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[compiler] enablePreserveExistingMemo memoizes primitive-returning functions (#34343)
`@enablePreserveExistingMemoizationGuarantees` mode currently does not guarantee memoization of primitive-returning functions. We're often able to infer that a function returns a primitive based on how its result is used, for example `foo() + 1` or `object[getIndex()]`, and by default we do not currently memoize computation that produces a primitive. The reasoning behind this is that the compiler is primarily focused on stopping cascading updates — it's fine to recompute a primitive since we can cheaply compare that primitive and avoid unnecessary downstream recomputation. But we've gotten a lot of feedback that people find this surprising, and that sometimes the computation can be expensive enough that it should be memoized. This PR changes `@enablePreserveExistingMemoizationGuarantees` mode to ensure that primitive-returning functions get memoized. Other modes will not memoize these functions. Separately from this we are considering enabling this mode by default. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34343). * #34347 * #34346 * __->__ #34343 * #34335
Joseph Savona committed
Sep 3, 2025 at 17:45 UTC
735e9ac54e5b46e5a963e0e436330baf20fb04c2
7 files changed
+376
-4
compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/PruneNonEscapingScopes.ts
+20
-4
@@ -411,7 +411,9 @@ class CollectDependenciesVisitor extends ReactiveFunctionVisitor<
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this.state = state;
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this.options = {
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memoizeJsxElements: !this.env.config.enableForest,
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- forceMemoizePrimitives: this.env.config.enableForest,
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+ forceMemoizePrimitives:
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+ this.env.config.enableForest ||
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+ this.env.config.enablePreserveExistingMemoizationGuarantees,
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};
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}
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@@ -534,9 +536,23 @@ class CollectDependenciesVisitor extends ReactiveFunctionVisitor<
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case 'JSXText':
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case 'BinaryExpression':
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case 'UnaryExpression': {
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- const level = options.forceMemoizePrimitives
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- ? MemoizationLevel.Memoized
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- : MemoizationLevel.Never;
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+ if (options.forceMemoizePrimitives) {
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+ /**
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+ * Because these instructions produce primitives we usually don't consider
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+ * them as escape points: they are known to copy, not return references.
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+ * However if we're forcing memoization of primitives then we mark these
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+ * instructions as needing memoization and walk their rvalues to ensure
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+ * any scopes transitively reachable from the rvalues are considered for
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+ * memoization. Note: we may still prune primitive-producing scopes if
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+ * they don't ultimately escape at all.
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+ */
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+ const level = MemoizationLevel.Memoized;
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+ return {
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+ lvalues: lvalue !== null ? [{place: lvalue, level}] : [],
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+ rvalues: [...eachReactiveValueOperand(value)],
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+ };
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+ }
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+ const level = MemoizationLevel.Never;
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return {
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// All of these instructions return a primitive value and never need to be memoized
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lvalues: lvalue !== null ? [{place: lvalue, level}] : [],
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping-useMemo.expect.md
new
+77
@@ -0,0 +1,77 @@
1
+
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+## Input
3
+
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+```javascript
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+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
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+import {useMemo} from 'react';
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+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
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+
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+function Component(props) {
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+ const result = useMemo(
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+ () => makeObject(props.value).value + 1,
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+ [props.value]
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+ );
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+ console.log(result);
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+ return 'ok';
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return {value};
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+}
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+
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+export const TODO_FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{value: 42}],
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+ sequentialRenders: [
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+ {value: 42},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ ],
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+};
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+
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+```
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+
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+## Code
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+
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+```javascript
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+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
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+import { useMemo } from "react";
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+import { makeObject_Primitives, ValidateMemoization } from "shared-runtime";
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+
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+function Component(props) {
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+ const result = makeObject(props.value).value + 1;
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+
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+ console.log(result);
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+ return "ok";
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return { value };
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+}
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+
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+export const TODO_FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{ value: 42 }],
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+ sequentialRenders: [
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+ { value: 42 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ { value: 3.14 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ ],
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+};
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+
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+```
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+
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+### Eval output
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+(kind: exception) Fixture not implemented
\ No newline at end of file
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping-useMemo.js
new
+32
@@ -0,0 +1,32 @@
1
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
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+import {useMemo} from 'react';
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+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
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+
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+function Component(props) {
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+ const result = useMemo(
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+ () => makeObject(props.value).value + 1,
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+ [props.value]
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+ );
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+ console.log(result);
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+ return 'ok';
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return {value};
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+}
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+
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+export const TODO_FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{value: 42}],
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+ sequentialRenders: [
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+ {value: 42},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ ],
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+};
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping.expect.md
new
+81
@@ -0,0 +1,81 @@
1
+
2
+## Input
3
+
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+```javascript
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+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
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+import {useMemo} from 'react';
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+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
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+
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+function Component(props) {
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+ const result = makeObject(props.value).value + 1;
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+ console.log(result);
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+ return 'ok';
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return {value};
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+}
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+
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+export const FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{value: 42}],
