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[Experiment] Reuse memo cache after interruption (#28878)
Adds an experimental feature flag to the implementation of useMemoCache, the internal cache used by the React Compiler (Forget). When enabled, instead of treating the cache as copy-on-write, like we do with fibers, we share the same cache instance across all render attempts, even if the component is interrupted before it commits. If an update is interrupted, either because it suspended or because of another update, we can reuse the memoized computations from the previous attempt. We can do this because the React Compiler performs atomic writes to the memo cache, i.e. it will not record the inputs to a memoization without also recording its output. This gives us a form of "resuming" within components and hooks. This only works when updating a component that already mounted. It has no impact during initial render, because the memo cache is stored on the fiber, and since we have not implemented resuming for fibers, it's always a fresh memo cache, anyway. However, this alone is pretty useful — it happens whenever you update the UI with fresh data after a mutation/action, which is extremely common in a Suspense-driven (e.g. RSC or Relay) app. So the impact of this feature is faster data mutations/actions (when the React Compiler is used).
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‎packages/react-reconciler/src/ReactFiberHooks.js‎

Lines changed: 27 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -46,6 +46,7 @@ import {
4646
enableAsyncActions,
4747
enableUseDeferredValueInitialArg,
4848
disableLegacyMode,
49+
enableNoCloningMemoCache,
4950
} from 'shared/ReactFeatureFlags';
5051
import {
5152
REACT_CONTEXT_TYPE,
@@ -1130,7 +1131,32 @@ function useMemoCache(size: number): Array<any> {
11301131
const currentMemoCache: ?MemoCache = currentUpdateQueue.memoCache;
11311132
if (currentMemoCache != null) {
11321133
memoCache = {
1133-
data: currentMemoCache.data.map(array => array.slice()),
1134+
// When enableNoCloningMemoCache is enabled, instead of treating the
1135+
// cache as copy-on-write, like we do with fibers, we share the same
1136+
// cache instance across all render attempts, even if the component
1137+
// is interrupted before it commits.
1138+
//
1139+
// If an update is interrupted, either because it suspended or
1140+
// because of another update, we can reuse the memoized computations
1141+
// from the previous attempt. We can do this because the React
1142+
// Compiler performs atomic writes to the memo cache, i.e. it will
1143+
// not record the inputs to a memoization without also recording its
1144+
// output.
1145+
//
1146+
// This gives us a form of "resuming" within components and hooks.
1147+
//
1148+
// This only works when updating a component that already mounted.
1149+
// It has no impact during initial render, because the memo cache is
1150+
// stored on the fiber, and since we have not implemented resuming
1151+
// for fibers, it's always a fresh memo cache, anyway.
1152+
//
1153+
// However, this alone is pretty useful — it happens whenever you
1154+
// update the UI with fresh data after a mutation/action, which is
1155+
// extremely common in a Suspense-driven (e.g. RSC or Relay) app.
1156+
data: enableNoCloningMemoCache
1157+
? currentMemoCache.data
1158+
: // Clone the memo cache before each render (copy-on-write)
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currentMemoCache.data.map(array => array.slice()),
11341160
index: 0,
11351161
};
11361162
}

‎packages/react-reconciler/src/__tests__/useMemoCache-test.js‎

Lines changed: 258 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -10,7 +10,9 @@
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1111
let React;
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let ReactNoop;
13+
let Scheduler;
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let act;
15+
let assertLog;
1416
let useState;
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let useMemoCache;
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let MemoCacheSentinel;
@@ -22,7 +24,9 @@ describe('useMemoCache()', () => {
2224

