[dart2wasm] Explicitly model functions that do not return

Similar to [0] which recognizes functions that return always nulls, we
recognize functions that never return and

* make the wasm function have no outputs
* make call sites emit `unreachable()` after the call (to inform
  binaryen & wasm runtime that this is unreachable)

Before we had an artificial construct where we made such functions
have a `w.RefType(HeapType.none, nullable: false)` return type (i.e.
bottom, i.e. no values) and encoded that way it's unreachable.

We also change some exported wasm functions to explicitly use wasm
types in the signature (namely `WasmVoid` instead of Dart `void`).

[0] https://dart-review.googlesource.com/c/sdk/+/497620

Change-Id: I3724e777cda23c0cf2c8a7dd2e473f3fef0a4f54
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/499240
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>

pkg/dart2wasm/lib/code_generator.dart

@@ -2077,6 +2077,11 @@ abstract class AstCodeGenerator
       b.ref_null(w.HeapType.none);
       return w.RefType(w.HeapType.none, nullable: true);
     }
+    if (selector.synthesizeNoReturn) {
+      assert(selector.signature.outputs.isEmpty);
+      b.unreachable();
+      return voidMarker;
+    }
 
     return translator.outputOrVoid(signature.outputs);
   }
@@ -6099,6 +6104,9 @@ abstract class CallTarget {
   /// Whether callers should synthesize a `null` return value.
   bool get synthesizeNullReturnValue => false;
 
+  /// Whether callee never returns and callers should emit `unreachable`.
+  bool get synthesizeNoReturn => false;
+
   /// Whether this call target supports inlining.
   bool get supportsInlining => false;
 
@@ -6133,6 +6141,9 @@ class AstCallTarget extends CallTarget {
   bool get synthesizeNullReturnValue =>
       _translator.synthesizeNullReturnValue(_reference);
 
+  @override
+  bool get synthesizeNoReturn => _translator.synthesizeNoReturn(_reference);
+
   @override
   String get name => _translator.functions.getFunctionName(_reference);
 

pkg/dart2wasm/lib/dispatch_table.dart

@@ -67,6 +67,12 @@ class SelectorInfo {
   /// Will be set during `_computeSignature`.
   late final bool synthesizeNullReturnValue;
 
+  /// Whether the call will never return and callers can emit an
+  /// `unreachable()` after the call.
+  ///
+  /// Will be set during `_computeSignature`.
+  late final bool synthesizeNoReturn;
+
   /// The selector's member's name.
   final String name;
 
@@ -210,14 +216,18 @@ class SelectorInfo {
       outputSets.length,
       (i) => _upperBound(outputSets[i], ensureBoxed: false),
     );
-    if (outputs case [w.RefType(heapType: w.HeapType.none, nullable: true)]) {
-      // All functions are guaranteed to return null.
-      // Will prune the signature and make call sites synthesize `null` if
-      // needed.
+    if (outputs case [
+      w.RefType(heapType: w.HeapType.none, nullable: final nullable),
+    ]) {
+      // All functions are guaranteed to return null or are unreachable.
+      // => Prune signature to not return anything
+      // => Tell callers to synthesize `null` or emit `unreachable`.
       outputs.clear();
-      synthesizeNullReturnValue = true;
+      synthesizeNullReturnValue = nullable;
+      synthesizeNoReturn = !nullable;
     } else {
       synthesizeNullReturnValue = isSetterOrIndexSetter;
+      synthesizeNoReturn = false;
     }
     return translator.typesBuilder.defineFunction([
       inputs[0],

pkg/dart2wasm/lib/functions.dart

@@ -107,6 +107,7 @@ class FunctionCollector {
             null,
             isImportOrExport: true,
             synthesizeNullReturnValue: false,
+            synthesizeNoReturn: false,
           );
           return _functions[member.reference] =
               translator
@@ -141,6 +142,7 @@ class FunctionCollector {
               null,
               isImportOrExport: true,
               synthesizeNullReturnValue: false,
+              synthesizeNoReturn: false,
             )
           : translator.signatureForDirectCall(target);
 
@@ -253,6 +255,7 @@ class FunctionCollector {
   w.FunctionType _getFunctionType(Reference target) {
     final Member member = target.asMember;
     final synthesizeNullReturnValue = this.synthesizeNullReturnValue(target);
+    final synthesizeNoReturn = this.synthesizeNoReturn(target);
 
     if (target.isBodyReference) {
       // This is the function body that is always called directly (never via
@@ -262,11 +265,16 @@ class FunctionCollector {
         translator,
         member,
         synthesizeNullReturnValue,
+        synthesizeNoReturn,
       );
     }
 
     return member.accept1(
-      _FunctionTypeGenerator(translator, synthesizeNullReturnValue),
+      _FunctionTypeGenerator(
+        translator,
+        synthesizeNullReturnValue,
+        synthesizeNoReturn,
+      ),
       target,
     );
   }
@@ -277,13 +285,25 @@ class FunctionCollector {
     if (member.name == indexSetName) return true;
 
     final returnType = translator.typeOfReturnValue(member);
-    final wasmType = translator.translateType(returnType);
+    final wasmType = translator.translateReturnType(returnType);
     if (wasmType case w.RefType(heapType: w.HeapType.none, nullable: true)) {
       return true;
     }
     return false;
   }
 
