Reland "[wasm-gc] Implement isorecursive canonicalization"

This is a reland of commit e76ad5c6

Changes compared to original:
- Move invocation of LAZY_INSTANCE_INITIALIZER to a static global
  variable, as some builds were failing with a function-level static.
- Drive-by: Improve documentation a bit.

Original change's description:
> [wasm-gc] Implement isorecursive canonicalization
>
> This implements isorecursive canonicalization for static types.
>
> Not implemented in this CL:
> - Runtime type canonicalization.
> - Cross-module signature canonicalization for purposes of call_indirect.
>
> Bug: v8:7748
> Change-Id: I6214f947444eea8d7b15a29b35c94c3d07ddb525
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3541925
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79665}

Bug: v8:7748
Change-Id: I493fba1906491762f7d8bae50108e3e4a743391d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3560480Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79692}

BUILD.bazel

@@ -2434,6 +2434,8 @@ filegroup(
             "src/wasm/baseline/liftoff-compiler.h",
             "src/wasm/baseline/liftoff-register.h",
             "src/wasm/branch-hint-map.h",
+            "src/wasm/canonical-types.cc",
+            "src/wasm/canonical-types.h",
             "src/wasm/code-space-access.cc",
             "src/wasm/code-space-access.h",
             "src/wasm/compilation-environment.h",

BUILD.gn

@@ -3513,6 +3513,7 @@ v8_header_set("v8_internal_headers") {
       "src/wasm/baseline/liftoff-assembler.h",
       "src/wasm/baseline/liftoff-compiler.h",
       "src/wasm/baseline/liftoff-register.h",
+      "src/wasm/canonical-types.h",
       "src/wasm/code-space-access.h",
       "src/wasm/compilation-environment.h",
       "src/wasm/decoder.h",
@@ -4540,6 +4541,7 @@ v8_source_set("v8_base_without_compiler") {
       "src/trap-handler/handler-shared.cc",
       "src/wasm/baseline/liftoff-assembler.cc",
       "src/wasm/baseline/liftoff-compiler.cc",
+      "src/wasm/canonical-types.cc",
       "src/wasm/code-space-access.cc",
       "src/wasm/function-body-decoder.cc",
       "src/wasm/function-compiler.cc",

src/flags/flag-definitions.h

@@ -1096,10 +1096,14 @@ DEFINE_BOOL(wasm_speculative_inlining, false,
 DEFINE_BOOL(trace_wasm_inlining, false, "trace wasm inlining")
 DEFINE_BOOL(trace_wasm_speculative_inlining, false,
             "trace wasm speculative inlining")
+DEFINE_BOOL(wasm_type_canonicalization, false,
+            "apply isorecursive canonicalization on wasm types")
 DEFINE_IMPLICATION(wasm_speculative_inlining, experimental_wasm_typed_funcref)
 DEFINE_IMPLICATION(wasm_speculative_inlining, wasm_dynamic_tiering)
 DEFINE_IMPLICATION(wasm_speculative_inlining, wasm_inlining)
 DEFINE_WEAK_IMPLICATION(experimental_wasm_gc, wasm_speculative_inlining)
+DEFINE_WEAK_IMPLICATION(experimental_wasm_typed_funcref,
+                        wasm_type_canonicalization)
 // Speculative inlining needs type feedback from Liftoff and compilation in
 // Turbofan.
 DEFINE_NEG_NEG_IMPLICATION(liftoff, wasm_speculative_inlining)

src/wasm/canonical-types.cc

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/wasm/canonical-types.h"
+
+#include "src/wasm/wasm-engine.h"
+
+namespace v8 {
+namespace internal {
+namespace wasm {
+
+V8_EXPORT_PRIVATE TypeCanonicalizer* GetTypeCanonicalizer() {
+  return GetWasmEngine()->type_canonicalizer();
+}
+
+void TypeCanonicalizer::AddRecursiveGroup(WasmModule* module, uint32_t size) {
+  // Multiple threads could try to register recursive groups concurrently.
+  // TODO(manoskouk): Investigate if we can fine-grain the synchronization.
+  base::MutexGuard mutex_guard(&mutex_);
+  DCHECK_GE(module->types.size(), size);
+  uint32_t start_index = static_cast<uint32_t>(module->types.size()) - size;
+  CanonicalGroup group;
+  group.types.resize(size);
+  for (uint32_t i = 0; i < size; i++) {
+    group.types[i] = CanonicalizeTypeDef(module, module->types[start_index + i],
+                                         start_index);
+  }
+  int canonical_index = FindCanonicalGroup(group);
+  if (canonical_index >= 0) {
+    // Identical group found. Map new types to the old types's canonical
+    // representatives.
+    for (uint32_t i = 0; i < size; i++) {
+      module->isorecursive_canonical_type_ids[start_index + i] =
+          canonical_index + i;
+    }
+  } else {
+    // Identical group not found. Add new canonical representatives for the new
+    // types.
+    uint32_t first_canonical_index =
+        static_cast<uint32_t>(canonical_supertypes_.size());
+    canonical_supertypes_.resize(first_canonical_index + size);
+    for (uint32_t i = 0; i < size; i++) {
+      CanonicalType& canonical_type = group.types[i];
+      // Compute the canonical index of the supertype: If it is relative, we
+      // need to add {first_canonical_index}.
+      canonical_supertypes_[first_canonical_index + i] =
+          canonical_type.is_relative_supertype
+              ? canonical_type.type_def.supertype + first_canonical_index
+              : canonical_type.type_def.supertype;
+      module->isorecursive_canonical_type_ids[start_index + i] =
+          first_canonical_index + i;
+    }
+    canonical_groups_.emplace(group, first_canonical_index);
+  }
+}
+
+// An index in a type gets mapped to a relative index if it is inside the new
+// canonical group, or the canonical representative if it is not.
+ValueType TypeCanonicalizer::CanonicalizeValueType(
+    const WasmModule* module, ValueType type,
+    uint32_t recursive_group_start) const {
+  if (!type.has_index()) return type;
+  return type.ref_index() >= recursive_group_start
+             ? ValueType::CanonicalWithRelativeIndex(
+                   type.kind(), type.ref_index() - recursive_group_start)
+             : ValueType::FromIndex(
+                   type.kind(),
+                   module->isorecursive_canonical_type_ids[type.ref_index()]);
+}
+
+bool TypeCanonicalizer::IsCanonicalSubtype(uint32_t sub_index,
+                                           uint32_t super_index,
+                                           const WasmModule* sub_module,
+                                           const WasmModule* super_module) {
+  // Multiple threads could try to register and access recursive groups
+  // concurrently.
+  // TODO(manoskouk): Investigate if we can improve this synchronization.
+  base::MutexGuard mutex_guard(&mutex_);
+  uint32_t canonical_super =
+      super_module->isorecursive_canonical_type_ids[super_index];
+  uint32_t canonical_sub =
+      sub_module->isorecursive_canonical_type_ids[sub_index];
+  while (canonical_sub != kNoSuperType) {
+    if (canonical_sub == canonical_super) return true;
+    canonical_sub = canonical_supertypes_[canonical_sub];
+  }
+  return false;
+}
+
+// Map all type indices (including supertype) inside {type} to indices relative
+// to {recursive_group_start}.
+TypeCanonicalizer::CanonicalType TypeCanonicalizer::CanonicalizeTypeDef(
+    const WasmModule* module, TypeDefinition type,
+    uint32_t recursive_group_start) {
+  uint32_t canonical_supertype = kNoSuperType;
+  bool is_relative_supertype = false;
+  if (type.supertype < recursive_group_start) {
+    canonical_supertype =
+        module->isorecursive_canonical_type_ids[type.supertype];
+  } else if (type.supertype != kNoSuperType) {
+    canonical_supertype = type.supertype - recursive_group_start;
+    is_relative_supertype = true;
+  }
+  TypeDefinition result;
+  switch (type.kind) {
+    case TypeDefinition::kFunction: {
+      const FunctionSig* original_sig = type.function_sig;
+      FunctionSig::Builder builder(&zone_, original_sig->return_count(),
+                                   original_sig->parameter_count());
+      for (ValueType ret : original_sig->returns()) {
+        builder.AddReturn(
+            CanonicalizeValueType(module, ret, recursive_group_start));
+      }
+      for (ValueType param : original_sig->parameters()) {
+        builder.AddParam(
+            CanonicalizeValueType(module, param, recursive_group_start));
+      }
+      result = TypeDefinition(builder.Build(), canonical_supertype);
+      break;
+    }
+    case TypeDefinition::kStruct: {
+      const StructType* original_type = type.struct_type;
+      StructType::Builder builder(&zone_, original_type->field_count());
+      for (uint32_t i = 0; i < original_type->field_count(); i++) {
+        builder.AddField(CanonicalizeValueType(module, original_type->field(i),
+                                               recursive_group_start),
+                         original_type->mutability(i));
+      }
+      result = TypeDefinition(builder.Build(), canonical_supertype);
+      break;
+    }
+    case TypeDefinition::kArray: {
+      ValueType element_type = CanonicalizeValueType(
+          module, type.array_type->element_type(), recursive_group_start);
+      result = TypeDefinition(
+          zone_.New<ArrayType>(element_type, type.array_type->mutability()),
+          canonical_supertype);
+      break;
+    }
+  }
+
+  return {result, is_relative_supertype};
+}
+
+// Returns the index of the canonical representative of the first type in this
+// group, or -1 if an identical group does not exist.
+int TypeCanonicalizer::FindCanonicalGroup(CanonicalGroup& group) const {
+  auto element = canonical_groups_.find(group);
+  return element == canonical_groups_.end() ? -1 : element->second;
+}
+
+}  // namespace wasm
+}  // namespace internal
+}  // namespace v8

