Cut the allocations of a tracking frame and of a TrackedValue

packages/@glimmer/validator/lib/tracking.ts

@@ -5,7 +5,14 @@ import type { Revision } from './validators';
 
 import { debug } from './debug';
 import { unwrap } from './utils';
-import { combine, CONSTANT_TAG, isConstTag, validateTag, valueForTag } from './validators';
+import {
+  combine,
+  CONSTANT_TAG,
+  isCombinationOf,
+  isConstTag,
+  validateTag,
+  valueForTag,
+} from './validators';
 
 /**
  * A tag with the `slot` field that `Tracker#add` reads and writes.
@@ -17,6 +24,30 @@ interface ConsumedTag extends Tag {
   slot: number;
 }
 
+/**
+ * Makes the tag of a frame that consumed two or more tags.
+ *
+ * `previous` is the tag that the frame produced the last time it ran.
+ * If the frame consumed the same tags again, in the same order,
+ * the result is `previous`.
+ * That tag keeps its memoized revision, and nothing is allocated.
+ *
+ *   1. last run:  consumed [ a, b, c ]    result: tag X.
+ *   2. this run:  consumed [ a, b, c ]    result: tag X again.
+ *   3. or:        consumed [ a, c ]       result: a new tag.
+ *
+ * This code is not in `Tracker#combineForCache`, to keep that method small.
+ * V8 puts a small method inline into the end of the frame.
+ * With this code in it, a frame that consumed one tag was 6% slower.
+ */
+function combineOrReuse(previous: Tag | undefined, tags: (Tag | null)[], size: number): Tag {
+  if (previous !== undefined && isCombinationOf(previous, tags, size)) {
+    return previous;
+  }
+
+  return combine(tags.slice(0, size) as Tag[]);
+}
+
 /**
  * An object that tracks @tracked properties that were consumed.
  *
@@ -109,6 +140,29 @@ class Tracker {
     return result;
   }
 
+  /**
+   * `combine()` for the frame of a cache.
+   *
+   * `previous` is the tag that the cache has from its last run.
+   * See `combineOrReuse`.
+   */
+  combineForCache(previous: Tag | undefined): Tag {
+    let { tags, size } = this;
+    let result: Tag;
+
+    if (size === 0) {
+      result = CONSTANT_TAG;
+    } else if (size === 1) {
+      result = tags[0] as Tag;
+    } else {
+      result = combineOrReuse(previous, tags, size);
+    }
+
+    this.clear();
+
+    return result;
+  }
+
   /**
    * Empties the tracker for the next frame at the same depth.
    *
@@ -208,6 +262,32 @@ export function endTrackFrame(): Tag {
   return unwrap(current).combine();
 }
 
+/**
+ * `endTrackFrame()` for the frame of a cache.
+ *
+ * `previous` is the tag that the cache has from its last run.
+ * If the cache consumed the same tags again, that tag is the result.
+ *
+ * Only `getValue` calls this.
+ * The frames of the render VM end in `endTrackFrame`,
+ * which has no code for the reuse.
+ */
+function endCacheFrame(previous: Tag | undefined): Tag {
+  let current = CURRENT_TRACKER;
+
+  if (DEBUG) {
+    if (OPEN_TRACK_FRAMES.length === 0) {
+      throw new Error('attempted to close a tracking frame, but one was not open');
+    }
+
+    unwrap(debug.endTrackingTransaction)();
+  }
+
+  CURRENT_TRACKER = OPEN_TRACK_FRAMES.pop() || null;
+
+  return unwrap(current).combineForCache(previous);
+}
+
 export function beginUntrackFrame(): void {
   OPEN_TRACK_FRAMES.push(CURRENT_TRACKER);
   CURRENT_TRACKER = null;
@@ -313,7 +393,7 @@ export function getValue<T>(cache: Cache<T>): T | undefined {
     try {
       cache[LAST_VALUE] = fn();
     } finally {
-      tag = endTrackFrame();
+      tag = endCacheFrame(tag);
       cache[TAG] = tag;
       cache[SNAPSHOT] = valueForTag(tag);
       consumeTag(tag);

packages/@glimmer/validator/lib/validators.ts

@@ -298,6 +298,23 @@ export const CURRENT_TAG = new CurrentTag();
 
 export const combine = MonomorphicTagImpl.combine;
 
+/**
+ * Whether `tag` is the combination of the first `size` entries of `tags`, in order.
+ */
+export function isCombinationOf(tag: Tag, tags: (Tag | null)[], size: number): boolean {
+  if (tag[TYPE] !== COMBINATOR_TAG_ID) return false;
+
+  let subtags = (tag as MonomorphicTagImpl).subtag as Tag[];
+
+  if (subtags.length !== size) return false;
+
+  for (let i = 0; i < size; i++) {
+    if (subtags[i] !== tags[i]) return false;
+  }
+
+  return true;
+}
+
 // Warm
 
 let tag1 = createUpdatableTag();

packages/@glimmer/validator/test/tracking-test.ts

@@ -448,6 +448,199 @@ module('@glimmer/validator: tracking', () => {
       assert.deepEqual(getValue(outerCache), [3, 2], 'memoized result returned correctly');
     });
 