+  bool synthesizeNoReturn(Reference target) {
+    final member = target.asMember;
+    if (member is! Procedure) return false;
+
+    final returnType = translator.typeOfReturnValue(member);
+    final wasmType = translator.translateReturnType(returnType);
+    if (wasmType case w.RefType(heapType: w.HeapType.none, nullable: false)) {
+      return true;
+    }
+    return false;
+  }
+
   String getFunctionName(Reference target) {
     final Member member = target.asMember;
     String memberName = member.toString();
@@ -433,8 +453,13 @@ class FunctionCollector {
 class _FunctionTypeGenerator extends MemberVisitor1<w.FunctionType, Reference> {
   final Translator translator;
   final bool synthesizeNullReturnValue;
+  final bool synthesizeNoReturn;
 
-  _FunctionTypeGenerator(this.translator, this.synthesizeNullReturnValue);
+  _FunctionTypeGenerator(
+    this.translator,
+    this.synthesizeNullReturnValue,
+    this.synthesizeNoReturn,
+  );
 
   @override
   w.FunctionType visitField(Field node, Reference target) {
@@ -445,6 +470,7 @@ class _FunctionTypeGenerator extends MemberVisitor1<w.FunctionType, Reference> {
         target,
         null,
         synthesizeNullReturnValue: synthesizeNullReturnValue,
+        synthesizeNoReturn: synthesizeNoReturn,
       );
     }
     assert(
@@ -465,6 +491,7 @@ class _FunctionTypeGenerator extends MemberVisitor1<w.FunctionType, Reference> {
       target,
       translator.translateType(receiverType),
       synthesizeNullReturnValue: synthesizeNullReturnValue,
+      synthesizeNoReturn: synthesizeNoReturn,
     );
   }
 
@@ -477,6 +504,7 @@ class _FunctionTypeGenerator extends MemberVisitor1<w.FunctionType, Reference> {
         target,
         null,
         synthesizeNullReturnValue: synthesizeNullReturnValue,
+        synthesizeNoReturn: synthesizeNoReturn,
       );
     }
 
@@ -502,6 +530,7 @@ class _FunctionTypeGenerator extends MemberVisitor1<w.FunctionType, Reference> {
       target,
       receiverType,
       synthesizeNullReturnValue: synthesizeNullReturnValue,
+      synthesizeNoReturn: synthesizeNoReturn,
     );
   }
 
@@ -738,6 +767,7 @@ w.FunctionType makeFunctionTypeForBody(
   Translator translator,
   Member member,
   bool synthesizeNullReturnValue,
+  bool synthesizeNoReturn,
 ) {
   assert(member.isInstanceMember);
   assert(member is Procedure);
@@ -758,7 +788,7 @@ w.FunctionType makeFunctionTypeForBody(
       translator.translateType(translator.typeOfCheckedParameterVariable(p)),
   ];
 
-  final hasNoReturnValue = synthesizeNullReturnValue;
+  final hasNoReturnValue = synthesizeNullReturnValue || synthesizeNoReturn;
   final outputs = [
     if (!hasNoReturnValue)
       translator.translateReturnType(translator.typeOfReturnValue(member)),
@@ -841,6 +871,7 @@ w.FunctionType _makeFunctionType(
   Reference target,
   w.ValueType? receiverType, {
   required bool synthesizeNullReturnValue,
+  required bool synthesizeNoReturn,
   bool isImportOrExport = false,
 }) {
   Member member = target.asMember;
@@ -881,7 +912,7 @@ w.FunctionType _makeFunctionType(
       (t is InterfaceType && t.classNode == translator.wasmVoidClass);
 
   final List<w.ValueType> outputs;
-  final hasNoReturnValue = target.isSetter || synthesizeNullReturnValue;
+  final hasNoReturnValue = synthesizeNullReturnValue || synthesizeNoReturn;
   if (hasNoReturnValue) {
     outputs = const [];
   } else {

pkg/dart2wasm/lib/translator.dart

@@ -783,6 +783,11 @@ class Translator with KernelNodes {
       b.ref_null(w.HeapType.none);
       return [w.RefType(w.HeapType.none, nullable: true)];
     }
+    if (callTarget.synthesizeNoReturn) {
+      assert(outputs.isEmpty);
+      b.unreachable();
+      return const [];
+    }
     return outputs;
   }
 