src/wasm/canonical-types.h

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#if !V8_ENABLE_WEBASSEMBLY
+#error This header should only be included if WebAssembly is enabled.
+#endif  // !V8_ENABLE_WEBASSEMBLY
+
+#ifndef V8_WASM_CANONICAL_TYPES_H_
+#define V8_WASM_CANONICAL_TYPES_H_
+
+#include <unordered_map>
+
+#include "src/base/lazy-instance.h"
+#include "src/wasm/wasm-module.h"
+
+namespace v8 {
+namespace internal {
+namespace wasm {
+
+// A singleton class, responsible for isorecursive canonicalization of wasm
+// types.
+// A recursive group is a subsequence of types explicitly marked in the type
+// section of a wasm module. Identical recursive groups have to be canonicalized
+// to a single canonical group and are are considered identical. Respective
+// types in two identical groups are considered identical for all purposes.
+// Two groups are considered identical if they have the same shape, and all
+// type indices referenced in the same position in both groups reference:
+// - identical types, if those do not belong to the rec. group,
+// - types in the same relative position in the group, if those belong to the
+//   rec. group.
+class TypeCanonicalizer {
+ public:
+  TypeCanonicalizer() = default;
+
+  // Singleton class; no copying or moving allowed.
+  TypeCanonicalizer(const TypeCanonicalizer& other) = delete;
+  TypeCanonicalizer& operator=(const TypeCanonicalizer& other) = delete;
+  TypeCanonicalizer(TypeCanonicalizer&& other) = delete;
+  TypeCanonicalizer& operator=(TypeCanonicalizer&& other) = delete;
+
+  // Registers the last {size} types of {module} as a recursive group, and
+  // possibly canonicalizes it if an identical one has been found.
+  // Modifies {module->isorecursive_canonical_type_ids}.
+  V8_EXPORT_PRIVATE void AddRecursiveGroup(WasmModule* module, uint32_t size);
+
+  // Returns if the type at {sub_index} in {sub_module} is a subtype of the
+  // type at {super_index} in {super_module} after canonicalization.
+  V8_EXPORT_PRIVATE bool IsCanonicalSubtype(uint32_t sub_index,
+                                            uint32_t super_index,
+                                            const WasmModule* sub_module,
+                                            const WasmModule* super_module);
+
+ private:
+  using TypeInModule = std::pair<const WasmModule*, uint32_t>;
+  struct CanonicalType {
+    TypeDefinition type_def;
+    bool is_relative_supertype;
+
+    bool operator==(const CanonicalType& other) const {
+      return type_def == other.type_def &&
+             is_relative_supertype == other.is_relative_supertype;
+    }
+
+    bool operator!=(const CanonicalType& other) const {
+      return type_def != other.type_def ||
+             is_relative_supertype != other.is_relative_supertype;
+    }
+
+    size_t hash_value() const {
+      return base::hash_combine(type_def.kind,
+                                base::hash_value(is_relative_supertype));
+    }
+  };
+  struct CanonicalGroup {
+    struct hash {
+      size_t operator()(const CanonicalGroup& group) const {
+        return group.hash_value();
+      }
+    };
+
+    bool operator==(const CanonicalGroup& other) const {
+      return types == other.types;
+    }
+
+    bool operator!=(const CanonicalGroup& other) const {
+      return types != other.types;
+    }
+
+    size_t hash_value() const {
+      size_t result = 0;
+      for (const CanonicalType& type : types) {
+        result = base::hash_combine(result, type.hash_value());
+      }
+      return result;
+    }
+
+    std::vector<CanonicalType> types;
+  };
+
+  int FindCanonicalGroup(CanonicalGroup&) const;
+
+  CanonicalType CanonicalizeTypeDef(const WasmModule* module,
+                                    TypeDefinition type,
+                                    uint32_t recursive_group_start);
+  ValueType CanonicalizeValueType(const WasmModule* module, ValueType type,
+                                  uint32_t recursive_group_start) const;
+
+  std::vector<uint32_t> canonical_supertypes_;
+  // group -> canonical id of first type
+  std::unordered_map<CanonicalGroup, uint32_t, CanonicalGroup::hash>
+      canonical_groups_;
+  AccountingAllocator allocator_;
+  Zone zone_{&allocator_, "canonical type zone"};
+  base::Mutex mutex_;
+};
+
+// Returns a reference to the TypeCanonicalizer shared by the entire process.
+V8_EXPORT_PRIVATE TypeCanonicalizer* GetTypeCanonicalizer();
+
+}  // namespace wasm
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_WASM_CANONICAL_TYPES_H_

src/wasm/module-decoder.cc

@@ -13,6 +13,7 @@
 #include "src/logging/metrics.h"
 #include "src/objects/objects-inl.h"
 #include "src/utils/ostreams.h"
+#include "src/wasm/canonical-types.h"
 #include "src/wasm/decoder.h"
 #include "src/wasm/function-body-decoder-impl.h"
 #include "src/wasm/init-expr-interface.h"
@@ -669,17 +670,21 @@ class ModuleDecoderImpl : public Decoder {
   }
 
   void DecodeTypeSection() {
+    TypeCanonicalizer* type_canon = GetTypeCanonicalizer();
     uint32_t types_count = consume_count("types count", kV8MaxWasmTypes);
 
