js/src/tests/compare_bench.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/tests/compare_bench.py	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,73 @@
+#!/usr/bin/env python2.4
+"""usage: %progname candidate_path baseline_path
+"""
+
+import json
+import optparse
+from contextlib import nested
+from operator import itemgetter
+
+
+def avg(seq):
+    return sum(seq) / len(seq)
+
+
+def compare(current, baseline):
+    percent_speedups = []
+    for key, current_result in current.iteritems():
+        try:
+            baseline_result = baseline[key]
+        except KeyError:
+            print key, 'missing from baseline'
+            continue
+        val_getter = itemgetter('average_ms', 'stddev_ms')
+        base_avg, base_stddev = val_getter(baseline_result)
+        current_avg, current_stddev = val_getter(current_result)
+        t_best, t_worst = current_avg - current_stddev, current_avg + current_stddev
+        base_t_best, base_t_worst = base_avg - base_stddev, base_avg + base_stddev
+        fmt = '%30s: %s'
+        if t_worst < base_t_best: # Worst takes less time (better) than baseline's best.
+            speedup = -((t_worst - base_t_best) / base_t_best) * 100
+            result = 'faster: %6.2fms < baseline %6.2fms (%+6.2f%%)' % \
+                    (t_worst, base_t_best, speedup)
+            percent_speedups.append(speedup)
+        elif t_best > base_t_worst: # Best takes more time (worse) than baseline's worst.
+            slowdown = -((t_best - base_t_worst) / base_t_worst) * 100
+            result = 'SLOWER: %6.2fms > baseline %6.2fms (%+6.2f%%) ' % \
+                    (t_best, base_t_worst, slowdown)
+            percent_speedups.append(slowdown)
+        else:
+            result = 'Meh.'
+        print '%30s: %s' % (key, result)
+    if percent_speedups:
+        print 'Average speedup: %.2f%%' % avg(percent_speedups)
+
+
+def compare_immediate(current_map, baseline_path):
+    baseline_file = open(baseline_path)
+    baseline_map = json.load(baseline_file)
+    baseline_file.close()
+    compare(current_map, baseline_map)
+
+
+def main(candidate_path, baseline_path):
+    candidate_file, baseline_file = open(candidate_path), open(baseline_path)
+    candidate = json.load(candidate_file)
+    baseline = json.load(baseline_file)
+    compare(candidate, baseline)
+    candidate_file.close()
+    baseline_file.close()
+
+
+if __name__ == '__main__':
+    parser = optparse.OptionParser(usage=__doc__.strip())
+    options, args = parser.parse_args()
+    try:
+        candidate_path = args.pop(0)
+    except IndexError:
+        parser.error('A JSON filepath to compare against baseline is required')
+    try:
+        baseline_path = args.pop(0)
+    except IndexError:
+        parser.error('A JSON filepath for baseline is required')
+    main(candidate_path, baseline_path)