Optimize readContext for Subsequent Reads of All Bits (#13248)

This is likely the common case because individual component authors
will casually call read on common contexts like the cache, or cache
provider.

Where as libraries like Relay only call read once per fragment and pass
all observed bits at once.

packages/react-reconciler/src/ReactFiberNewContext.js

@@ -38,14 +38,14 @@ if (__DEV__) {
 
 let currentlyRenderingFiber: Fiber | null = null;
 let lastContextDependency: ContextDependency<mixed> | null = null;
-let lastContext: ReactContext<any> | null = null;
+let lastContextWithAllBitsObserved: ReactContext<any> | null = null;
 
 export function resetContextDependences(): void {
   // This is called right before React yields execution, to ensure `readContext`
   // cannot be called outside the render phase.
   currentlyRenderingFiber = null;
   lastContextDependency = null;
-  lastContext = null;
+  lastContextWithAllBitsObserved = null;
 }
 
 export function pushProvider(providerFiber: Fiber): void {
@@ -221,7 +221,7 @@ export function prepareToReadContext(
 ): boolean {
   currentlyRenderingFiber = workInProgress;
   lastContextDependency = null;
-  lastContext = null;
+  lastContextWithAllBitsObserved = null;
 
   const firstContextDependency = workInProgress.firstContextDependency;
   if (firstContextDependency !== null) {
@@ -264,46 +264,41 @@ export function readContext<T>(
   context: ReactContext<T>,
   observedBits: void | number | boolean,
 ): T {
-  if (typeof observedBits !== 'number') {
-    if (observedBits === false) {
-      // Do not observe updates
-      observedBits = 0;
+  if (lastContextWithAllBitsObserved === context) {
+    // Nothing to do. We already observe everything in this context.
+  } else if (observedBits === false || observedBits === 0) {
+    // Do not observe any updates.
+  } else {
+    let resolvedObservedBits; // Avoid deopting on observable arguments or heterogeneous types.
+    if (
+      typeof observedBits !== 'number' ||
+      observedBits === maxSigned31BitInt
+    ) {
+      // Observe all updates.
+      lastContextWithAllBitsObserved = ((context: any): ReactContext<mixed>);
+      resolvedObservedBits = maxSigned31BitInt;
     } else {
-      // Observe all updates
-      observedBits = maxSigned31BitInt;
+      resolvedObservedBits = observedBits;
     }
-  }
 
-  if (lastContext === null) {
-    invariant(
-      currentlyRenderingFiber !== null,
-      'Context.unstable_read(): Context can only be read while React is ' +
-        'rendering, e.g. inside the render method or getDerivedStateFromProps.',
-    );
-    // This is the first dependency in the list
-    currentlyRenderingFiber.firstContextDependency = lastContextDependency = {
+    let contextItem = {
       context: ((context: any): ReactContext<mixed>),
-      observedBits,
+      observedBits: resolvedObservedBits,
       next: null,
     };
-    lastContext = context;
-  } else {
-    // `lastContextDependency` is always non-null if `lastContext is.
-    const lastDependency: ContextDependency<T> = (lastContextDependency: any);
-    if (lastContext === context) {
-      // Fast path. The previous context has the same type. We can reuse
-      // the same node.
-      lastDependency.observedBits |= observedBits;
+
+    if (lastContextDependency === null) {
+      invariant(
+        currentlyRenderingFiber !== null,
+        'Context.unstable_read(): Context can only be read while React is ' +
+          'rendering, e.g. inside the render method or getDerivedStateFromProps.',
+      );
+      // This is the first dependency in the list
+      currentlyRenderingFiber.firstContextDependency = lastContextDependency = contextItem;
     } else {
       // Append a new context item.
-      lastContextDependency = lastDependency.next = {
-        context: ((context: any): ReactContext<mixed>),
-        observedBits,
-        next: null,
-      };
-      lastContext = context;
+      lastContextDependency = lastContextDependency.next = contextItem;
     }
   }
-
   return isPrimaryRenderer ? context._currentValue : context._currentValue2;
 }