     // Non wasm-gc type section decoding.
     if (!enabled_features_.has_gc()) {
-      for (uint32_t i = 0; ok() && i < types_count; ++i) {
+      for (uint32_t i = 0; i < types_count; ++i) {
         TRACE("DecodeSignature[%d] module+%d\n", i,
               static_cast<int>(pc_ - start_));
         expect_u8("signature definition", kWasmFunctionTypeCode);
         const FunctionSig* sig = consume_sig(module_->signature_zone.get());
         if (!ok()) break;
         module_->add_signature(sig, kNoSuperType);
+        if (FLAG_wasm_type_canonicalization) {
+          type_canon->AddRecursiveGroup(module_.get(), 1);
+        }
       }
       return;
     }
@@ -700,6 +705,9 @@ class ModuleDecoderImpl : public Decoder {
           TypeDefinition type = consume_nominal_type_definition();
           if (ok()) module_->add_type(type);
         }
+        if (ok() && FLAG_wasm_type_canonicalization) {
+          type_canon->AddRecursiveGroup(module_.get(), types_count);
+        }
       } else {
         // wasm-gc isorecursive type section decoding.
         for (uint32_t i = 0; ok() && i < types_count; ++i) {
@@ -722,9 +730,17 @@ class ModuleDecoderImpl : public Decoder {
               TypeDefinition type = consume_subtype_definition();
               if (ok()) module_->add_type(type);
             }
+            if (ok() && FLAG_wasm_type_canonicalization) {
+              type_canon->AddRecursiveGroup(module_.get(), group_size);
+            }
           } else {
             TypeDefinition type = consume_subtype_definition();
-            if (ok()) module_->add_type(type);
+            if (ok()) {
+              module_->add_type(type);
+              if (FLAG_wasm_type_canonicalization) {
+                type_canon->AddRecursiveGroup(module_.get(), 1);
+              }
+            }
           }
         }
       }

src/wasm/struct-types.h

@@ -132,8 +132,12 @@ class ArrayType : public ZoneObject {
   ValueType element_type() const { return rep_; }
   bool mutability() const { return mutability_; }
 
-  bool operator==(const ArrayType& other) const { return rep_ == other.rep_; }
-  bool operator!=(const ArrayType& other) const { return rep_ != other.rep_; }
+  bool operator==(const ArrayType& other) const {
+    return rep_ == other.rep_ && mutability_ == other.mutability_;
+  }
+  bool operator!=(const ArrayType& other) const {
+    return rep_ != other.rep_ || mutability_ != other.mutability_;
+  }
 
  private:
   const ValueType rep_;

src/wasm/value-type.h

@@ -279,12 +279,14 @@ constexpr bool is_defaultable(ValueKind kind) {
   return kind != kRef && !is_rtt(kind);
 }
 
-// A ValueType is encoded by three components: A ValueKind, a heap
-// representation (for reference types), and an inheritance depth (for rtts
-// only). Those are encoded into 32 bits using base::BitField. The underlying
-// ValueKind enumeration includes four elements which do not strictly correspond
-// to value types: the two packed types i8 and i16, the void type (for control
-// structures), and a bottom value (for internal use).
+// A ValueType is encoded by two components: a ValueKind and a heap
+// representation (for reference types/rtts). Those are encoded into 32 bits
+// using base::BitField. The underlying ValueKind enumeration includes four
+// elements which do not strictly correspond to value types: the two packed
+// types i8 and i16, the void type (for control structures), and a bottom value
+// (for internal use).
+// ValueType encoding includes an additional bit marking the index of a type as
+// relative. This should only be used during type canonicalization.
 class ValueType {
  public:
   /******************************* Constructors *******************************/
@@ -309,6 +311,11 @@ class ValueType {
                      HeapTypeField::encode(type_index));
   }
 
+  static constexpr ValueType FromIndex(ValueKind kind, uint32_t index) {
+    DCHECK(kind == kOptRef || kind == kRef || kind == kRtt);
+    return ValueType(KindField::encode(kind) | HeapTypeField::encode(index));
+  }
+
   // Useful when deserializing a type stored in a runtime object.
   static constexpr ValueType FromRawBitField(uint32_t bit_field) {
     return ValueType(bit_field);
@@ -491,8 +498,6 @@ class ValueType {
     }
   }
 
-  static constexpr int kLastUsedBit = 24;
-
   /****************************** Pretty-printing *****************************/
   constexpr char short_name() const { return wasm::short_name(kind()); }
 
@@ -517,23 +522,40 @@ class ValueType {
     return buf.str();
   }
 
-  // We only use 31 bits so ValueType fits in a Smi. This can be changed if
-  // needed.
+  /********************** Type canonicalization utilities *********************/
+  static constexpr ValueType CanonicalWithRelativeIndex(ValueKind kind,
+                                                        uint32_t index) {
+    return ValueType(KindField::encode(kind) | HeapTypeField::encode(index) |
+                     CanonicalRelativeField::encode(true));
+  }
+
+  constexpr bool is_canonical_relative() const {
+    return has_index() && CanonicalRelativeField::decode(bit_field_);
+  }
+
+  /**************************** Static constants ******************************/
+  static constexpr int kLastUsedBit = 25;
   static constexpr int kKindBits = 5;
   static constexpr int kHeapTypeBits = 20;
 
  private:
-  STATIC_ASSERT(kV8MaxWasmTypes < (1u << kHeapTypeBits));
-
   // {hash_value} directly reads {bit_field_}.
   friend size_t hash_value(ValueType type);
 
   using KindField = base::BitField<ValueKind, 0, kKindBits>;
   using HeapTypeField = KindField::Next<uint32_t, kHeapTypeBits>;
+  // Marks a type as a canonical type which uses an index relative to its
+  // recursive group start. Used only during type canonicalization.
+  using CanonicalRelativeField = HeapTypeField::Next<bool, 1>;
 
+  static_assert(kV8MaxWasmTypes < (1u << kHeapTypeBits),
+                "Type indices fit in kHeapTypeBits");
   // This is implemented defensively against field order changes.
-  STATIC_ASSERT(kLastUsedBit ==
-                std::max(KindField::kLastUsedBit, HeapTypeField::kLastUsedBit));
+  static_assert(kLastUsedBit ==
+                    std::max(KindField::kLastUsedBit,
+                             std::max(HeapTypeField::kLastUsedBit,
+                                      CanonicalRelativeField::kLastUsedBit)),
+                "kLastUsedBit is consistent");
 
   constexpr explicit ValueType(uint32_t bit_field) : bit_field_(bit_field) {}
 

src/wasm/wasm-engine.h

@@ -19,6 +19,7 @@
 #include "src/base/platform/mutex.h"
 #include "src/tasks/cancelable-task.h"
 #include "src/tasks/operations-barrier.h"
+#include "src/wasm/canonical-types.h"
 #include "src/wasm/wasm-code-manager.h"
 #include "src/wasm/wasm-tier.h"
 #include "src/zone/accounting-allocator.h"
@@ -357,6 +358,8 @@ class V8_EXPORT_PRIVATE WasmEngine {
   void SampleRethrowEvent(Isolate*);
   void SampleCatchEvent(Isolate*);
 
