refactor(core): global epoch to optimize non-live signal reads (angular#52420)

This commit adds a global epoch to the reactive graph, which can optimize
non-live reads.

When a non-live read occurs, a computed must poll its dependencies to check
if they've changed, and this operation is transitive and not cacheable.
Since non-live computeds don't receive dirty notifications, they're forced
to assume potential dirtiness on each and every read.

Using a global epoch, we can add an important optimization: if *no* signals
have been set globally since the last time it polled its dependencies, then
we *can* assume a clean state. This significantly improves performance of
large unwatched graphs when repeatedly reading values.

PR Close angular#52420

Author: alxhub

goldens/public-api/core/primitives/signals/index.md

@@ -64,6 +64,7 @@ export interface ReactiveNode {
     consumerMarkedDirty(node: unknown): void;
     consumerOnSignalRead(node: unknown): void;
     dirty: boolean;
+    lastCleanEpoch: Version;
     liveConsumerIndexOfThis: number[] | undefined;
     liveConsumerNode: ReactiveNode[] | undefined;
     nextProducerIndex: number;

packages/core/primitives/signals/src/graph.ts

@@ -21,6 +21,11 @@ let inNotificationPhase = false;
 
 type Version = number&{__brand: 'Version'};
 
+/**
+ * Global epoch counter. Incremented whenever a source signal is set.
+ */
+let epoch: Version = 1 as Version;
+
 /**
  * Symbol used to tell `Signal`s apart from other functions.
  *
@@ -52,6 +57,7 @@ export function isReactive(value: unknown): value is Reactive {
 
 export const REACTIVE_NODE: ReactiveNode = {
   version: 0 as Version,
+  lastCleanEpoch: 0 as Version,
   dirty: false,
   producerNode: undefined,
   producerLastReadVersion: undefined,
@@ -88,6 +94,14 @@ export interface ReactiveNode {
    */
   version: Version;
 
+  /**
+   * Epoch at which this node is verified to be clean.
+   *
+   * This allows skipping of some polling operations in the case where no signals have been set
+   * since this node was last read.
+   */
+  lastCleanEpoch: Version;
+
   /**
    * Whether this node (in its consumer capacity) is dirty.
    *
@@ -231,6 +245,15 @@ export function producerAccessed(node: ReactiveNode): void {
   activeConsumer.producerLastReadVersion[idx] = node.version;
 }
 
+/**
+ * Increment the global epoch counter.
+ *
+ * Called by source producers (that is, not computeds) whenever their values change.
+ */
+export function producerIncrementEpoch(): void {
+  epoch++;
+}
+
 /**
  * Ensure this producer's `version` is up-to-date.
  */
@@ -241,17 +264,26 @@ export function producerUpdateValueVersion(node: ReactiveNode): void {
     return;
   }
 
+  if (!node.dirty && node.lastCleanEpoch === epoch) {
+    // Even non-live consumers can skip polling if they previously found themselves to be clean at
+    // the current epoch, since their dependencies could not possibly have changed (such a change
+    // would've increased the epoch).
+    return;
+  }
+
   if (!node.producerMustRecompute(node) && !consumerPollProducersForChange(node)) {
     // None of our producers report a change since the last time they were read, so no
     // recomputation of our value is necessary, and we can consider ourselves clean.
     node.dirty = false;
+    node.lastCleanEpoch = epoch;
     return;
   }
 
   node.producerRecomputeValue(node);
 
   // After recomputing the value, we're no longer dirty.
   node.dirty = false;
+  node.lastCleanEpoch = epoch;
 }
 
 /**

packages/core/primitives/signals/src/signal.ts

@@ -8,7 +8,7 @@
 
 import {defaultEquals, ValueEqualityFn} from './equality';
 import {throwInvalidWriteToSignalError} from './errors';
-import {producerAccessed, producerNotifyConsumers, producerUpdatesAllowed, REACTIVE_NODE, ReactiveNode, SIGNAL} from './graph';
+import {producerAccessed, producerIncrementEpoch, producerNotifyConsumers, producerUpdatesAllowed, REACTIVE_NODE, ReactiveNode, SIGNAL} from './graph';
 
 /**
  * If set, called after `WritableSignal`s are updated.
@@ -98,6 +98,7 @@ const SIGNAL_NODE: object = /* @__PURE__ */ (() => {
 
 function signalValueChanged<T>(node: SignalNode<T>): void {
   node.version++;
+  producerIncrementEpoch();
   producerNotifyConsumers(node);
   postSignalSetFn?.();
 }

packages/core/test/signals/signal_spec.ts

@@ -7,7 +7,7 @@
  */
 
 import {computed, signal} from '@angular/core';
-import {setPostSignalSetFn} from '@angular/core/primitives/signals';
+import {ReactiveNode, setPostSignalSetFn, SIGNAL} from '@angular/core/primitives/signals';
 
 describe('signals', () => {
   it('should be a getter which reflects the set value', () => {
@@ -104,6 +104,39 @@ describe('signals', () => {
     expect(double()).toBe(4);
   });
 
+  describe('optimizations', () => {
+    it('should not repeatedly poll status of a non-live node if no signals have changed', () => {
+      const unrelated = signal(0);
+      const source = signal(1);
+      let computations = 0;
+      const derived = computed(() => {
+        computations++;
+        return source() * 2;
+      });
+
+      expect(derived()).toBe(2);
+      expect(computations).toBe(1);
+
+      const sourceNode = source[SIGNAL] as ReactiveNode;
+      // Forcibly increment the version of the source signal. This will cause a mismatch during
+      // polling, and will force the derived signal to recompute if polled (which we should observe
+      // in this test).
+      sourceNode.version++;
+
+      // Read the derived signal again. This should not recompute (even with the forced version
+      // update) as no signals have been set since the last read.
+      expect(derived()).toBe(2);
+      expect(computations).toBe(1);
+
+      // Set the `unrelated` signal, which now means that `derived` should poll if read again.
+      // Because of the forced version, that poll will cause a recomputation which we will observe.
+      unrelated.set(1);
+
+      expect(derived()).toBe(2);
+      expect(computations).toBe(2);
+    });
+  });
+
   describe('post-signal-set functions', () => {
     let prevPostSignalSetFn: (() => void)|null = null;
     let log: number;