Experiment with using an object literal for Fiber creation

packages/react-reconciler/src/ReactFiber.js

@@ -38,6 +38,7 @@ import {
   enableDO_NOT_USE_disableStrictPassiveEffect,
   enableRenderableContext,
   disableLegacyMode,
+  enableObjectFiber,
   enableOwnerStacks,
 } from 'shared/ReactFeatureFlags';
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
@@ -232,7 +233,7 @@ function FiberNode(
 //    is faster.
 // 5) It should be easy to port this to a C struct and keep a C implementation
 //    compatible.
-function createFiber(
+function createFiberImplClass(
   tag: WorkTag,
   pendingProps: mixed,
   key: null | string,
@@ -242,6 +243,75 @@ function createFiber(
   return new FiberNode(tag, pendingProps, key, mode);
 }
 
+function createFiberImplObject(
+  tag: WorkTag,
+  pendingProps: mixed,
+  key: null | string,
+  mode: TypeOfMode,
+): Fiber {
+  const fiber: Fiber = {
+    // Instance
+    // tag, key - defined at the bottom as dynamic properties
+    elementType: null,
+    type: null,
+    stateNode: null,
+
+    // Fiber
+    return: null,
+    child: null,
+    sibling: null,
+    index: 0,
+
+    ref: null,
+    refCleanup: null,
+
+    // pendingProps - defined at the bottom as dynamic properties
+    memoizedProps: null,
+    updateQueue: null,
+    memoizedState: null,
+    dependencies: null,
+
+    // Effects
+    flags: NoFlags,
+    subtreeFlags: NoFlags,
+    deletions: null,
+
+    lanes: NoLanes,
+    childLanes: NoLanes,
+
+    alternate: null,
+
+    // dynamic properties at the end for more efficient hermes bytecode
+    tag,
+    key,
+    pendingProps,
+    mode,
+  };
+
+  if (enableProfilerTimer) {
+    fiber.actualDuration = 0;
+    fiber.actualStartTime = -1;
+    fiber.selfBaseDuration = 0;
+    fiber.treeBaseDuration = 0;
+  }
+
+  if (__DEV__) {
+    fiber._debugInfo = null;
+    fiber._debugOwner = null;
+    fiber._debugNeedsRemount = false;
+    fiber._debugHookTypes = null;
+
+    if (!hasBadMapPolyfill && typeof Object.preventExtensions === 'function') {
+      Object.preventExtensions(fiber);
+    }
+  }
+  return fiber;
+}
+
+const createFiber = enableObjectFiber
+  ? createFiberImplObject
+  : createFiberImplClass;
+
 function shouldConstruct(Component: Function) {
   const prototype = Component.prototype;
   return !!(prototype && prototype.isReactComponent);

packages/shared/ReactFeatureFlags.js

@@ -87,6 +87,11 @@ export const enableTaint = __EXPERIMENTAL__;
 
 export const enablePostpone = __EXPERIMENTAL__;
 
+/**
+ * Switches Fiber creation to a simple object instead of a constructor.
+ */
+export const enableObjectFiber = false;
+
 export const enableTransitionTracing = false;
 
 // No known bugs, but needs performance testing

packages/shared/forks/ReactFeatureFlags.native-fb-dynamic.js

@@ -22,5 +22,6 @@ export const consoleManagedByDevToolsDuringStrictMode = __VARIANT__;
 export const disableDefaultPropsExceptForClasses = __VARIANT__;
 export const enableAddPropertiesFastPath = __VARIANT__;
 export const enableFastJSX = __VARIANT__;
+export const enableObjectFiber = __VARIANT__;
 export const enableShallowPropDiffing = __VARIANT__;
 export const passChildrenWhenCloningPersistedNodes = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -24,6 +24,7 @@ export const {
   disableDefaultPropsExceptForClasses,
   enableAddPropertiesFastPath,
   enableFastJSX,
+  enableObjectFiber,
   enableShallowPropDiffing,
   passChildrenWhenCloningPersistedNodes,
 } = dynamicFlags;

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -55,6 +55,7 @@ export const enableLegacyCache = false;
 export const enableLegacyFBSupport = false;
 export const enableLegacyHidden = false;
 export const enableNoCloningMemoCache = false;
+export const enableObjectFiber = false;
 export const enableOwnerStacks = __EXPERIMENTAL__;
 export const enablePostpone = false;
 export const enableReactTestRendererWarning = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -96,6 +96,7 @@ export const enableRenderableContext = true;
 export const enableReactTestRendererWarning = true;
 export const disableDefaultPropsExceptForClasses = true;
 
+export const enableObjectFiber = false;
 export const enableOwnerStacks = false;
 
