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feat: add support for tail calls #3020

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038cf8b
feat: add initial tail-call support behind feature flag
JairusSW Apr 24, 2026
9a03329
feat: add support for tail-calls on `call_indirect`
JairusSW Apr 24, 2026
5e0b2fe
tests: add coverage for `call_indirect`
JairusSW Apr 24, 2026
c33df89
chore: remove random formatting artifacts
JairusSW Apr 24, 2026
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89 changes: 73 additions & 16 deletions src/compiler.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -223,6 +223,8 @@ import {
lowerRequiresExportRuntime
} from "./bindings/js";

import * as binaryen from "./glue/binaryen";

/** Features enabled by default. */
export const defaultFeatures = Feature.MutableGlobals
| Feature.SignExtension
Expand Down Expand Up @@ -375,7 +377,9 @@ export const enum Constraints {
/** Indicates that static data is preferred. */
PreferStatic = 1 << 4,
/** Indicates that the value will become `this` of a property access or instance call. */
IsThis = 1 << 5
IsThis = 1 << 5,
/** Indicates that the expression is compiled for an immediate return position. */
WillReturn = 1 << 6
}

/** Runtime features to be activated by the compiler. */
Expand Down Expand Up @@ -2825,6 +2829,7 @@ export class Compiler extends DiagnosticEmitter {
if (valueExpression) {
let constraints = Constraints.ConvImplicit;
if (flow.sourceFunction.is(CommonFlags.ModuleExport)) constraints |= Constraints.MustWrap;
if (!flow.isInline && this.options.hasFeature(Feature.TailCalls)) constraints |= Constraints.WillReturn;

expr = this.compileExpression(valueExpression, returnType, constraints);
if (!flow.canOverflow(expr, returnType)) flow.set(FlowFlags.ReturnsWrapped);
Expand Down Expand Up @@ -2854,6 +2859,8 @@ export class Compiler extends DiagnosticEmitter {
: module.br(inlineReturnLabel);
}

if (expr && this.options.hasFeature(Feature.TailCalls) && this.isTailReturnCallExpression(expr)) return expr;

// Otherwise emit a normal return
return expr
? this.currentType == Type.void
Expand Down Expand Up @@ -6145,6 +6152,7 @@ export class Compiler extends DiagnosticEmitter {
Constraints.ConvImplicit | Constraints.IsThis
);
}
if (!this.canTailReturn(constraints, contextualType, functionInstance.signature.returnType)) constraints &= ~Constraints.WillReturn;
return this.compileCallDirect(
functionInstance,
expression.args,
Expand All @@ -6159,13 +6167,15 @@ export class Compiler extends DiagnosticEmitter {
let functionArg = this.compileExpression(expression.expression, Type.auto);
let signature = this.currentType.getSignature();
if (signature) {
if (!this.canTailReturn(constraints, contextualType, signature.returnType)) constraints &= ~Constraints.WillReturn;
return this.compileCallIndirect(
signature,
functionArg,
expression.args,
expression,
0,
contextualType == Type.void
contextualType == Type.void,
constraints
);
}
this.error(
Expand All @@ -6184,6 +6194,38 @@ export class Compiler extends DiagnosticEmitter {
return module.unreachable();
}

private canTailReturn(constraints: Constraints, contextType: Type, returnType: Type): bool {
if ((constraints & Constraints.WillReturn) == 0) return false;
// Tail calls are only valid as long as no result conversions are needed.
// `compileExpression` skips conversion for:
// 1. `currentType == nonnull<contextType>`, and
// 2. nullable reference identity (`T | null` -> `T | null`) where conversion is a no-op.
let sameWithoutNullable = returnType.equals(contextType.nonNullableType);
let sameNullableRef = returnType.isReference && returnType.equals(contextType);
if (!sameWithoutNullable && !sameNullableRef) return false;
if ((constraints & Constraints.MustWrap) == 0) return true;
switch (returnType.kind) {
case TypeKind.I8:
case TypeKind.I16:
case TypeKind.U8:
case TypeKind.U16:
return false;
default:
return true;
}
}

private isTailReturnCallExpression(expr: ExpressionRef): bool {
switch (getExpressionId(expr)) {
case ExpressionId.Call:
return binaryen._BinaryenCallIsReturn(expr);
case ExpressionId.CallIndirect:
return binaryen._BinaryenCallIndirectIsReturn(expr);
default:
return false;
}
}