+  TypeCanonicalizer* type_canonicalizer() { return &type_canonicalizer_; }
+
   // Call on process start and exit.
   static void InitializeOncePerProcess();
   static void GlobalTearDown();
@@ -392,6 +395,8 @@ class V8_EXPORT_PRIVATE WasmEngine {
 
   std::atomic<int> next_compilation_id_{0};
 
+  TypeCanonicalizer type_canonicalizer_;
+
   // This mutex protects all information which is mutated concurrently or
   // fields that are initialized lazily on the first access.
   base::Mutex mutex_;

src/wasm/wasm-module-builder.cc

@@ -274,6 +274,8 @@ WasmModuleBuilder::WasmModuleBuilder(Zone* zone)
       globals_(zone),
       exceptions_(zone),
       signature_map_(zone),
+      current_recursive_group_start_(-1),
+      recursive_groups_(zone),
       start_function_index_(-1),
       min_memory_size_(16),
       max_memory_size_(0),
@@ -593,10 +595,24 @@ void WasmModuleBuilder::WriteTo(ZoneBuffer* buffer) const {
   // == Emit types =============================================================
   if (types_.size() > 0) {
     size_t start = EmitSection(kTypeSectionCode, buffer);
-    buffer->write_size(types_.size());
+    size_t type_count = types_.size();
+    for (auto pair : recursive_groups_) {
+      // Every rec. group counts as one type entry.
+      type_count -= pair.second - 1;
+    }
+
+    buffer->write_size(type_count);
+
+    for (uint32_t i = 0; i < types_.size(); i++) {
+      auto recursive_group = recursive_groups_.find(i);
+
+      if (recursive_group != recursive_groups_.end()) {
+        buffer->write_u8(kWasmRecursiveTypeGroupCode);
+        buffer->write_u32v(recursive_group->second);
+      }
+
+      const TypeDefinition& type = types_[i];
 
-    // TODO(7748): Add support for recursive groups.
-    for (const TypeDefinition& type : types_) {
       if (type.supertype != kNoSuperType) {
         buffer->write_u8(kWasmSubtypeCode);
         buffer->write_u8(1);  // The supertype count is always 1.

src/wasm/wasm-module-builder.h

@@ -359,6 +359,22 @@ class V8_EXPORT_PRIVATE WasmModuleBuilder : public ZoneObject {
   void SetMaxMemorySize(uint32_t value);
   void SetHasSharedMemory();
 
+  void StartRecursiveTypeGroup() {
+    DCHECK_EQ(current_recursive_group_start_, -1);
+    current_recursive_group_start_ = static_cast<int>(types_.size());
+  }
+
+  void EndRecursiveTypeGroup() {
+    // Make sure we are in a recursive group.
+    DCHECK_NE(current_recursive_group_start_, -1);
+    // Make sure the current recursive group has at least one element.
+    DCHECK_GT(static_cast<int>(types_.size()), current_recursive_group_start_);
+    recursive_groups_.emplace(
+        current_recursive_group_start_,
+        static_cast<uint32_t>(types_.size()) - current_recursive_group_start_);
+    current_recursive_group_start_ = -1;
+  }
+
   // Writing methods.
   void WriteTo(ZoneBuffer* buffer) const;
   void WriteAsmJsOffsetTable(ZoneBuffer* buffer) const;
@@ -455,6 +471,9 @@ class V8_EXPORT_PRIVATE WasmModuleBuilder : public ZoneObject {
   ZoneVector<WasmGlobal> globals_;
   ZoneVector<int> exceptions_;
   ZoneUnorderedMap<FunctionSig, uint32_t> signature_map_;
+  int current_recursive_group_start_;
+  // first index -> size
+  ZoneUnorderedMap<uint32_t, uint32_t> recursive_groups_;
   int start_function_index_;
   uint32_t min_memory_size_;
   uint32_t max_memory_size_;

src/wasm/wasm-module.h

@@ -364,6 +364,25 @@ struct TypeDefinition {
     const StructType* struct_type;
     const ArrayType* array_type;
   };
+
+  bool operator==(const TypeDefinition& other) const {
+    if (supertype != other.supertype || kind != other.kind) {
+      return false;
+    }
+    switch (kind) {
+      case kFunction:
+        return *function_sig == *other.function_sig;
+      case kStruct:
+        return *struct_type == *other.struct_type;
+      case kArray:
+        return *array_type == *other.array_type;
+    }
+  }
+
+  bool operator!=(const TypeDefinition& other) const {
+    return !(*this == other);
+  }
+
   uint32_t supertype;
   Kind kind;
 };
@@ -424,15 +443,14 @@ struct V8_EXPORT_PRIVATE WasmModule {
                                 ? signature_map.FindOrInsert(*type.function_sig)
                                 : 0;
     canonicalized_type_ids.push_back(canonical_id);
+    isorecursive_canonical_type_ids.push_back(-1);  // Will be computed later.
   }
 
   bool has_type(uint32_t index) const { return index < types.size(); }
 
   void add_signature(const FunctionSig* sig, uint32_t supertype) {
-    types.push_back(TypeDefinition(sig, supertype));
     DCHECK_NOT_NULL(sig);
-    uint32_t canonical_id = signature_map.FindOrInsert(*sig);
-    canonicalized_type_ids.push_back(canonical_id);
+    add_type(TypeDefinition(sig, supertype));
   }
   bool has_signature(uint32_t index) const {
     return index < types.size() &&
@@ -444,9 +462,8 @@ struct V8_EXPORT_PRIVATE WasmModule {
   }
 
   void add_struct_type(const StructType* type, uint32_t supertype) {
-    types.push_back(TypeDefinition(type, supertype));
-    // No canonicalization for structs.
-    canonicalized_type_ids.push_back(0);
+    DCHECK_NOT_NULL(type);
+    add_type(TypeDefinition(type, supertype));
   }
   bool has_struct(uint32_t index) const {
     return index < types.size() && types[index].kind == TypeDefinition::kStruct;
@@ -457,9 +474,8 @@ struct V8_EXPORT_PRIVATE WasmModule {
   }
 
   void add_array_type(const ArrayType* type, uint32_t supertype) {
-    types.push_back(TypeDefinition(type, supertype));
-    // No canonicalization for arrays.
-    canonicalized_type_ids.push_back(0);
+    DCHECK_NOT_NULL(type);
+    add_type(TypeDefinition(type, supertype));
   }
   bool has_array(uint32_t index) const {
     return index < types.size() && types[index].kind == TypeDefinition::kArray;
@@ -478,11 +494,12 @@ struct V8_EXPORT_PRIVATE WasmModule {
   }
 
   std::vector<TypeDefinition> types;  // by type index
-  // Map from each type index to the index of its corresponding canonical index.
-  // Canonical indices do not correspond to types.
-  // Note: right now, only functions are canonicalized, and arrays and structs
-  // map to 0.
+  // TODO(7748): Unify the following two arrays.
+  // Maps each type index to a canonical index for purposes of call_indirect.
   std::vector<uint32_t> canonicalized_type_ids;
+  // Maps each type index to its global (cross-module) canonical index as per
+  // isorecursive type canonicalization.
+  std::vector<uint32_t> isorecursive_canonical_type_ids;
   // Canonicalizing map for signature indexes.
   SignatureMap signature_map;
   std::vector<WasmFunction> functions;

src/wasm/wasm-subtyping.cc

@@ -5,6 +5,7 @@
 #include "src/wasm/wasm-subtyping.h"
 