+    /**
+     * The tag that a cache has after a read.
+     *
+     * The outer frame consumes only the tag of the cache,
+     * and a frame with one tag returns that tag.
+     */
+    function tagOf(cache: ReturnType<typeof createCache>) {
+      beginTrackFrame();
+      getValue(cache);
+
+      return endTrackFrame();
+    }
+
+    test('it keeps its tag if it consumes the same tags in the next run', (assert) => {
+      let tag1 = createTag();
+      let tag2 = createTag();
+
+      let cache = createCache(() => {
+        assert.step('cache reads tag1 and tag2');
+
+        consumeTag(tag1);
+        consumeTag(tag2);
+      });
+
+      let first = tagOf(cache);
+
+      assert.verifySteps(['cache reads tag1 and tag2']);
+
+      dirtyTag(tag1);
+
+      let second = tagOf(cache);
+
+      assert.verifySteps(['cache reads tag1 and tag2'], 'the cache ran again');
+      assert.strictEqual(second, first, 'the cache has the tag of its first run');
+    });
+
+    test('it gets a new tag if it consumes other tags in the next run', (assert) => {
+      let tag1 = createTag();
+      let tag2 = createTag();
+      let tag3 = createTag();
+      let useThird = false;
+
+      let cache = createCache(() => {
+        consumeTag(tag1);
+
+        if (useThird) {
+          assert.step('cache reads tag1 and tag3');
+          consumeTag(tag3);
+        } else {
+          assert.step('cache reads tag1 and tag2');
+          consumeTag(tag2);
+        }
+      });
+
+      let first = tagOf(cache);
+
+      assert.verifySteps(['cache reads tag1 and tag2']);
+
+      useThird = true;
+      dirtyTag(tag1);
+
+      let second = tagOf(cache);
+
+      assert.verifySteps(['cache reads tag1 and tag3'], 'the cache ran again');
+      assert.notStrictEqual(second, first);
+    });
+
+    test('it gets a new tag if it consumes the same tags in another order', (assert) => {
+      let tag1 = createTag();
+      let tag2 = createTag();
+      let swap = false;
+
+      let cache = createCache(() => {
+        if (swap) {
+          assert.step('cache reads tag2, then tag1');
+          consumeTag(tag2);
+          consumeTag(tag1);
+        } else {
+          assert.step('cache reads tag1, then tag2');
+          consumeTag(tag1);
+          consumeTag(tag2);
+        }
+      });
+
+      let first = tagOf(cache);
+
+      assert.verifySteps(['cache reads tag1, then tag2']);
+
+      swap = true;
+      dirtyTag(tag1);
+
+      let second = tagOf(cache);
+
+      assert.verifySteps(['cache reads tag2, then tag1'], 'the cache ran again');
+      assert.notStrictEqual(second, first);
+
+      let snapshot = valueForTag(second);
+
+      dirtyTag(tag2);
+      assert.notOk(validateTag(second, snapshot), 'the new tag follows both tags');
+    });
+
+    test('it tracks the new tags if the tags change between two runs', (assert) => {
+      let tag1 = createTag();
+      let tag2 = createTag();
+      let tag3 = createTag();
+      let useThird = false;
+
+      let cache = createCache(() => {
+        consumeTag(tag1);
+
+        if (useThird) {
+          assert.step('cache reads tag1 and tag3');
+          consumeTag(tag3);
+        } else {
+          assert.step('cache reads tag1 and tag2');
+          consumeTag(tag2);
+        }
+      });
+
+      getValue(cache);
+      assert.verifySteps(['cache reads tag1 and tag2']);
+
+      dirtyTag(tag2);
+      getValue(cache);
+      assert.verifySteps(['cache reads tag1 and tag2'], 'the cache ran again with the same tags');
+
+      dirtyTag(tag2);
+      getValue(cache);
+      assert.verifySteps(['cache reads tag1 and tag2'], 'the reused tag still follows its tags');
+
+      useThird = true;
+      dirtyTag(tag1);
+      getValue(cache);
+      assert.verifySteps(['cache reads tag1 and tag3']);
+
+      dirtyTag(tag2);
+      getValue(cache);
+      assert.verifySteps([], 'the cache does not depend on the old tag');
+
+      dirtyTag(tag3);
+      getValue(cache);
+      assert.verifySteps(['cache reads tag1 and tag3'], 'the cache depends on the new tag');
+    });
+
+    test('an outer cache runs again if a tag of a reused inner tag changes', (assert) => {
+      let tag1 = createTag();
+      let tag2 = createTag();
+      let tag3 = createTag();
+
+      let inner = createCache(() => {
+        assert.step('inner reads tag1 and tag2');
+
+        consumeTag(tag1);
+        consumeTag(tag2);
+      });
+
+      let outer = createCache(() => {
+        assert.step('outer reads tag3 and inner');
+
+        consumeTag(tag3);
+        getValue(inner);
+      });
+
+      getValue(outer);
+      assert.verifySteps(['outer reads tag3 and inner', 'inner reads tag1 and tag2']);
+
+      dirtyTag(tag1);
+      getValue(outer);
+      assert.verifySteps(
+        ['outer reads tag3 and inner', 'inner reads tag1 and tag2'],
+        'the inner cache ran again with the same tags'
+      );
+
+      dirtyTag(tag2);
+      getValue(outer);
+      assert.verifySteps(
+        ['outer reads tag3 and inner', 'inner reads tag1 and tag2'],
+        'the outer cache follows the reused inner tag'
+      );
+
+      dirtyTag(tag3);
+      getValue(outer);
+      assert.verifySteps(['outer reads tag3 and inner'], 'the inner cache did not run again');
+
+      dirtyTag(tag1);
+      getValue(outer);
+      assert.verifySteps(
+        ['outer reads tag3 and inner', 'inner reads tag1 and tag2'],
+        'the outer cache still follows the inner tag'
+      );
+    });
+
     test('isTracking works within a memoized function and untrack frame', (assert) => {
       assert.notOk(isTracking());