/** Compiles the given arguments like a call expression according to the specified context. */
private compileCallExpressionLike(
/** Called expression. */
Expand Down Expand Up @@ -6439,7 +6481,7 @@ export class Compiler extends DiagnosticEmitter {
operands[index] = paramExpr;
}
assert(index == numArgumentsInclThis);
return this.makeCallDirect(instance, operands, reportNode, (constraints & Constraints.WillDrop) != 0);
return this.makeCallDirect(instance, operands, reportNode, (constraints & Constraints.WillDrop) != 0, (constraints & Constraints.WillReturn) != 0);
}

makeCallInline(
Expand Down Expand Up @@ -6883,7 +6925,8 @@ export class Compiler extends DiagnosticEmitter {
instance: Function,
operands: ExpressionRef[] | null,
reportNode: Node,
immediatelyDropped: bool = false
immediatelyDropped: bool = false,
isReturn: bool = false
): ExpressionRef {
if (instance.hasDecorator(DecoratorFlags.Inline)) {
if (!instance.is(CommonFlags.Overridden)) {
Expand Down Expand Up @@ -6982,8 +7025,10 @@ export class Compiler extends DiagnosticEmitter {
lastOperand
], lastOperandType.toRef());
this.operandsTostack(instance.signature, operands);
let expr = module.call(instance.internalName, operands, returnTypeRef);
if (returnType != Type.void && immediatelyDropped) {
let expr = isReturn
? module.return_call(instance.internalName, operands, returnTypeRef)
: module.call(instance.internalName, operands, returnTypeRef);
if (!isReturn && returnType != Type.void && immediatelyDropped) {
expr = module.drop(expr);
this.currentType = Type.void;
} else {
Expand All @@ -6999,7 +7044,9 @@ export class Compiler extends DiagnosticEmitter {
}

if (operands) this.operandsTostack(instance.signature, operands);
let expr = module.call(instance.internalName, operands, returnType.toRef());
let expr = isReturn
? module.return_call(instance.internalName, operands, returnType.toRef())
: module.call(instance.internalName, operands, returnType.toRef());
this.currentType = returnType;
return expr;
}
Expand All @@ -7011,7 +7058,8 @@ export class Compiler extends DiagnosticEmitter {
argumentExpressions: Expression[],
reportNode: Node,
thisArg: ExpressionRef = 0,
immediatelyDropped: bool = false
immediatelyDropped: bool = false,
constraints: Constraints = Constraints.None
): ExpressionRef {
let numArguments = argumentExpressions.length;

Expand Down Expand Up @@ -7041,7 +7089,7 @@ export class Compiler extends DiagnosticEmitter {
);
}
assert(index == numArgumentsInclThis);
return this.makeCallIndirect(signature, functionArg, reportNode, operands, immediatelyDropped);
return this.makeCallIndirect(signature, functionArg, reportNode, operands, immediatelyDropped, (constraints & Constraints.WillReturn) != 0);
}

/** Creates an indirect call to a first-class function. */
Expand All @@ -7051,6 +7099,7 @@ export class Compiler extends DiagnosticEmitter {
reportNode: Node,
operands: ExpressionRef[] | null = null,
immediatelyDropped: bool = false,
isReturn: bool = false,
): ExpressionRef {
let module = this.module;
let numOperands = operands ? operands.length : 0;
Expand Down Expand Up @@ -7101,13 +7150,21 @@ export class Compiler extends DiagnosticEmitter {
], sizeTypeRef);
}
if (operands) this.operandsTostack(signature, operands);
let expr = module.call_indirect(
null, // TODO: handle multiple tables
module.load(4, false, functionArg, TypeRef.I32), // ._index
operands,
signature.paramRefs,
signature.resultRefs
);
let expr = isReturn
? module.return_call_indirect(
null,
module.load(4, false, functionArg, TypeRef.I32),
operands,
signature.paramRefs,
signature.resultRefs
)
: module.call_indirect(
null,
module.load(4, false, functionArg, TypeRef.I32),
operands,
signature.paramRefs,
signature.resultRefs
);
this.currentType = returnType;
return expr;
}
Expand Down
21 changes: 11 additions & 10 deletions src/module.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1842,21 +1842,16 @@ export class Module {
index: ExpressionRef,
operands: ExpressionRef[] | null,
params: TypeRef,
results: TypeRef,
isReturn: bool = false
results: TypeRef
): ExpressionRef {
let cStr = this.allocStringCached(tableName != null
? tableName
: CommonNames.DefaultTable
);
let cArr = allocPtrArray(operands);
let ret = isReturn
? binaryen._BinaryenReturnCallIndirect(
this.ref, cStr, index, cArr, operands ? operands.length : 0, params, results
)
: binaryen._BinaryenCallIndirect(
this.ref, cStr, index, cArr, operands ? operands.length : 0, params, results
);
let ret = binaryen._BinaryenCallIndirect(
this.ref, cStr, index, cArr, operands ? operands.length : 0, params, results
);
binaryen._free(cArr);
return ret;
}
Expand All @@ -1868,7 +1863,13 @@ export class Module {
params: TypeRef,
results: TypeRef
): ExpressionRef {
return this.call_indirect(tableName, index, operands, params, results, true);
let cStr = this.allocStringCached(tableName != null ? tableName : CommonNames.DefaultTable);
let cArr = allocPtrArray(operands);
let ret = binaryen._BinaryenReturnCallIndirect(
this.ref, cStr, index, cArr, operands ? operands.length : 0, params, results
);
binaryen._free(cArr);
return ret;
}

unreachable(): ExpressionRef {
Expand Down
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