 #include "src/base/platform/mutex.h"
+#include "src/wasm/canonical-types.h"
 #include "src/wasm/wasm-module.h"
 #include "src/zone/zone-containers.h"
 
@@ -18,17 +19,15 @@ V8_INLINE bool EquivalentIndices(uint32_t index1, uint32_t index2,
                                  const WasmModule* module1,
                                  const WasmModule* module2) {
   DCHECK(index1 != index2 || module1 != module2);
-  // TODO(7748): Canonicalize types.
-  return false;
+  if (!FLAG_wasm_type_canonicalization) return false;
+  return module1->isorecursive_canonical_type_ids[index1] ==
+         module2->isorecursive_canonical_type_ids[index2];
 }
 
 bool ValidStructSubtypeDefinition(uint32_t subtype_index,
                                   uint32_t supertype_index,
                                   const WasmModule* sub_module,
                                   const WasmModule* super_module) {
-  // TODO(7748): Figure out the cross-module story.
-  if (sub_module != super_module) return false;
-
   const StructType* sub_struct = sub_module->types[subtype_index].struct_type;
   const StructType* super_struct =
       super_module->types[supertype_index].struct_type;
@@ -56,9 +55,6 @@ bool ValidArraySubtypeDefinition(uint32_t subtype_index,
                                  uint32_t supertype_index,
                                  const WasmModule* sub_module,
                                  const WasmModule* super_module) {
-  // TODO(7748): Figure out the cross-module story.
-  if (sub_module != super_module) return false;
-
   const ArrayType* sub_array = sub_module->types[subtype_index].array_type;
   const ArrayType* super_array =
       super_module->types[supertype_index].array_type;
@@ -78,9 +74,6 @@ bool ValidFunctionSubtypeDefinition(uint32_t subtype_index,
                                     uint32_t supertype_index,
                                     const WasmModule* sub_module,
                                     const WasmModule* super_module) {
-  // TODO(7748): Figure out the cross-module story.
-  if (sub_module != super_module) return false;
-
   const FunctionSig* sub_func = sub_module->types[subtype_index].function_sig;
   const FunctionSig* super_func =
       super_module->types[supertype_index].function_sig;
@@ -219,9 +212,10 @@ V8_NOINLINE V8_EXPORT_PRIVATE bool IsSubtypeOfImpl(
   // equality; here we catch (ref $x) being a subtype of (ref null $x).
   if (sub_module == super_module && sub_index == super_index) return true;
 
-  // TODO(7748): Figure out cross-module story.
-  if (sub_module != super_module) return false;
-
+  if (FLAG_wasm_type_canonicalization) {
+    return GetTypeCanonicalizer()->IsCanonicalSubtype(sub_index, super_index,
+                                                      sub_module, super_module);
+  } else {
     uint32_t explicit_super = sub_module->supertype(sub_index);
     while (true) {
       if (explicit_super == super_index) return true;
@@ -229,6 +223,7 @@ V8_NOINLINE V8_EXPORT_PRIVATE bool IsSubtypeOfImpl(
       if (explicit_super == kNoSuperType) return false;
       explicit_super = sub_module->supertype(explicit_super);
     }
+  }
 }
 
 V8_NOINLINE bool EquivalentTypes(ValueType type1, ValueType type2,

src/wasm/wasm-subtyping.h

@@ -27,14 +27,16 @@ V8_NOINLINE V8_EXPORT_PRIVATE bool IsSubtypeOfImpl(
 // - Two numeric types are equivalent iff they are equal.
 // - T(ht1) ~ T(ht2) iff ht1 ~ ht2 for T in {ref, optref, rtt}.
 // Equivalence of heap types ht1 ~ ht2 is defined as follows:
-// - Two heap types are equivalent iff they are equal.
-// - TODO(7748): Implement iso-recursive canonicalization.
-V8_NOINLINE bool EquivalentTypes(ValueType type1, ValueType type2,
+// - Two non-index heap types are equivalent iff they are equal.
+// - Two indexed heap types are equivalent iff they are iso-recursive
+//   equivalent.
+V8_NOINLINE V8_EXPORT_PRIVATE bool EquivalentTypes(ValueType type1,
+                                                   ValueType type2,
                                                    const WasmModule* module1,
                                                    const WasmModule* module2);
 
-// Checks if subtype, defined in module1, is a subtype of supertype, defined in
-// module2.
+// Checks if {subtype}, defined in {module1}, is a subtype of {supertype},
+// defined in {module2}.
 // Subtyping between value types is described by the following rules
 // (structural subtyping):
 // - numeric types are subtype-related iff they are equal.
@@ -54,7 +56,7 @@ V8_NOINLINE bool EquivalentTypes(ValueType type1, ValueType type2,
 // - All structs are subtypes of data.
 // - All arrays are subtypes of array.
 // - An indexed heap type h1 is a subtype of indexed heap type h2 if h2 is
-//   transitively an explicit supertype of h1.
+//   transitively an explicit canonical supertype of h1.
 V8_INLINE bool IsSubtypeOf(ValueType subtype, ValueType supertype,
                            const WasmModule* sub_module,
                            const WasmModule* super_module) {
@@ -62,7 +64,7 @@ V8_INLINE bool IsSubtypeOf(ValueType subtype, ValueType supertype,
   return IsSubtypeOfImpl(subtype, supertype, sub_module, super_module);
 }
 
-// Checks if 'subtype' is a subtype of 'supertype' (both defined in module).
+// Checks if {subtype} is a subtype of {supertype} (both defined in {module}).
 V8_INLINE bool IsSubtypeOf(ValueType subtype, ValueType supertype,
                            const WasmModule* module) {
   // If the types are trivially identical, exit early.

test/cctest/wasm/test-gc.cc

@@ -1304,11 +1304,14 @@ WASM_COMPILED_EXEC_TEST(WasmArrayCopy) {
   tester.CheckResult(kZeroLength, 0);  // Does not throw.
 }
 
-/* TODO(7748): This test requires for recursive groups.
 WASM_COMPILED_EXEC_TEST(NewDefault) {
   WasmGCTester tester(execution_tier);
+
+  tester.builder()->StartRecursiveTypeGroup();
   const byte struct_type = tester.DefineStruct(
       {F(wasm::kWasmI32, true), F(wasm::kWasmF64, true), F(optref(0), true)});
+  tester.builder()->EndRecursiveTypeGroup();
+
   const byte array_type = tester.DefineArray(wasm::kWasmI32, true);
   // Returns: struct[0] + f64_to_i32(struct[1]) + (struct[2].is_null ^ 1) == 0.
   const byte allocate_struct = tester.DefineFunction(
@@ -1338,7 +1341,6 @@ WASM_COMPILED_EXEC_TEST(NewDefault) {
   tester.CheckResult(allocate_struct, 0);
   tester.CheckResult(allocate_array, 0);
 }
-*/
 
