feat: improve tree-shaking by propagate const parameter

src/ast/nodes/ConditionalExpression.ts

@@ -1,5 +1,5 @@
 import type MagicString from 'magic-string';
-import { BLANK, EMPTY_ARRAY } from '../../utils/blank';
+import { BLANK } from '../../utils/blank';
 import type { NodeRenderOptions, RenderOptions } from '../../utils/renderHelpers';
 import {
 	findFirstOccurrenceOutsideComment,
@@ -46,15 +46,16 @@ export default class ConditionalExpression extends NodeBase implements Deoptimiz
 	}
 
 	deoptimizeCache(): void {
+		this.isBranchResolutionAnalysed = false;
+		const { expressionsToBeDeoptimized } = this;
+		this.expressionsToBeDeoptimized = [];
+		for (const expression of expressionsToBeDeoptimized) {
+			expression.deoptimizeCache();
+		}
 		if (this.usedBranch !== null) {
 			const unusedBranch = this.usedBranch === this.consequent ? this.alternate : this.consequent;
 			this.usedBranch = null;
 			unusedBranch.deoptimizePath(UNKNOWN_PATH);
-			const { expressionsToBeDeoptimized } = this;
-			this.expressionsToBeDeoptimized = EMPTY_ARRAY as unknown as DeoptimizableEntity[];
-			for (const expression of expressionsToBeDeoptimized) {
-				expression.deoptimizeCache();
-			}
 		}
 	}
 
@@ -73,9 +74,9 @@ export default class ConditionalExpression extends NodeBase implements Deoptimiz
 		recursionTracker: PathTracker,
 		origin: DeoptimizableEntity
 	): LiteralValueOrUnknown {
+		this.expressionsToBeDeoptimized.push(origin);
 		const usedBranch = this.getUsedBranch();
 		if (!usedBranch) return UnknownValue;
-		this.expressionsToBeDeoptimized.push(origin);
 		return usedBranch.getLiteralValueAtPath(path, recursionTracker, origin);
 	}
 

src/ast/nodes/FunctionDeclaration.ts

@@ -14,6 +14,11 @@ export default class FunctionDeclaration extends FunctionNode {
 		}
 	}
 
+	protected onlyFunctionCallUsed(): boolean {
+		// call super.onlyFunctionCallUsed for export default anonymous function
+		return this.id?.variable.getOnlyFunctionCallUsed() ?? super.onlyFunctionCallUsed();
+	}
+
 	parseNode(esTreeNode: GenericEsTreeNode): this {
 		if (esTreeNode.id !== null) {
 			this.id = new Identifier(this, this.scope.parent as ChildScope).parseNode(

src/ast/nodes/Identifier.ts

@@ -60,10 +60,12 @@ export default class Identifier extends NodeBase implements PatternNode {
 		}
 	}
 
+	private isReferenceVariable = false;
 	bind(): void {
 		if (!this.variable && isReference(this, this.parent as NodeWithFieldDefinition)) {
 			this.variable = this.scope.findVariable(this.name);
 			this.variable.addReference(this);
+			this.isReferenceVariable = true;
 		}
 	}
 
@@ -295,6 +297,9 @@ export default class Identifier extends NodeBase implements PatternNode {
 			this.variable.consolidateInitializers();
 			this.scope.context.requestTreeshakingPass();
 		}
+		if (this.isReferenceVariable) {
+			this.variable!.addUsedPlace(this);
+		}
 	}
 
 	private getVariableRespectingTDZ(): ExpressionEntity | null {

src/ast/nodes/IfStatement.ts

@@ -7,7 +7,7 @@ import { EMPTY_PATH, SHARED_RECURSION_TRACKER } from '../utils/PathTracker';
 import BlockStatement from './BlockStatement';
 import type Identifier from './Identifier';
 import * as NodeType from './NodeType';
-import { type LiteralValueOrUnknown, UnknownValue } from './shared/Expression';
+import { type LiteralValueOrUnknown } from './shared/Expression';
 import {
 	type ExpressionNode,
 	type GenericEsTreeNode,
@@ -29,7 +29,7 @@ export default class IfStatement extends StatementBase implements DeoptimizableE
 	private testValue: LiteralValueOrUnknown | typeof unset = unset;
 
 	deoptimizeCache(): void {
-		this.testValue = UnknownValue;
+		this.testValue = unset;
 	}
 
