SuspenseList Optimizations

packages/react-reconciler/src/ReactChildFiber.js

@@ -33,6 +33,7 @@ import warningWithoutStack from 'shared/warningWithoutStack';
 
 import {
   createWorkInProgress,
+  resetWorkInProgress,
   createFiberFromElement,
   createFiberFromFragment,
   createFiberFromText,
@@ -1386,3 +1387,15 @@ export function cloneChildFibers(
   }
   newChild.sibling = null;
 }
+
+// Reset a workInProgress child set to prepare it for a second pass.
+export function resetChildFibers(
+  workInProgress: Fiber,
+  renderExpirationTime: ExpirationTime,
+): void {
+  let child = workInProgress.child;
+  while (child !== null) {
+    resetWorkInProgress(child, renderExpirationTime);
+    child = child.sibling;
+  }
+}

packages/react-reconciler/src/ReactFiber.js

@@ -481,6 +481,79 @@ export function createWorkInProgress(
   return workInProgress;
 }
 
+// Used to reuse a Fiber for a second pass.
+export function resetWorkInProgress(
+  workInProgress: Fiber,
+  renderExpirationTime: ExpirationTime,
+) {
+  // This resets the Fiber to what createFiber or createWorkInProgress would
+  // have set the values to before during the first pass. Ideally this wouldn't
+  // be necessary but unfortunately many code paths reads from the workInProgress
+  // when they should be reading from current and writing to workInProgress.
+
+  // We assume pendingProps, index, key, ref, return are still untouched to
+  // avoid doing another reconciliation.
+
+  // Reset the effect tag.
+  workInProgress.effectTag = NoEffect;
+
+  // The effect list is no longer valid.
+  workInProgress.nextEffect = null;
+  workInProgress.firstEffect = null;
+  workInProgress.lastEffect = null;
+
+  let current = workInProgress.alternate;
+  if (current === null) {
+    // Reset to createFiber's initial values.
+    workInProgress.childExpirationTime = NoWork;
+    workInProgress.expirationTime = renderExpirationTime;
+
+    workInProgress.child = null;
+    workInProgress.memoizedProps = null;
+    workInProgress.memoizedState = null;
+    workInProgress.updateQueue = null;
+
+    workInProgress.dependencies = null;
+
+    if (enableProfilerTimer) {
+      // Note: We don't reset the actualTime counts. It's useful to accumulate
+      // actual time across multiple render passes.
+      workInProgress.selfBaseDuration = 0;
+      workInProgress.treeBaseDuration = 0;
+    }
+  } else {
+    // Reset to the cloned values that createWorkInProgress would've.
+    workInProgress.childExpirationTime = current.childExpirationTime;
+    workInProgress.expirationTime = current.expirationTime;
+
+    workInProgress.child = current.child;
+    workInProgress.memoizedProps = current.memoizedProps;
+    workInProgress.memoizedState = current.memoizedState;
+    workInProgress.updateQueue = current.updateQueue;
+
+    // Clone the dependencies object. This is mutated during the render phase, so
+    // it cannot be shared with the current fiber.
+    const currentDependencies = current.dependencies;
+    workInProgress.dependencies =
+      currentDependencies === null
+        ? null
+        : {
+            expirationTime: currentDependencies.expirationTime,
+            firstContext: currentDependencies.firstContext,
+            events: currentDependencies.events,
+          };
+
+    if (enableProfilerTimer) {
+      // Note: We don't reset the actualTime counts. It's useful to accumulate
+      // actual time across multiple render passes.
+      workInProgress.selfBaseDuration = current.selfBaseDuration;
+      workInProgress.treeBaseDuration = current.treeBaseDuration;
+    }
+  }
+
+  return workInProgress;
+}
+
 export function createHostRootFiber(tag: RootTag): Fiber {
   let mode;
   if (tag === ConcurrentRoot) {