2325
React = require('react');
2426
ReactNoop = require('react-noop-renderer');
27+
Scheduler = require('scheduler');
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act = require('internal-test-utils').act;
29+
assertLog = require('internal-test-utils').assertLog;
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useState = React.useState;
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useMemoCache = React.unstable_useMemoCache;
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MemoCacheSentinel = Symbol.for('react.memo_cache_sentinel');
@@ -363,4 +367,258 @@ describe('useMemoCache()', () => {
363367
expect(Text).toBeCalledTimes(3);
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expect(data).toBe(data1); // confirm that the cache persisted across renders
365369
});
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// @gate enableUseMemoCacheHook
372+
test('reuses computations from suspended/interrupted render attempts during an update', async () => {
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// This test demonstrates the benefit of a shared memo cache. By "shared" I
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// mean multiple concurrent render attempts of the same component/hook use
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// the same cache. (When the feature flag is off, we don't do this — the
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// cache is copy-on-write.)
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//
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// If an update is interrupted, either because it suspended or because of
379+
// another update, we can reuse the memoized computations from the previous
380+
// attempt. We can do this because the React Compiler performs atomic writes
381+
// to the memo cache, i.e. it will not record the inputs to a memoization
382+
// without also recording its output.
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//
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// This gives us a form of "resuming" within components and hooks.
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//
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// This only works when updating a component that already mounted. It has no
387+
// impact during initial render, because the memo cache is stored on the
388+
// fiber, and since we have not implemented resuming for fibers, it's always
389+
// a fresh memo cache, anyway.
390+
//
391+
// However, this alone is pretty useful — it happens whenever you update the
392+
// UI with fresh data after a mutation/action, which is extremely common in
393+
// a Suspense-driven (e.g. RSC or Relay) app. That's the scenario that this
394+
// test simulates.
395+
//
396+
// So the impact of this feature is faster data mutations/actions.
397+
398+
function someExpensiveProcessing(t) {
399+
Scheduler.log(`Some expensive processing... [${t}]`);
400+
return t;
401+
}
402+
403+
function useWithLog(t, msg) {
404+
try {
405+
return React.use(t);
406+
} catch (x) {
407+
Scheduler.log(`Suspend! [${msg}]`);
408+
throw x;
409+
}
410+
}
411+
412+
// Original code:
413+
//
414+
// function Data({chunkA, chunkB}) {
415+
// const a = someExpensiveProcessing(useWithLog(chunkA, 'chunkA'));
416+
// const b = useWithLog(chunkB, 'chunkB');
417+
// return (
418+
// <>
419+
// {a}
420+
// {b}
421+
// </>
422+
// );
423+
// }
424+
//
425+
// function Input() {
426+
// const [input, _setText] = useState('');
427+
// return input;
428+
// }
429+
//
430+
// function App({chunkA, chunkB}) {
431+
// return (
432+
// <>
433+
// <div>
434+
// Input: <Input />
435+
// </div>
436+
// <div>
437+
// Data: <Data chunkA={chunkA} chunkB={chunkB} />
438+
// </div>
439+
// </>
440+
// );
441+
// }
442+
function Data(t0) {
443+
const $ = useMemoCache(5);
444+
const {chunkA, chunkB} = t0;
445+
const t1 = useWithLog(chunkA, 'chunkA');
446+
let t2;
447+
448+
if ($[0] !== t1) {
449+
t2 = someExpensiveProcessing(t1);
450+
$[0] = t1;
451+
$[1] = t2;
452+
} else {
453+
t2 = $[1];
454+
}
455+
456+
const a = t2;
457+
const b = useWithLog(chunkB, 'chunkB');
458+
let t3;
459+
460+
if ($[2] !== a || $[3] !== b) {
461+
t3 = (
462+
<>
463+
{a}
464+
{b}
465+
</>
466+
);
467+
$[2] = a;
468+