 	hasEffects(context: HasEffectsContext): boolean {

src/ast/nodes/LogicalExpression.ts

@@ -1,5 +1,5 @@
 import type MagicString from 'magic-string';
-import { BLANK, EMPTY_ARRAY } from '../../utils/blank';
+import { BLANK } from '../../utils/blank';
 import {
 	findFirstOccurrenceOutsideComment,
 	findNonWhiteSpace,
@@ -57,22 +57,25 @@ export default class LogicalExpression extends NodeBase implements Deoptimizable
 	}
 
 	deoptimizeCache(): void {
-		if (this.usedBranch) {
-			const unusedBranch = this.usedBranch === this.left ? this.right : this.left;
-			this.usedBranch = null;
-			unusedBranch.deoptimizePath(UNKNOWN_PATH);
+		this.isBranchResolutionAnalysed = false;
+		if (this.expressionsToBeDeoptimized.length > 0) {
 			const {
 				scope: { context },
 				expressionsToBeDeoptimized
 			} = this;
-			this.expressionsToBeDeoptimized = EMPTY_ARRAY as unknown as DeoptimizableEntity[];
+			this.expressionsToBeDeoptimized = [];
 			for (const expression of expressionsToBeDeoptimized) {
 				expression.deoptimizeCache();
 			}
 			// Request another pass because we need to ensure "include" runs again if
 			// it is rendered
 			context.requestTreeshakingPass();
 		}
+		if (this.usedBranch) {
+			const unusedBranch = this.usedBranch === this.left ? this.right : this.left;
+			this.usedBranch = null;
+			unusedBranch.deoptimizePath(UNKNOWN_PATH);
+		}
 	}
 
 	deoptimizePath(path: ObjectPath): void {
@@ -90,9 +93,9 @@ export default class LogicalExpression extends NodeBase implements Deoptimizable
 		recursionTracker: PathTracker,
 		origin: DeoptimizableEntity
 	): LiteralValueOrUnknown {
+		this.expressionsToBeDeoptimized.push(origin);
 		const usedBranch = this.getUsedBranch();
 		if (!usedBranch) return UnknownValue;
-		this.expressionsToBeDeoptimized.push(origin);
 		return usedBranch.getLiteralValueAtPath(path, recursionTracker, origin);
 	}
 

src/ast/nodes/MemberExpression.ts

@@ -1,7 +1,7 @@
 import type MagicString from 'magic-string';
 import type { AstContext } from '../../Module';
 import type { NormalizedTreeshakingOptions } from '../../rollup/types';
-import { BLANK, EMPTY_ARRAY } from '../../utils/blank';
+import { BLANK } from '../../utils/blank';
 import { LOGLEVEL_WARN } from '../../utils/logging';
 import { logIllegalImportReassignment, logMissingExport } from '../../utils/logs';
 import type { NodeRenderOptions, RenderOptions } from '../../utils/renderHelpers';
@@ -185,7 +185,7 @@ export default class MemberExpression
 
 	deoptimizeCache(): void {
 		const { expressionsToBeDeoptimized, object } = this;
-		this.expressionsToBeDeoptimized = EMPTY_ARRAY as unknown as DeoptimizableEntity[];
+		this.expressionsToBeDeoptimized = [];
 		this.propertyKey = UnknownKey;
 		object.deoptimizePath(UNKNOWN_PATH);
 		for (const expression of expressionsToBeDeoptimized) {
@@ -396,6 +396,9 @@ export default class MemberExpression
 			);
 			this.scope.context.requestTreeshakingPass();
 		}
+		if (this.variable) {
+			this.variable.addUsedPlace(this);
+		}
 	}
 
 	private applyAssignmentDeoptimization(): void {

src/ast/nodes/UpdateExpression.ts

@@ -87,7 +87,7 @@ export default class UpdateExpression extends NodeBase {
 		this.argument.deoptimizePath(EMPTY_PATH);
 		if (this.argument instanceof Identifier) {
 			const variable = this.scope.findVariable(this.argument.name);
-			variable.isReassigned = true;
+			variable.markReassigned();
 		}
 		this.scope.context.requestTreeshakingPass();
 	}