 // Flow magic to verify the exports of this file match the original version.

packages/shared/forks/ReactFeatureFlags.test-renderer.native-fb.js

@@ -47,6 +47,7 @@ export const enableLegacyCache = false;
 export const enableLegacyFBSupport = false;
 export const enableLegacyHidden = false;
 export const enableNoCloningMemoCache = false;
+export const enableObjectFiber = false;
 export const enableOwnerStacks = false;
 export const enablePostpone = false;
 export const enableProfilerCommitHooks = __PROFILE__;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -90,6 +90,7 @@ export const enableAddPropertiesFastPath = false;
 
 export const renameElementSymbol = false;
 
+export const enableObjectFiber = false;
 export const enableOwnerStacks = false;
 export const enableShallowPropDiffing = false;
 

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -13,26 +13,27 @@
 // Use __VARIANT__ to simulate a GK. The tests will be run twice: once
 // with the __VARIANT__ set to `true`, and once set to `false`.
 
+export const alwaysThrottleRetries = true;
+export const disableDefaultPropsExceptForClasses = __VARIANT__;
+export const disableLegacyMode = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
-export const enableLazyContextPropagation = __VARIANT__;
-export const forceConcurrentByDefaultForTesting = __VARIANT__;
-export const enableTransitionTracing = __VARIANT__;
+export const enableAddPropertiesFastPath = __VARIANT__;
 export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
-export const alwaysThrottleRetries = true;
 export const enableDO_NOT_USE_disableStrictPassiveEffect = __VARIANT__;
-export const enableUseDeferredValueInitialArg = __VARIANT__;
-export const enableRenderableContext = __VARIANT__;
 export const enableFastJSX = __VARIANT__;
+export const enableLazyContextPropagation = __VARIANT__;
+export const enableNoCloningMemoCache = __VARIANT__;
+export const enableObjectFiber = __VARIANT__;
+export const enableRenderableContext = __VARIANT__;
 export const enableRetryLaneExpiration = __VARIANT__;
+export const enableTransitionTracing = __VARIANT__;
+export const enableUseDeferredValueInitialArg = __VARIANT__;
 export const favorSafetyOverHydrationPerf = __VARIANT__;
-export const disableDefaultPropsExceptForClasses = __VARIANT__;
-export const enableNoCloningMemoCache = __VARIANT__;
+export const forceConcurrentByDefaultForTesting = __VARIANT__;
+export const renameElementSymbol = __VARIANT__;
 export const retryLaneExpirationMs = 5000;
 export const syncLaneExpirationMs = 250;
 export const transitionLaneExpirationMs = 5000;
-export const enableAddPropertiesFastPath = __VARIANT__;
-export const disableLegacyMode = __VARIANT__;
-export const renameElementSymbol = __VARIANT__;
 
 // Enable this flag to help with concurrent mode debugging.
 // It logs information to the console about React scheduling, rendering, and commit phases.

packages/shared/forks/ReactFeatureFlags.www.js

@@ -15,27 +15,28 @@ import typeof * as DynamicFeatureFlags from './ReactFeatureFlags.www-dynamic';
 const dynamicFeatureFlags: DynamicFeatureFlags = require('ReactFeatureFlags');
 
 export const {
-  enableTrustedTypesIntegration,
+  alwaysThrottleRetries,
+  disableDefaultPropsExceptForClasses,
+  disableSchedulerTimeoutInWorkLoop,
+  enableAddPropertiesFastPath,
   enableDebugTracing,
+  enableDeferRootSchedulingToMicrotask,
+  enableDO_NOT_USE_disableStrictPassiveEffect,
+  enableFastJSX,
+  enableInfiniteRenderLoopDetection,
   enableLazyContextPropagation,
+  enableNoCloningMemoCache,
+  enableObjectFiber,
+  enableRenderableContext,
   enableRetryLaneExpiration,
   enableTransitionTracing,
-  enableDeferRootSchedulingToMicrotask,
-  alwaysThrottleRetries,
-  enableDO_NOT_USE_disableStrictPassiveEffect,
-  disableSchedulerTimeoutInWorkLoop,
+  enableTrustedTypesIntegration,
   enableUseDeferredValueInitialArg,
+  favorSafetyOverHydrationPerf,
+  renameElementSymbol,
   retryLaneExpirationMs,
   syncLaneExpirationMs,
   transitionLaneExpirationMs,
-  enableInfiniteRenderLoopDetection,
-  enableRenderableContext,
-  favorSafetyOverHydrationPerf,
-  disableDefaultPropsExceptForClasses,
-  enableNoCloningMemoCache,
-  enableAddPropertiesFastPath,
-  enableFastJSX,
-  renameElementSymbol,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.