@@ -1782,16 +1787,24 @@ class Translator with KernelNodes {
       final table = dispatchTable;
       final selector = table.selectorForTarget(target);
       if (selector.containsTarget(target)) {
-        assert(
-          !selector.synthesizeNullReturnValue ||
-              selector.signature.outputs.isEmpty,
-        );
         return selector.synthesizeNullReturnValue;
       }
     }
     return functions.synthesizeNullReturnValue(target);
   }
 
+  bool synthesizeNoReturn(Reference target) {
+    final member = target.asMember;
+    if (member.isInstanceMember) {
+      final table = dispatchTable;
+      final selector = table.selectorForTarget(target);
+      if (selector.containsTarget(target)) {
+        return selector.synthesizeNoReturn;
+      }
+    }
+    return functions.synthesizeNoReturn(target);
+  }
+
   ParameterInfo paramInfoForDirectCall(Reference target) {
     if (target.asMember.isInstanceMember) {
       final selector = dispatchTable.selectorForTarget(target);
@@ -2920,9 +2933,6 @@ class _ClosureDynamicEntryGenerator implements CodeGenerator {
         outputs.single,
         translator.outputOrVoid(function.type.outputs),
       );
-    } else if (function.type.outputs.isNotEmpty) {
-      assert(target.synthesizeNullReturnValue);
-      b.ref_null(w.HeapType.none);
     }
 
     b.end(); // end function

pkg/dart2wasm/test/ir_tests/always_throws.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// functionFilter=foo
+// typeFilter=NoMatch
+// globalFilter=NoMatch
+// compilerOption=-O0
+
+void main() {
+  foo();
+}
+
+void foo() {
+  print('foo');
+  print(fooAlwaysThrows());
+}
+
+Never fooAlwaysThrows() {
+  print('fooAlwaysThrows');
+  throw Object();
+}

pkg/dart2wasm/test/ir_tests/always_throws.wat

@@ -0,0 +1,27 @@
+(module $module0
+  (type $#Top <...>)
+  (type $JSExternWrapper <...>)
+  (type $Object <...>)
+  (global $"\"fooAlwaysThrows\"" (ref $JSExternWrapper) <...>)
+  (global $"\"foo\"" (ref $JSExternWrapper) <...>)
+  (func $Error._throwWithCurrentStackTrace <noInline> (param $object (ref $#Top)) <...>)
+  (func $Object (result (ref $Object)) <...>)
+  (func $foo (result (ref null $#Top))
+    global.get $"\"foo\""
+    call $print
+    ref.null none
+    drop
+    call $fooAlwaysThrows
+    unreachable
+  )
+  (func $fooAlwaysThrows
+    global.get $"\"fooAlwaysThrows\""
+    call $print
+    ref.null none
+    drop
+    call $Object
+    call $"Error._throwWithCurrentStackTrace <noInline>"
+    unreachable
+  )
+  (func $print (param $object (ref null $#Top)) <...>)
+)

pkg/dart2wasm/test/ir_tests/deferred.fine_grained_module3.wat

@@ -211,6 +211,7 @@
       global.get $"\"[]\""
       i32.const 21
       call_indirect $module0.cross-module-funcs-0 (param i64 i64 (ref null $JSExternWrapper))
+      unreachable
     end
     local.get $var1
     struct.get $WasmListBase $_data

pkg/dart2wasm/test/ir_tests/deferred.type_checks_module1.wat

@@ -8,7 +8,6 @@
   (type $_InterfaceType <...>)
   (type $_Type <...>)
   (global $"\")\"_11" (import "$" "2") (ref $JSExternWrapper))
-  (global $"\"Attempt to execute code remove<...>\"" (import "$" "(") (ref $JSExternWrapper))
   (global $_InterfaceType (import "$" "0") (ref $_InterfaceType))
   (table $$.% (import "$" "%") 742 funcref)
   (table $$.' (import "$" "'") 20 funcref)
@@ -158,9 +157,6 @@
     if
       i32.const 2
       call_indirect $$.' 
-      global.get $"\"Attempt to execute code remove<...>\""
-      i32.const 3
-      call_indirect $$.' (param (ref $#Top))
       unreachable
     end
     local.get $var0

pkg/dart2wasm/test/ir_tests/dispatch_table_reuse_module1.wat

@@ -77,6 +77,7 @@
           local.get $var3
           i32.const 5
           call_indirect $module0.cross-module-funcs-0 (param (ref null $#Top) (ref $_Type))
+          unreachable
         end
         local.get $var1
         ref.cast $Object