src/ast/nodes/shared/FunctionBase.ts

@@ -9,12 +9,23 @@ import {
 } from '../../NodeInteractions';
 import type ReturnValueScope from '../../scopes/ReturnValueScope';
 import type { ObjectPath, PathTracker } from '../../utils/PathTracker';
-import { UNKNOWN_PATH, UnknownKey } from '../../utils/PathTracker';
+import {
+	EMPTY_PATH,
+	SHARED_RECURSION_TRACKER,
+	UNKNOWN_PATH,
+	UnknownKey
+} from '../../utils/PathTracker';
+import { UNDEFINED_EXPRESSION } from '../../values';
 import type ParameterVariable from '../../variables/ParameterVariable';
+import type Variable from '../../variables/Variable';
 import BlockStatement from '../BlockStatement';
+import type CallExpression from '../CallExpression';
+import type ExportDefaultDeclaration from '../ExportDefaultDeclaration';
 import Identifier from '../Identifier';
+import * as NodeType from '../NodeType';
 import RestElement from '../RestElement';
 import type SpreadElement from '../SpreadElement';
+import type VariableDeclarator from '../VariableDeclarator';
 import { Flag, isFlagSet, setFlag } from './BitFlags';
 import type { ExpressionEntity, LiteralValueOrUnknown } from './Expression';
 import { UNKNOWN_EXPRESSION, UNKNOWN_RETURN_EXPRESSION } from './Expression';
@@ -27,6 +38,13 @@ import {
 import type { ObjectEntity } from './ObjectEntity';
 import type { PatternNode } from './Pattern';
 
+type InteractionCalledArguments = NodeInteractionCalled['args'];
+
+// This handler does nothing.
+// Since we always re-evaluate argument values in a new tree-shaking pass,
+// we don't need to get notified if it is deoptimized.
+const EMPTY_DEOPTIMIZABLE_HANDLER = { deoptimizeCache() {} };
+
 export default abstract class FunctionBase extends NodeBase {
 	declare body: BlockStatement | ExpressionNode;
 	declare params: PatternNode[];
@@ -57,6 +75,107 @@ export default abstract class FunctionBase extends NodeBase {
 		this.flags = setFlag(this.flags, Flag.generator, value);
 	}
 
+	private knownParameterValues: (ExpressionEntity | undefined)[] = [];
+	private allArguments: InteractionCalledArguments[] = [];
+	/**
+	 * update knownParameterValues when a call is made to this function
+	 * @param newArguments arguments of the call
+	 */
+	private updateKnownParameterValues(newArguments: InteractionCalledArguments): void {
+		for (let position = 0; position < this.params.length; position++) {
+			// only the "this" argument newArguments[0] can be null
+			// it's possible that some arguments are empty, so the value is undefined
+			const argument = newArguments[position + 1] ?? UNDEFINED_EXPRESSION;
+			const parameter = this.params[position];
+			// RestElement can be, and can only be, the last parameter
+			if (parameter instanceof RestElement) {
+				return;
+			}
+
+			const knownParameterValue = this.knownParameterValues[position];
+			if (knownParameterValue === undefined) {
+				this.knownParameterValues[position] = argument;
+				continue;
+			}
+			if (
+				knownParameterValue === UNKNOWN_EXPRESSION ||
+				knownParameterValue === argument ||
+				(knownParameterValue instanceof Identifier &&
+					argument instanceof Identifier &&
+					knownParameterValue.variable === argument.variable)
+			) {
+				continue;
+			}
+
+			const oldValue = knownParameterValue.getLiteralValueAtPath(
+				EMPTY_PATH,
+				SHARED_RECURSION_TRACKER,
+				EMPTY_DEOPTIMIZABLE_HANDLER
+			);
+			const newValue = argument.getLiteralValueAtPath(
+				EMPTY_PATH,
+				SHARED_RECURSION_TRACKER,
+				EMPTY_DEOPTIMIZABLE_HANDLER
+			);
+			if (oldValue !== newValue || typeof oldValue === 'symbol') {
+				this.knownParameterValues[position] = UNKNOWN_EXPRESSION;
+			} // else both are the same literal, no need to update
+		}
+	}
+
+	private forwardArgumentsForFunctionCalledOnce(newArguments: InteractionCalledArguments): void {
+		for (let position = 0; position < this.params.length; position++) {
+			const parameter = this.params[position];
+			if (parameter instanceof Identifier) {
+				const ParameterVariable = parameter.variable as ParameterVariable | null;
+				const argument = newArguments[position + 1] ?? UNDEFINED_EXPRESSION;
+				ParameterVariable?.setKnownValue(argument);
+			}
+		}
+	}
+
+	/**
+	 * each time tree-shake starts, this method should be called to reoptimize the parameters
+	 * a parameter's state will change at most twice:
+	 *   `undefined` (no call is made) -> an expression -> `UnknownArgument`
+	 * we are sure it will converge, and can use state from last iteration
+	 */
+	private applyFunctionParameterOptimization() {
+		if (this.allArguments.length === 0) {
+			return;
+		}
+
+		if (this.allArguments.length === 1) {
+			// we are sure what knownParameterValues will be, so skip it and do setKnownValue
+			this.forwardArgumentsForFunctionCalledOnce(this.allArguments[0]);
+			return;
+		}
+
+		// reoptimize all arguments, that's why we save them
+		for (const argumentsList of this.allArguments) {
+			this.updateKnownParameterValues(argumentsList);
+		}
+		for (let position = 0; position < this.params.length; position++) {
+			const parameter = this.params[position];
+			// Parameters without default values
+			if (parameter instanceof Identifier) {
+				const parameterVariable = parameter.variable as ParameterVariable | null;
+				// Only the RestElement may be undefined
+				const knownParameterValue = this.knownParameterValues[position]!;
+				parameterVariable?.setKnownValue(knownParameterValue);
+			}
+		}
+	}
+
+	private deoptimizeFunctionParameters() {
+		for (const parameter of this.params) {
+			if (parameter instanceof Identifier) {
+				const parameterVariable = parameter.variable as ParameterVariable | null;
+				parameterVariable?.markReassigned();
+			}
+		}
+	}
+
 	protected objectEntity: ObjectEntity | null = null;
 
