diff --git a/working/0546-patterns/exhaustiveness_prototype/lib/equal.dart b/working/0546-patterns/exhaustiveness_prototype/lib/equal.dart
index e9da1a4ac9..c85d5d5230 100644
--- a/working/0546-patterns/exhaustiveness_prototype/lib/equal.dart
+++ b/working/0546-patterns/exhaustiveness_prototype/lib/equal.dart
@@ -1,13 +1,16 @@
 // Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
+import 'profile.dart' as profile;
 import 'space.dart';
 
 /// Returns `true` if [left] and [right] are equivalent spaces.
 ///
 /// Equality is defined purely structurally/syntactically.
-bool equal(Space left, Space right) {
-  if (left == right) return true;
+bool equal(Space left, Space right, String reason) {
+  profile.count('equal', reason);
+
+  if (identical(left, right)) return true;
 
   // Empty is only equal to itself (and will get caught by the previous check).
   if (left == Space.empty) return false;
@@ -35,7 +38,7 @@ bool _equalUnions(UnionSpace left, UnionSpace right) {
   for (var leftArm in left.arms) {
     var found = false;
     for (var rightArm in right.arms) {
-      if (equal(leftArm, rightArm)) {
+      if (equal(leftArm, rightArm, 'recurse union')) {
         found = true;
         break;
       }
@@ -58,7 +61,9 @@ bool _equalExtracts(ExtractSpace left, ExtractSpace right) {
   if (right.fields.length != fields.length) return false;
 
   for (var field in fields) {
-    if (!equal(left.fields[field]!, right.fields[field]!)) return false;
+    if (!equal(left.fields[field]!, right.fields[field]!, 'recurse extract')) {
+      return false;
+    }
   }
 
   return true;
diff --git a/working/0546-patterns/exhaustiveness_prototype/lib/intersect.dart b/working/0546-patterns/exhaustiveness_prototype/lib/intersect.dart
index a3c1448eae..2f18e04e53 100644
--- a/working/0546-patterns/exhaustiveness_prototype/lib/intersect.dart
+++ b/working/0546-patterns/exhaustiveness_prototype/lib/intersect.dart
@@ -1,15 +1,17 @@
 // Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-import 'package:exhaustiveness_prototype/static_type.dart';
-
+import 'profile.dart' as profile;
 import 'space.dart';
+import 'static_type.dart';
 
 /// Calculates the intersection of [left] and [right].
 ///
 /// This is used to tell if two field spaces on a pair of spaces being
 /// subtracted have no common values.
 Space intersect(Space left, Space right) {
+  profile.count('intersect');
+
   // The intersection with an empty space is always empty.
   if (left == Space.empty) return Space.empty;
   if (right == Space.empty) return Space.empty;
diff --git a/working/0546-patterns/exhaustiveness_prototype/lib/profile.dart b/working/0546-patterns/exhaustiveness_prototype/lib/profile.dart
new file mode 100644
index 0000000000..dbc9d4a215
--- /dev/null
+++ b/working/0546-patterns/exhaustiveness_prototype/lib/profile.dart
@@ -0,0 +1,47 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:math';
+
+var enabled = false;
+
+final _counts = <String, int>{};
+
+void count(String name, [String? subname]) {
+  if (!enabled) return;
+  _counts.putIfAbsent(name, () => 0);
+  _counts[name] = _counts[name]! + 1;
+
+  if (subname != null) {
+    count('$name/$subname');
+  }
+}
+
+void run(void Function() callback) {
+  reset();
+  try {
+    callback();
+  } finally {
+    log();
+    reset();
+  }
+}
+
+void reset() {
+  _counts.clear();
+}
+
+void log() {
+  var names = _counts.keys.toList();
+  names.sort();
+  var nameLength =
+      names.fold<int>(0, (length, name) => max(length, name.length));
+  var countLength = _counts.values
+      .fold<int>(0, (length, count) => max(length, count.toString().length));
+
+  for (var name in names) {
+    print('${name.padRight(nameLength)} = '
+        '${_counts[name].toString().padLeft(countLength)}');
+  }
+}
diff --git a/working/0546-patterns/exhaustiveness_prototype/lib/space.dart b/working/0546-patterns/exhaustiveness_prototype/lib/space.dart
index 7739daef02..c9f6dc0694 100644
--- a/working/0546-patterns/exhaustiveness_prototype/lib/space.dart
+++ b/working/0546-patterns/exhaustiveness_prototype/lib/space.dart
@@ -28,7 +28,7 @@ abstract class Space {
       // implementation would likely define hash code too and then simply
       // create a hash set to merge duplicates in O(n) time.
       for (var existing in allArms) {
-        if (equal(existing, space)) return;
+        if (equal(existing, space, 'dedupe union')) return;
       }
 
       allArms.add(space);
diff --git a/working/0546-patterns/exhaustiveness_prototype/lib/subtract.dart b/working/0546-patterns/exhaustiveness_prototype/lib/subtract.dart
index 62a0d1ace0..788723cfb6 100644
--- a/working/0546-patterns/exhaustiveness_prototype/lib/subtract.dart
+++ b/working/0546-patterns/exhaustiveness_prototype/lib/subtract.dart
@@ -1,14 +1,17 @@
 // Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-import 'package:exhaustiveness_prototype/intersect_empty.dart';
-import 'package:exhaustiveness_prototype/static_type.dart';
+import 'intersect_empty.dart';
+import 'profile.dart' as profile;
+import 'static_type.dart';
 
 import 'space.dart';
 
 /// Returns a new [Space] that contains all of the values of [left] that are
 /// not also in [right].
 Space subtract(Space left, Space right) {
+  profile.count('subtract');
+
   // Subtracting from empty is still empty.
   if (left == Space.empty) return Space.empty;
 
@@ -96,7 +99,6 @@ List<Space> _subtractExtractAtType(StaticType type, ExtractSpace left,
 
   // Walk the fields and see which ones are modified by the right-hand fields.
   var fixed = <String, Space>{};
-  var changedLeft = <String, Space>{};
   var changedDifference = <String, Space>{};
   for (var name in fieldNames) {
     var difference = subtract(leftFields[name]!, rightFields[name]!);
@@ -108,18 +110,17 @@ List<Space> _subtractExtractAtType(StaticType type, ExtractSpace left,
       // If the resulting field matches everything, simply discard it since
       // it's equivalent to omitting the field.
     } else {
-      changedLeft[name] = leftFields[name]!;
       changedDifference[name] = difference;
     }
   }
 
   // If no fields are affected by the subtraction, just return a single arm
   // with all of the fields.
-  if (changedLeft.isEmpty) return [Space(type, fixed)];
+  if (changedDifference.isEmpty) return [Space(type, fixed)];
 
   // For each field whose `left - right` is different, include an arm that
   // includes that one difference.
-  var changedFields = changedLeft.keys.toList();
+  var changedFields = changedDifference.keys.toList();
   var spaces = <Space>[];
   for (var i = 0; i < changedFields.length; i++) {
     var fields = {...fixed};
@@ -129,7 +130,7 @@ List<Space> _subtractExtractAtType(StaticType type, ExtractSpace left,
       if (i == j) {
         fields[name] = changedDifference[name]!;
       } else {
-        fields[name] = changedLeft[name]!;
+        fields[name] = leftFields[name]!;
       }
     }