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compiler: Promote pruned scope declarations to temporaries if used in…
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… a later scope

There's a category of bug currently where pruned reactive scopes whose outputs are non-reactive can have their code end up inlining into another scope, moving the location of the instruction. Any value that had a scope assigned has to have its order of evaluation preserved, despite the fact that it got pruned, so naively we could just force every pruned scope to have its declarations promoted to named variables.

However, that ends up assigning names to _tons_ of scope declarations that don't really need to be promoted. For example, a scope with just a hook call ends up with:

```
const x = useFoo();

=>

scope {
 $t0 = Call read useFoo$ (...);
}
$t1 = StoreLocal 'x' = read $t0;
```

Where t0 doesn't need to be promoted since it's used immediately to assign to another value which is a non-temporary.

So the idea of this PR is that we can track outputs of pruned scopes which are directly referenced from inside a later scope. This fixes one of the two cases of the above pattern. We'll also likely have to consider values from pruned scopes as always reactive, i'll do that in the next PR.

ghstack-source-id: 2ef7fb05eb488188c5fad63f21d4968215e08bba
Pull Request resolved: #29789
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josephsavona committed Jun 6, 2024
1 parent 82631e0 commit 942c9b4
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Showing 4 changed files with 94 additions and 27 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,7 @@ import {
getHookKind,
makeIdentifierName,
} from "../HIR/HIR";
import { printPlace } from "../HIR/PrintHIR";
import { printIdentifier, printPlace } from "../HIR/PrintHIR";
import { eachPatternOperand } from "../HIR/visitors";
import { Err, Ok, Result } from "../Utils/Result";
import { GuardKind } from "../Utils/RuntimeDiagnosticConstants";
Expand Down Expand Up @@ -524,8 +524,8 @@ function codegenReactiveScope(
}

CompilerError.invariant(identifier.name != null, {
reason: `Expected identifier '@${identifier.id}' to be named`,
description: null,
reason: `Expected scope declaration identifier to be named`,
description: `Declaration \`${printIdentifier(identifier)}\` is unnamed in scope @${scope.id}`,
loc: null,
suggestions: null,
});
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -12,21 +12,20 @@ import {
IdentifierId,
InstructionId,
Place,
PrunedReactiveScopeBlock,
ReactiveFunction,
ReactiveScopeBlock,
ReactiveValue,
ScopeId,
promoteTemporary,
promoteTemporaryJsxTag,
} from "../HIR/HIR";
import { ReactiveFunctionVisitor, visitReactiveFunction } from "./visitors";

type VisitorState = {
tags: JsxExpressionTags;
};
class Visitor extends ReactiveFunctionVisitor<VisitorState> {
override visitScope(block: ReactiveScopeBlock, state: VisitorState): void {
this.traverseScope(block, state);
for (const dep of block.scope.dependencies) {
class Visitor extends ReactiveFunctionVisitor<State> {
override visitScope(scopeBlock: ReactiveScopeBlock, state: State): void {
this.traverseScope(scopeBlock, state);
for (const dep of scopeBlock.scope.dependencies) {
const { identifier } = dep;
if (identifier.name == null) {
promoteIdentifier(identifier, state);
Expand All @@ -39,14 +38,29 @@ class Visitor extends ReactiveFunctionVisitor<VisitorState> {
* Many of our current test fixtures do not return a value, so
* it is better for now to promote (and memoize) every output.
*/
for (const [, declaration] of block.scope.declarations) {
for (const [, declaration] of scopeBlock.scope.declarations) {
if (declaration.identifier.name == null) {
promoteIdentifier(declaration.identifier, state);
}
}
}

override visitParam(place: Place, state: VisitorState): void {
override visitPrunedScope(
scopeBlock: PrunedReactiveScopeBlock,
state: State
): void {
this.traversePrunedScope(scopeBlock, state);
for (const [, declaration] of scopeBlock.scope.declarations) {
if (
declaration.identifier.name == null &&
state.pruned.get(declaration.identifier.id)?.usedOutsideScope === true
) {
promoteIdentifier(declaration.identifier, state);
}
}
}

override visitParam(place: Place, state: State): void {
if (place.identifier.name === null) {
promoteIdentifier(place.identifier, state);
}
Expand All @@ -55,7 +69,7 @@ class Visitor extends ReactiveFunctionVisitor<VisitorState> {
override visitValue(
id: InstructionId,
value: ReactiveValue,
state: VisitorState
state: State
): void {
this.traverseValue(id, value, state);
if (value.kind === "FunctionExpression" || value.kind === "ObjectMethod") {
Expand All @@ -67,7 +81,7 @@ class Visitor extends ReactiveFunctionVisitor<VisitorState> {
_id: InstructionId,
_dependencies: Array<Place>,
fn: ReactiveFunction,
state: VisitorState
state: State
): void {
for (const operand of fn.params) {
const place = operand.kind === "Identifier" ? operand : operand.place;
Expand All @@ -80,25 +94,65 @@ class Visitor extends ReactiveFunctionVisitor<VisitorState> {
}