 WASM_COMPILED_EXEC_TEST(BasicRtt) {
   WasmGCTester tester(execution_tier);

test/mjsunit/wasm/call-ref.js

@@ -96,11 +96,10 @@ d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
 
   print("--imported function from another module--");
   assertEquals(57, instance.exports.test_wasm_import());
-  /* TODO(7748): Implement cross-module type canonicalization.
   print("--not imported function defined in another module--");
   assertEquals(19, instance.exports.main(
     exporting_instance.exports.addition, 12, 7));
-*/
+
   print("--imported WebAssembly.Function--")
   assertEquals(21, instance.exports.test_js_api_import());
   print("--not imported WebAssembly.Function--")

test/mjsunit/wasm/imported-function-types.js

@@ -35,9 +35,8 @@ var importing_module = function(imported_function) {
   return builder.instantiate({other: {func: imported_function}});
 };
 
-// TODO(7748): Implement cross-module subtyping.
 // Same form/different index should be fine.
-// importing_module(exporting_module.exports.func2);
+importing_module(exporting_module.exports.func2);
 // Same index/different form should throw.
 assertThrows(
     () => importing_module(exporting_module.exports.func1),

test/mjsunit/wasm/reference-globals.js

@@ -6,7 +6,6 @@
 
 d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
 
-/* TODO(7748): Implement cross-module subtyping.
 (function TestReferenceGlobals() {
   print(arguments.callee.name);
 
@@ -106,7 +105,6 @@ d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
   // The correct function reference has been passed.
   assertEquals(66, instance.exports.test_import(42, 24));
 })();
-*/
 
 (function TestStructInitExpr() {
   print(arguments.callee.name);

test/mjsunit/wasm/reference-tables.js

@@ -5,7 +5,6 @@
 // Flags: --experimental-wasm-gc
 
 d8.file.execute('test/mjsunit/wasm/wasm-module-builder.js');
-/* TODO(7748): Implement cross-module subtyping.
 (function TestTables() {
   print(arguments.callee.name);
   var exporting_instance = (function() {
@@ -100,9 +99,8 @@ d8.file.execute('test/mjsunit/wasm/wasm-module-builder.js');
   assertThrows(
       () => instance.exports.table.set(0, exporting_instance.exports.addition),
       TypeError,
-      /Argument 1 must be null or a WebAssembly function of type compatible to/);
+      /Argument 1 is invalid for table of type \(ref null 0\)/);
 })();
-*/
 
 (function TestNonNullableTables() {
   print(arguments.callee.name);

test/mjsunit/wasm/speculative-inlining.js

@@ -155,7 +155,6 @@ d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
   assertEquals(8, instance.exports.main(10, 0));
 })();
 
-/* TODO(7748): Implement cross-module subtyping.
 (function CallRefImportedFunction() {
   print(arguments.callee.name);
 
@@ -196,7 +195,6 @@ d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
   // The function f1 defined in another module should not be inlined.
   assertEquals(1, instance2.exports.main(0, instance1.exports.f1));
 })();
-*/
 
 // Check that we handle WasmJSFunctions properly and do not inline them, both
 // in the monomorphic and polymorphic case.

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -8,6 +8,7 @@
 #include "src/objects/objects-inl.h"
 #include "src/objects/objects.h"
 #include "src/utils/ostreams.h"
+#include "src/wasm/canonical-types.h"
 #include "src/wasm/function-body-decoder-impl.h"
 #include "src/wasm/leb-helper.h"
 #include "src/wasm/local-decl-encoder.h"
@@ -89,6 +90,7 @@ class TestModuleBuilder {
   byte AddSignature(const FunctionSig* sig, uint32_t supertype = kNoSuperType) {
     mod.add_signature(sig, supertype);
     CHECK_LE(mod.types.size(), kMaxByteSizedLeb128);
+    GetTypeCanonicalizer()->AddRecursiveGroup(module(), 1);
     return static_cast<byte>(mod.types.size() - 1);
   }
   byte AddFunction(const FunctionSig* sig, bool declared = true) {
@@ -131,12 +133,14 @@ class TestModuleBuilder {
       type_builder.AddField(field.first, field.second);
     }
     mod.add_struct_type(type_builder.Build(), supertype);
+    GetTypeCanonicalizer()->AddRecursiveGroup(module(), 1);
     return static_cast<byte>(mod.types.size() - 1);
   }
 
   byte AddArray(ValueType type, bool mutability) {
     ArrayType* array = mod.signature_zone->New<ArrayType>(type, mutability);
     mod.add_array_type(array, kNoSuperType);
+    GetTypeCanonicalizer()->AddRecursiveGroup(module(), 1);
     return static_cast<byte>(mod.types.size() - 1);
   }
 
@@ -3639,32 +3643,6 @@ TEST_F(FunctionBodyDecoderTest, StructNewDefaultWithRtt) {
   }
 }
 
-TEST_F(FunctionBodyDecoderTest, NominalStructSubtyping) {
-  WASM_FEATURE_SCOPE(typed_funcref);
-  WASM_FEATURE_SCOPE(gc);
-  byte structural_type = builder.AddStruct({F(kWasmI32, true)});
-  byte nominal_type = builder.AddStruct({F(kWasmI32, true)});
-  AddLocals(optref(structural_type), 1);
-  AddLocals(optref(nominal_type), 1);
-  // Try to assign a nominally-typed value to a structurally-typed local.
-  ExpectFailure(sigs.v_v(),
-                {WASM_LOCAL_SET(0, WASM_STRUCT_NEW_DEFAULT(nominal_type))},
-                kAppendEnd, "expected type (ref null 0)");
-  // Try to assign a structurally-typed value to a nominally-typed local.
-  ExpectFailure(sigs.v_v(),
-                {WASM_LOCAL_SET(
-                    1, WASM_STRUCT_NEW_DEFAULT_WITH_RTT(
-                           structural_type, WASM_RTT_CANON(structural_type)))},
-                kAppendEnd, "expected type (ref null 1)");
-  // But assigning to the correctly typed local works.
-  ExpectValidates(sigs.v_v(),
-                  {WASM_LOCAL_SET(1, WASM_STRUCT_NEW_DEFAULT(nominal_type))});
-  ExpectValidates(sigs.v_v(),
-                  {WASM_LOCAL_SET(0, WASM_STRUCT_NEW_DEFAULT_WITH_RTT(
-                                         structural_type,
-                                         WASM_RTT_CANON(structural_type)))});
-}
-
 TEST_F(FunctionBodyDecoderTest, DefaultableLocal) {
   WASM_FEATURE_SCOPE(typed_funcref);
   AddLocals(kWasmAnyRef, 1);