$[3] = b;
469+
$[4] = t3;
470+
} else {
471+
t3 = $[4];
472+
}
473+
474+
return t3;
475+
}
476+
477+
let setInput;
478+
function Input() {
479+
const [input, _set] = useState('');
480+
setInput = _set;
481+
return input;
482+
}
483+
484+
function App(t0) {
485+
const $ = useMemoCache(4);
486+
const {chunkA, chunkB} = t0;
487+
let t1;
488+
489+
if ($[0] === Symbol.for('react.memo_cache_sentinel')) {
490+
t1 = (
491+
<div>
492+
Input: <Input />
493+
</div>
494+
);
495+
$[0] = t1;
496+
} else {
497+
t1 = $[0];
498+
}
499+
500+
let t2;
501+
502+
if ($[1] !== chunkA || $[2] !== chunkB) {
503+
t2 = (
504+
<>
505+
{t1}
506+
<div>
507+
Data: <Data chunkA={chunkA} chunkB={chunkB} />
508+
</div>
509+
</>
510+
);
511+
$[1] = chunkA;
512+
$[2] = chunkB;
513+
$[3] = t2;
514+
} else {
515+
t2 = $[3];
516+
}
517+
518+
return t2;
519+
}
520+
521+
function createInstrumentedResolvedPromise(value) {
522+
return {
523+
then() {},
524+
status: 'fulfilled',
525+
value,
526+
};
527+
}
528+
529+
function createDeferred() {
530+
let resolve;
531+
const p = new Promise(res => {
532+
resolve = res;
533+
});
534+
p.resolve = resolve;
535+
return p;
536+
}
537+
538+
// Initial render. We pass the data in as two separate "chunks" to simulate
539+
// a stream (e.g. RSC).
540+
const root = ReactNoop.createRoot();
541+
const initialChunkA = createInstrumentedResolvedPromise('A1');
542+
const initialChunkB = createInstrumentedResolvedPromise('B1');
543+
await act(() =>
544+
root.render(<App chunkA={initialChunkA} chunkB={initialChunkB} />),
545+
);
546+
assertLog(['Some expensive processing... [A1]']);
547+
expect(root).toMatchRenderedOutput(
548+
<>
549+
<div>Input: </div>
550+
<div>Data: A1B1</div>
551+
</>,
552+
);
553+
554+
// Update the UI in a transition. This would happen after a data mutation.
555+
const updatedChunkA = createDeferred();
556+
const updatedChunkB = createDeferred();
557+
await act(() => {
558+
React.startTransition(() => {
559+
root.render(<App chunkA={updatedChunkA} chunkB={updatedChunkB} />);
560+
});
561+
});
562+
assertLog(['Suspend! [chunkA]']);
563+
564+
// The data starts to stream in. Loading the data in the first chunk
565+
// triggers an expensive computation in the UI. Later, we'll test whether
566+
// this computation is reused.
567+
await act(() => updatedChunkA.resolve('A2'));
568+
assertLog(['Some expensive processing... [A2]', 'Suspend! [chunkB]']);
569+
570+
// The second chunk hasn't loaded yet, so we're still showing the
571+
// initial UI.
572+
expect(root).toMatchRenderedOutput(
573+
<>
574+
<div>Input: </div>
575+
<div>Data: A1B1</div>
576+
</>,
577+
);
578+
579+
// While waiting for the data to finish loading, update a different part of
580+
// the screen. This interrupts the refresh transition.
581+
//
582+
// In a real app, this might be an input or hover event.
583+
await act(() => setInput('hi!'));
584+
585+
// Once the input has updated, we go back to rendering the transition.
586+
if (gate(flags => flags.enableNoCloningMemoCache)) {
587+
// We did not have process the first chunk again. We reused the
588+
// computation from the earlier attempt.
589+
assertLog(['Suspend! [chunkB]']);
590+
} else {
591+
// Because we clone/reset the memo cache after every aborted attempt, we
592+
// must process the first chunk again.
593+
assertLog(['Some expensive processing... [A2]', 'Suspend! [chunkB]']);
594+
}
595+
596+
expect(root).toMatchRenderedOutput(
597+
<>
598+
<div>Input: hi!</div>
599+
<div>Data: A1B1</div>
600+
</>,
601+
);
602+
603+
// Finish loading the data.
604+
await act(() => updatedChunkB.resolve('B2'));
605+
if (gate(flags => flags.enableNoCloningMemoCache)) {
606+
// We did not have process the first chunk again. We reused the
607+
// computation from the earlier attempt.
608+
assertLog([]);
609+
} else {
610+
// Because we clone/reset the memo cache after every aborted attempt, we
611+
// must process the first chunk again.
612+
//
613+
// That's three total times we've processed the first chunk, compared to
614+
// just once when enableNoCloningMemoCache is on.
615+
assertLog(['Some expensive processing... [A2]']);
616+
}
617+
expect(root).toMatchRenderedOutput(
618+
<>
619+
<div>Input: hi!</div>
620+
<div>Data: A2B2</div>
621+
</>,
622+
);
623+
});
366624
});