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+ sequentialRenders: [
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+ {value: 42},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ ],
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+};
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+
35
+```
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+
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+## Code
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+
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+```javascript
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+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
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+import { useMemo } from "react";
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+import { makeObject_Primitives, ValidateMemoization } from "shared-runtime";
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+
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+function Component(props) {
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+ const result = makeObject(props.value).value + 1;
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+ console.log(result);
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+ return "ok";
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return { value };
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+}
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+
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+export const FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{ value: 42 }],
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+ sequentialRenders: [
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+ { value: 42 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ { value: 3.14 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ ],
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+};
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+
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+```
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+
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+### Eval output
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+(kind: ok) "ok"
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+"ok"
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+"ok"
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+"ok"
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+"ok"
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+"ok"
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+"ok"
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+"ok"
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+logs: [42,43,42,43,3.14,4.140000000000001,3.14,4.140000000000001,42,43,3.14,4.140000000000001,42,43,3.14,4.140000000000001]
\ No newline at end of file
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/dont-memoize-primitive-function-call-non-escaping.js
new
+29
@@ -0,0 +1,29 @@
1
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
2
+import {useMemo} from 'react';
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+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
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+
5
+function Component(props) {
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+ const result = makeObject(props.value).value + 1;
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+ console.log(result);
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+ return 'ok';
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return {value};
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+}
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+
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+export const FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{value: 42}],
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+ sequentialRenders: [
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+ {value: 42},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ ],
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+};
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/memoize-primitive-function-calls.expect.md
new
+107
@@ -0,0 +1,107 @@
1
+
2
+## Input
3
+
4
+```javascript
5
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
6
+import {useMemo} from 'react';
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+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
8
+
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+function Component(props) {
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+ const result = useMemo(() => {
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+ return makeObject(props.value).value + 1;
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+ }, [props.value]);
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+ return <ValidateMemoization inputs={[props.value]} output={result} />;
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return {value};
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+}
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+
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+export const FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{value: 42}],
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+ sequentialRenders: [
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+ {value: 42},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ ],
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+};
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+
36
+```
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+
38
+## Code
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+
40
+```javascript
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+import { c as _c } from "react/compiler-runtime"; // @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
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+import { useMemo } from "react";
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+import { makeObject_Primitives, ValidateMemoization } from "shared-runtime";
44
+
45
+function Component(props) {
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+ const $ = _c(7);
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+ let t0;
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+ if ($[0] !== props.value) {
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+ t0 = makeObject(props.value);
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+ $[0] = props.value;
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+ $[1] = t0;
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+ } else {
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+ t0 = $[1];
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+ }
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+ const result = t0.value + 1;
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+ let t1;
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+ if ($[2] !== props.value) {
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+ t1 = [props.value];
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+ $[2] = props.value;
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+ $[3] = t1;
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+ } else {
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+ t1 = $[3];
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+ }
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+ let t2;
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+ if ($[4] !== result || $[5] !== t1) {
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+ t2 = <ValidateMemoization inputs={t1} output={result} />;
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+ $[4] = result;
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+ $[5] = t1;
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+ $[6] = t2;
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+ } else {
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+ t2 = $[6];
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+ }
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+ return t2;
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+}
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+
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+function makeObject(value) {
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+ console.log(value);
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+ return { value };
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+}
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+
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+export const FIXTURE_ENTRYPOINT = {
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+ fn: Component,
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+ params: [{ value: 42 }],
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+ sequentialRenders: [
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+ { value: 42 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ { value: 3.14 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ { value: 42 },
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+ { value: 3.14 },
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+ ],
94
+};
95
+
96
+```
97
+
98
+### Eval output
99
+(kind: ok) <div>{"inputs":[42],"output":43}</div>
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+<div>{"inputs":[42],"output":43}</div>
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+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
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+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
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+<div>{"inputs":[42],"output":43}</div>
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+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
105
+<div>{"inputs":[42],"output":43}</div>
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+<div>{"inputs":[3.14],"output":4.140000000000001}</div>
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+logs: [42,3.14,42,3.14,42,3.14]
\ No newline at end of file
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/memoize-primitive-function-calls.js
new
+30
@@ -0,0 +1,30 @@
1
+// @compilationMode:"infer" @enablePreserveExistingMemoizationGuarantees @validatePreserveExistingMemoizationGuarantees
2
+import {useMemo} from 'react';
3
+import {makeObject_Primitives, ValidateMemoization} from 'shared-runtime';
4
+
5
+function Component(props) {
6
+ const result = useMemo(() => {
7
+ return makeObject(props.value).value + 1;
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+ }, [props.value]);
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+ return <ValidateMemoization inputs={[props.value]} output={result} />;
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+}
11
+
12
+function makeObject(value) {
13
+ console.log(value);
14
+ return {value};
15
+}
16
+
17
+export const FIXTURE_ENTRYPOINT = {
18
+ fn: Component,
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+ params: [{value: 42}],
20
+ sequentialRenders: [
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+ {value: 42},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 3.14},
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+ {value: 42},
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+ {value: 3.14},
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+ {value: 42},
28
+ {value: 3.14},
29
+ ],
30
+};