test/unittests/wasm/module-decoder-unittest.cc

@@ -169,7 +169,9 @@ namespace module_decoder_unittest {
 #define EXPECT_NOT_OK(result, msg)                           \
   do {                                                       \
     EXPECT_FALSE(result.ok());                               \
+    if (!result.ok()) {                                      \
       EXPECT_THAT(result.error().message(), HasSubstr(msg)); \
+    }                                                        \
   } while (false)
 
 static size_t SizeOfVarInt(size_t value) {
@@ -803,7 +805,7 @@ TEST_F(WasmModuleVerifyTest, RttCanonGlobalTypeError) {
   static const byte data[] = {
       SECTION(Type, ENTRY_COUNT(2),
               WASM_STRUCT_DEF(FIELD_COUNT(1), STRUCT_FIELD(kI32Code, true)),
-              WASM_STRUCT_DEF(FIELD_COUNT(1), STRUCT_FIELD(kI32Code, true))),
+              WASM_STRUCT_DEF(FIELD_COUNT(1), STRUCT_FIELD(kI64Code, true))),
       SECTION(Global, ENTRY_COUNT(1), WASM_RTT(0), 1, WASM_RTT_CANON(1),
               kExprEnd)};
   ModuleResult result = DecodeModule(data, data + sizeof(data));
@@ -1189,6 +1191,46 @@ TEST_F(WasmModuleVerifyTest, InvalidArrayTypeDef) {
   EXPECT_VERIFIES(immutable);
 }
 
+TEST_F(WasmModuleVerifyTest, TypeCanonicalization) {
+  WASM_FEATURE_SCOPE(typed_funcref);
+  WASM_FEATURE_SCOPE(gc);
+  FLAG_SCOPE(wasm_type_canonicalization);
+  static const byte identical_group[] = {
+      SECTION(Type,            // --
+              ENTRY_COUNT(2),  // two identical rec. groups
+              kWasmRecursiveTypeGroupCode, ENTRY_COUNT(1),  // --
+              kWasmArrayTypeCode, kI32Code, 0,              // --
+              kWasmRecursiveTypeGroupCode, ENTRY_COUNT(1),  // --
+              kWasmArrayTypeCode, kI32Code, 0),
+      SECTION(Global,                          // --
+              ENTRY_COUNT(1), kRefCode, 0, 0,  // Type, mutability
+              WASM_ARRAY_INIT_STATIC(1, 1, WASM_I32V(10)),
+              kExprEnd)  // Init. expression
+  };
+
+  // Global initializer should verify as identical type in other group
+  EXPECT_VERIFIES(identical_group);
+
+  static const byte non_identical_group[] = {
+      SECTION(Type,            // --
+              ENTRY_COUNT(2),  // two distrinct rec. groups
+              kWasmRecursiveTypeGroupCode, ENTRY_COUNT(1),  // --
+              kWasmArrayTypeCode, kI32Code, 0,              // --
+              kWasmRecursiveTypeGroupCode, ENTRY_COUNT(2),  // --
+              kWasmArrayTypeCode, kI32Code, 0,              // --
+              kWasmStructTypeCode, ENTRY_COUNT(0)),
+      SECTION(Global,                          // --
+              ENTRY_COUNT(1), kRefCode, 0, 0,  // Type, mutability
+              WASM_ARRAY_INIT_STATIC(1, 1, WASM_I32V(10)),
+              kExprEnd)  // Init. expression
+  };
+
+  // Global initializer should not verify as type in distinct rec. group.
+  EXPECT_FAILURE_WITH_MSG(
+      non_identical_group,
+      "type error in init. expression[0] (expected (ref 0), got (ref 1))");
+}
+
 TEST_F(WasmModuleVerifyTest, ZeroExceptions) {
   static const byte data[] = {SECTION(Tag, ENTRY_COUNT(0))};
   FAIL_IF_NO_EXPERIMENTAL_EH(data);
@@ -3389,13 +3431,13 @@ TEST_F(WasmModuleVerifyTest, DataCountSegmentCount_omitted) {
   EXPECT_NOT_OK(result, "data segments count 0 mismatch (1 expected)");
 }
 
-/* TODO(7748): Add support for rec. groups.
 TEST_F(WasmModuleVerifyTest, GcStructIdsPass) {
   WASM_FEATURE_SCOPE(gc);
   WASM_FEATURE_SCOPE(typed_funcref);
 
   static const byte data[] = {SECTION(
-      Type, ENTRY_COUNT(3),
+      Type, ENTRY_COUNT(1),                         // One recursive group...
+      kWasmRecursiveTypeGroupCode, ENTRY_COUNT(3),  // with three entries.
       WASM_STRUCT_DEF(FIELD_COUNT(3), STRUCT_FIELD(kI32Code, true),
                       STRUCT_FIELD(WASM_OPT_REF(0), true),
                       STRUCT_FIELD(WASM_OPT_REF(1), true)),
@@ -3404,7 +3446,7 @@ TEST_F(WasmModuleVerifyTest, GcStructIdsPass) {
       WASM_ARRAY_DEF(WASM_OPT_REF(0), true))};
   ModuleResult result = DecodeModule(data, data + sizeof(data));
   EXPECT_OK(result);
-}*/
+}
 
 TEST_F(WasmModuleVerifyTest, OutOfBoundsTypeInGlobal) {
   WASM_FEATURE_SCOPE(typed_funcref);
@@ -3424,7 +3466,6 @@ TEST_F(WasmModuleVerifyTest, OutOfBoundsTypeInType) {
   EXPECT_NOT_OK(result, "Type index 1 is out of bounds");
 }
 
-// TODO(7748): Add support for rec. groups.
 TEST_F(WasmModuleVerifyTest, ForwardSupertype) {
   WASM_FEATURE_SCOPE(typed_funcref);
   WASM_FEATURE_SCOPE(gc);

test/unittests/wasm/subtyping-unittest.cc

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "src/wasm/canonical-types.h"
 #include "src/wasm/wasm-subtyping.h"
+#include "test/common/flag-utils.h"
 #include "test/common/wasm/flag-utils.h"
 #include "test/unittests/test-utils.h"
 
@@ -25,29 +27,41 @@ FieldInit mut(ValueType type) { return FieldInit(type, true); }
 FieldInit immut(ValueType type) { return FieldInit(type, false); }
 
 void DefineStruct(WasmModule* module, std::initializer_list<FieldInit> fields,
-                  uint32_t supertype = kNoSuperType) {
+                  uint32_t supertype = kNoSuperType,
+                  bool in_singleton_rec_group = true) {
   StructType::Builder builder(module->signature_zone.get(),
                               static_cast<uint32_t>(fields.size()));
   for (FieldInit field : fields) {
     builder.AddField(field.first, field.second);
   }
-  return module->add_struct_type(builder.Build(), supertype);
+  module->add_struct_type(builder.Build(), supertype);
+  if (in_singleton_rec_group) {
+    GetTypeCanonicalizer()->AddRecursiveGroup(module, 1);
+  }
 }
 
 void DefineArray(WasmModule* module, FieldInit element_type,
-                 uint32_t supertype = kNoSuperType) {
+                 uint32_t supertype = kNoSuperType,
+                 bool in_singleton_rec_group = true) {
   module->add_array_type(module->signature_zone->New<ArrayType>(
                              element_type.first, element_type.second),
                          supertype);
+  if (in_singleton_rec_group) {
+    GetTypeCanonicalizer()->AddRecursiveGroup(module, 1);
+  }
 }
 
 void DefineSignature(WasmModule* module,
                      std::initializer_list<ValueType> params,
                      std::initializer_list<ValueType> returns,
-                     uint32_t supertype = kNoSuperType) {
+                     uint32_t supertype = kNoSuperType,
+                     bool in_singleton_rec_group = true) {
   module->add_signature(
       FunctionSig::Build(module->signature_zone.get(), returns, params),
       supertype);
+  if (in_singleton_rec_group) {
+    GetTypeCanonicalizer()->AddRecursiveGroup(module, 1);
+  }
 }
 