‎packages/shared/ReactFeatureFlags.js‎

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Original file line numberDiff line numberDiff line change
@@ -105,6 +105,8 @@ export const enableCPUSuspense = __EXPERIMENTAL__;
105105
// Enables unstable_useMemoCache hook, intended as a compilation target for
106106
// auto-memoization.
107107
export const enableUseMemoCacheHook = __EXPERIMENTAL__;
108+
// Test this at Meta before enabling.
109+
export const enableNoCloningMemoCache = false;
108110

109111
export const enableUseEffectEventHook = __EXPERIMENTAL__;
110112

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

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Original file line numberDiff line numberDiff line change
@@ -62,6 +62,7 @@ export const enableSuspenseAvoidThisFallback = false;
6262
export const enableSuspenseAvoidThisFallbackFizz = false;
6363
export const enableCPUSuspense = true;
6464
export const enableUseMemoCacheHook = true;
65+
export const enableNoCloningMemoCache = false;
6566
export const enableUseEffectEventHook = false;
6667
export const favorSafetyOverHydrationPerf = true;
6768
export const enableLegacyFBSupport = false;

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

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Original file line numberDiff line numberDiff line change
@@ -62,6 +62,7 @@ export const debugRenderPhaseSideEffectsForStrictMode = __DEV__;
6262

6363
// TODO: decide on React 19
6464
export const enableUseMemoCacheHook = false;
65+
export const enableNoCloningMemoCache = false;
6566
export const enableUseDeferredValueInitialArg = __EXPERIMENTAL__;
6667

6768
// -----------------------------------------------------------------------------

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

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Original file line numberDiff line numberDiff line change
@@ -37,6 +37,7 @@ export const enableSuspenseAvoidThisFallback = false;
3737
export const enableSuspenseAvoidThisFallbackFizz = false;
3838
export const enableCPUSuspense = false;
3939
export const enableUseMemoCacheHook = true;
40+
export const enableNoCloningMemoCache = false;
4041
export const enableUseEffectEventHook = false;
4142
export const favorSafetyOverHydrationPerf = true;
4243
export const enableComponentStackLocations = true;

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

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -42,6 +42,7 @@ export const enableSuspenseAvoidThisFallback = false;
4242
export const enableSuspenseAvoidThisFallbackFizz = false;
4343
export const enableCPUSuspense = true;
4444
export const enableUseMemoCacheHook = true;
45+
export const enableNoCloningMemoCache = false;
4546
export const enableUseEffectEventHook = false;
4647
export const favorSafetyOverHydrationPerf = true;
4748
export const enableInfiniteRenderLoopDetection = false;

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

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -38,6 +38,7 @@ export const enableSuspenseAvoidThisFallback = true;
3838
export const enableSuspenseAvoidThisFallbackFizz = false;
3939
export const enableCPUSuspense = false;
4040
export const enableUseMemoCacheHook = true;
41+
export const enableNoCloningMemoCache = false;
4142
export const enableUseEffectEventHook = false;
4243
export const favorSafetyOverHydrationPerf = true;
4344
export const enableComponentStackLocations = true;

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

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Original file line numberDiff line numberDiff line change
@@ -27,6 +27,7 @@ export const enableRefAsProp = __VARIANT__;
2727
export const enableRetryLaneExpiration = __VARIANT__;
2828
export const favorSafetyOverHydrationPerf = __VARIANT__;
2929
export const disableDefaultPropsExceptForClasses = __VARIANT__;
30+
export const enableNoCloningMemoCache = __VARIANT__;
3031
export const retryLaneExpirationMs = 5000;
3132
export const syncLaneExpirationMs = 250;
3233
export const transitionLaneExpirationMs = 5000;

‎packages/shared/forks/ReactFeatureFlags.www.js‎

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Original file line numberDiff line numberDiff line change
@@ -34,6 +34,7 @@ export const {
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enableRefAsProp,
3535
favorSafetyOverHydrationPerf,
3636
disableDefaultPropsExceptForClasses,
37+
enableNoCloningMemoCache,
3738
} = dynamicFeatureFlags;
3839

3940
// On WWW, __EXPERIMENTAL__ is used for a new modern build.

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