type JsxExpressionTags = Set<IdentifierId>;
class CollectJsxTagsVisitor extends ReactiveFunctionVisitor<JsxExpressionTags> {
type State = {
tags: JsxExpressionTags;
pruned: Map<
IdentifierId,
{ activeScopes: Array<ScopeId>; usedOutsideScope: boolean }
>; // true if referenced within another scope, false if only accessed outside of scopes
};

class CollectPromotableTemporaries extends ReactiveFunctionVisitor<State> {
activeScopes: Array<ScopeId> = [];

override visitPlace(_id: InstructionId, place: Place, state: State): void {
if (
this.activeScopes.length !== 0 &&
state.pruned.has(place.identifier.id)
) {
const prunedPlace = state.pruned.get(place.identifier.id)!;
if (prunedPlace.activeScopes.indexOf(this.activeScopes.at(-1)!) === -1) {
prunedPlace.usedOutsideScope = true;
}
}
}

override visitValue(
id: InstructionId,
value: ReactiveValue,
state: JsxExpressionTags
state: State
): void {
this.traverseValue(id, value, state);
if (value.kind === "JsxExpression" && value.tag.kind === "Identifier") {
state.add(value.tag.identifier.id);
state.tags.add(value.tag.identifier.id);
}
}

override visitPrunedScope(
scopeBlock: PrunedReactiveScopeBlock,
state: State
): void {
for (const [id] of scopeBlock.scope.declarations) {
state.pruned.set(id, {
activeScopes: [...this.activeScopes],
usedOutsideScope: false,
});
}
}

override visitScope(scopeBlock: ReactiveScopeBlock, state: State): void {
this.activeScopes.push(scopeBlock.scope.id);
this.traverseScope(scopeBlock, state);
this.activeScopes.pop();
}
}

export function promoteUsedTemporaries(fn: ReactiveFunction): void {
const tags: JsxExpressionTags = new Set();
visitReactiveFunction(fn, new CollectJsxTagsVisitor(), tags);
const state: VisitorState = {
tags,
const state: State = {
tags: new Set(),
pruned: new Map(),
};
visitReactiveFunction(fn, new CollectPromotableTemporaries(), state);
for (const operand of fn.params) {
const place = operand.kind === "Identifier" ? operand : operand.place;
if (place.identifier.name === null) {
Expand All @@ -108,7 +162,7 @@ export function promoteUsedTemporaries(fn: ReactiveFunction): void {
visitReactiveFunction(fn, new Visitor(), state);
}

function promoteIdentifier(identifier: Identifier, state: VisitorState): void {
function promoteIdentifier(identifier: Identifier, state: State): void {
CompilerError.invariant(identifier.name === null, {
reason:
"promoteTemporary: Expected to be called only for temporary variables",
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@ import {
isUseRefType,
makeInstructionId,
Place,
PrunedReactiveScopeBlock,
ReactiveFunction,
ReactiveInstruction,
ReactiveScope,
Expand Down Expand Up @@ -687,6 +688,17 @@ class PropagationVisitor extends ReactiveFunctionVisitor<Context> {
scope.scope.dependencies = scopeDependencies;
}

override visitPrunedScope(
scopeBlock: PrunedReactiveScopeBlock,
context: Context
): void {
// NOTE: we explicitly throw away the deps, we only enter() the scope to record its
// declarations
const _scopeDepdencies = context.enter(scopeBlock.scope, () => {
this.visitBlock(scopeBlock.instructions, context);
});
}

override visitInstruction(
instruction: ReactiveInstruction,
context: Context
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -71,14 +71,15 @@ function Foo() {
useNoAlias();

const shouldCaptureObj = obj != null && CONST_TRUE;
let t0;
const t0 = shouldCaptureObj ? identity(obj) : null;
let t1;
if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
t0 = [shouldCaptureObj ? identity(obj) : null, obj];
$[0] = t0;
t1 = [t0, obj];
$[0] = t1;
} else {
t0 = $[0];
t1 = $[0];
}
const result = t0;
const result = t1;

useNoAlias(result, obj);
if (shouldCaptureObj && result[0] !== obj) {
Expand Down

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