 TEST_F(WasmSubtypingTest, Subtyping) {
@@ -79,6 +93,37 @@ TEST_F(WasmSubtypingTest, Subtyping) {
     /* 14 */ DefineSignature(module, {ref(0)}, {kWasmI32}, 13);
     /* 15 */ DefineSignature(module, {ref(0)}, {ref(4)}, 16);
     /* 16 */ DefineSignature(module, {ref(0)}, {ref(0)});
+    /* 17 */ DefineStruct(module, {mut(kWasmI32), immut(optRef(17))});
+
+    // Rec. group.
+    /* 18 */ DefineStruct(module, {mut(kWasmI32), immut(optRef(17))}, 17,
+                          false);
+    /* 19 */ DefineArray(module, {mut(optRef(21))}, kNoSuperType, false);
+    /* 20 */ DefineSignature(module, {kWasmI32}, {kWasmI32}, kNoSuperType,
+                             false);
+    /* 21 */ DefineSignature(module, {kWasmI32}, {kWasmI32}, 20, false);
+    GetTypeCanonicalizer()->AddRecursiveGroup(module, 4);
+
+    // Identical rec. group.
+    /* 22 */ DefineStruct(module, {mut(kWasmI32), immut(optRef(17))}, 17,
+                          false);
+    /* 23 */ DefineArray(module, {mut(optRef(25))}, kNoSuperType, false);
+    /* 24 */ DefineSignature(module, {kWasmI32}, {kWasmI32}, kNoSuperType,
+                             false);
+    /* 25 */ DefineSignature(module, {kWasmI32}, {kWasmI32}, 24, false);
+    GetTypeCanonicalizer()->AddRecursiveGroup(module, 4);
+
+    // Nonidentical rec. group: the last function extends a type outside the
+    // recursive group.
+    /* 26 */ DefineStruct(module, {mut(kWasmI32), immut(optRef(17))}, 17,
+                          false);
+    /* 27 */ DefineArray(module, {mut(optRef(29))}, kNoSuperType, false);
+    /* 28 */ DefineSignature(module, {kWasmI32}, {kWasmI32}, kNoSuperType,
+                             false);
+    /* 29 */ DefineSignature(module, {kWasmI32}, {kWasmI32}, 20, false);
+    GetTypeCanonicalizer()->AddRecursiveGroup(module, 4);
+
+    /* 30 */ DefineStruct(module, {mut(kWasmI32), immut(optRef(18))}, 18);
   }
 
   constexpr ValueType numeric_types[] = {kWasmI32, kWasmI64, kWasmF32, kWasmF64,
@@ -88,6 +133,7 @@ TEST_F(WasmSubtypingTest, Subtyping) {
                                      optRef(0),    ref(0),        optRef(2),
                                      ref(2),       optRef(11),    ref(11)};
 
+// Some macros to help managing types and modules.
 #define SUBTYPE(type1, type2) \
   EXPECT_TRUE(IsSubtypeOf(type1, type2, module1, module))
 #define SUBTYPE_IFF(type1, type2, condition) \
@@ -102,10 +148,20 @@ TEST_F(WasmSubtypingTest, Subtyping) {
 #define NOT_VALID_SUBTYPE(type1, type2)                                     \
   EXPECT_FALSE(ValidSubtypeDefinition(type1.ref_index(), type2.ref_index(), \
                                       module1, module));
+#define IDENTICAL(index1, index2)                                         \
+  EXPECT_TRUE(EquivalentTypes(ValueType::Ref(index1, kNullable),          \
+                              ValueType::Ref(index2, kNullable), module1, \
+                              module));
+#define DISTINCT(index1, index2)                                           \
+  EXPECT_FALSE(EquivalentTypes(ValueType::Ref(index1, kNullable),          \
+                               ValueType::Ref(index2, kNullable), module1, \
+                               module));
 
+  for (WasmModule* module : {module1, module2}) {
+    // For cross module subtyping, we need to enable type canonicalization.
     // Type judgements across modules should work the same as within one module.
-  // TODO(7748): add module2 once we have a cross-module story.
-  for (WasmModule* module : {module1 /* , module2 */}) {
+    FLAG_VALUE_SCOPE(wasm_type_canonicalization, module == module2);
+
     // Value types are unrelated, except if they are equal.
     for (ValueType subtype : numeric_types) {
       for (ValueType supertype : numeric_types) {
@@ -183,9 +239,6 @@ TEST_F(WasmSubtypingTest, Subtyping) {
     SUBTYPE(ValueType::Rtt(5), ValueType::Rtt(5));
     // Rtts of unrelated types are unrelated.
     NOT_SUBTYPE(ValueType::Rtt(1), ValueType::Rtt(2));
-    // Rtts of identical types are subtype-related.
-    // TODO(7748): Implement type canonicalization.
-    // SUBTYPE(ValueType::Rtt(8), ValueType::Rtt(9));
     // Rtts of subtypes are not related.
     NOT_SUBTYPE(ValueType::Rtt(1), ValueType::Rtt(0));
 
@@ -201,10 +254,42 @@ TEST_F(WasmSubtypingTest, Subtyping) {
     // Identical types are subtype-related.
     VALID_SUBTYPE(ref(10), ref(10));
     VALID_SUBTYPE(ref(11), ref(11));
+
+    {
+      // Canonicalization tests.
+      FLAG_SCOPE(wasm_type_canonicalization);
+
+      // Groups should only be canonicalized to identical groups.
+      IDENTICAL(18, 22);
+      IDENTICAL(19, 23);
+      IDENTICAL(20, 24);
+      IDENTICAL(21, 25);
+
+      DISTINCT(18, 26);
+      DISTINCT(19, 27);
+      DISTINCT(20, 28);
+      DISTINCT(21, 29);
+
+      // A type should not be canonicalized to an identical one with a different
+      // group structure.
+      DISTINCT(18, 17);
+
+      // A subtype should also be subtype of an equivalent type.
+      VALID_SUBTYPE(ref(30), ref(18));
+      VALID_SUBTYPE(ref(30), ref(22));
+      NOT_SUBTYPE(ref(30), ref(26));
+
+      // Rtts of identical types are subtype-related.
+      SUBTYPE(ValueType::Rtt(8), ValueType::Rtt(17));
+    }
   }
 #undef SUBTYPE
 #undef NOT_SUBTYPE
 #undef SUBTYPE_IFF
+#undef VALID_SUBTYPE
+#undef NOT_VALID_SUBTYPE
+#undef IDENTICAL
+#undef DISTINCT
 }
 
 }  // namespace subtyping_unittest