 	deoptimizeArgumentsOnInteractionAtPath(
@@ -84,6 +203,7 @@ export default abstract class FunctionBase extends NodeBase {
 					this.addArgumentToBeDeoptimized(argument);
 				}
 			}
+			this.allArguments.push(args);
 		} else {
 			this.getObjectEntity().deoptimizeArgumentsOnInteractionAtPath(
 				interaction,
@@ -102,6 +222,7 @@ export default abstract class FunctionBase extends NodeBase {
 			for (const parameterList of this.scope.parameters) {
 				for (const parameter of parameterList) {
 					parameter.deoptimizePath(UNKNOWN_PATH);
+					parameter.markReassigned();
 				}
 			}
 		}
@@ -180,7 +301,33 @@ export default abstract class FunctionBase extends NodeBase {
 		return false;
 	}
 
+	/**
+	 * If the function (expression or declaration) is only used as function calls
+	 */
+	protected onlyFunctionCallUsed(): boolean {
+		let variable: Variable | null = null;
+		if (this.parent.type === NodeType.VariableDeclarator) {
+			variable = (this.parent as VariableDeclarator).id.variable ?? null;
+		}
+		if (this.parent.type === NodeType.ExportDefaultDeclaration) {
+			variable = (this.parent as ExportDefaultDeclaration).variable;
+		}
+		return variable?.getOnlyFunctionCallUsed() ?? false;
+	}
+
+	private functionParametersOptimized = false;
 	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren): void {
+		const isIIFE =
+			this.parent.type === NodeType.CallExpression &&
+			(this.parent as CallExpression).callee === this;
+		const shoulOptimizeFunctionParameters = isIIFE || this.onlyFunctionCallUsed();
+		if (shoulOptimizeFunctionParameters) {
+			this.applyFunctionParameterOptimization();
+		} else if (this.functionParametersOptimized) {
+			this.deoptimizeFunctionParameters();
+		}
+		this.functionParametersOptimized = shoulOptimizeFunctionParameters;
+
 		if (!this.deoptimized) this.applyDeoptimizations();
 		this.included = true;
 		const { brokenFlow } = context;

src/ast/variables/ExportDefaultVariable.ts

@@ -3,6 +3,7 @@ import ClassDeclaration from '../nodes/ClassDeclaration';
 import type ExportDefaultDeclaration from '../nodes/ExportDefaultDeclaration';
 import FunctionDeclaration from '../nodes/FunctionDeclaration';
 import Identifier, { type IdentifierWithVariable } from '../nodes/Identifier';
+import type { NodeBase } from '../nodes/shared/Node';
 import LocalVariable from './LocalVariable';
 import UndefinedVariable from './UndefinedVariable';
 import type Variable from './Variable';
@@ -37,6 +38,15 @@ export default class ExportDefaultVariable extends LocalVariable {
 		}
 	}
 
+	addUsedPlace(usedPlace: NodeBase): void {
+		const original = this.getOriginalVariable();
+		if (original === this) {
+			super.addUsedPlace(usedPlace);
+		} else {
+			original.addUsedPlace(usedPlace);
+		}
+	}
+
 	forbidName(name: string) {
 		const original = this.getOriginalVariable();
 		if (original === this) {