[wasm-gc] Fix empty structs bug

Drive-by: add const modifier to some StructType fields

Bug: v8:7748
Change-Id: Ic0b4bb51ed01502f19d082c669683f69b85e76e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2419015
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70029}

src/wasm/struct-types.h

@@ -62,10 +62,11 @@ class StructType : public ZoneObject {
     return field_offsets_[index - 1];
   }
   uint32_t total_fields_size() const {
-    return field_offsets_[field_count() - 1];
+    return field_count() == 0 ? 0 : field_offsets_[field_count() - 1];
   }
 
   void InitializeOffsets() {
+    if (field_count() == 0) return;
     uint32_t offset = field(0).element_size_bytes();
     for (uint32_t i = 1; i < field_count(); i++) {
       uint32_t field_size = field(i).element_size_bytes();
@@ -102,17 +103,17 @@ class StructType : public ZoneObject {
 
    private:
     const uint32_t field_count_;
-    Zone* zone_;
+    Zone* const zone_;
     uint32_t cursor_;
-    ValueType* buffer_;
-    bool* mutabilities_;
+    ValueType* const buffer_;
+    bool* const mutabilities_;
   };
 
  private:
-  uint32_t field_count_;
-  uint32_t* field_offsets_;
-  const ValueType* reps_;
-  const bool* mutabilities_;
+  const uint32_t field_count_;
+  uint32_t* const field_offsets_;
+  const ValueType* const reps_;
+  const bool* const mutabilities_;
 };
 
 class ArrayType : public ZoneObject {

test/cctest/wasm/test-gc.cc

@@ -181,9 +181,12 @@ TEST(WasmBasicStruct) {
   WasmGCTester tester;
   const byte type_index =
       tester.DefineStruct({F(kWasmI32, true), F(kWasmI32, true)});
-  ValueType kRefTypes[] = {ref(type_index)};
+  const byte empty_struct_index = tester.DefineStruct({});
+  ValueType kRefType = ref(type_index);
+  ValueType kEmptyStructType = ref(empty_struct_index);
   ValueType kOptRefType = optref(type_index);
-  FunctionSig sig_q_v(1, 0, kRefTypes);
+  FunctionSig sig_q_v(1, 0, &kRefType);
+  FunctionSig sig_qe_v(1, 0, &kEmptyStructType);
 
   // Test struct.new and struct.get.
   const byte kGet1 = tester.DefineFunction(
@@ -210,6 +213,13 @@ TEST(WasmBasicStruct) {
                                 WASM_RTT_CANON(type_index)),
        kExprEnd});
 
+  // Test struct.new, returning reference to an empty struct.
+  const byte kGetEmptyStruct = tester.DefineFunction(
+      &sig_qe_v, {},
+      {WASM_STRUCT_NEW_WITH_RTT(empty_struct_index,
+                                WASM_RTT_CANON(empty_struct_index)),
+       kExprEnd});
+
   // Test struct.set, struct refs types in locals.
   const byte j_local_index = 0;
   const byte j_field_index = 0;
@@ -230,6 +240,9 @@ TEST(WasmBasicStruct) {
   tester.CheckResult(kGet1, 42);
   tester.CheckResult(kGet2, 64);
   CHECK(tester.GetResultObject(kGetStruct).ToHandleChecked()->IsWasmStruct());
+  CHECK(tester.GetResultObject(kGetEmptyStruct)
+            .ToHandleChecked()
+            ->IsWasmStruct());
   tester.CheckResult(kSet, -99);
 }
 

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

@@ -834,6 +834,17 @@ TEST_F(WasmModuleVerifyTest, RttSubGlobalTypeError) {
                 "(rtt 2 i31)");
 }
 
+TEST_F(WasmModuleVerifyTest, EmptyStruct) {
+  WASM_FEATURE_SCOPE(reftypes);
+  WASM_FEATURE_SCOPE(typed_funcref);
+  WASM_FEATURE_SCOPE(gc);
+  static const byte empty_struct[] = {SECTION(Type, ENTRY_COUNT(1),  // --
+                                              kWasmStructTypeCode,   // --
+                                              U32V_1(0))};  // field count
+
+  EXPECT_VERIFIES(empty_struct);
+}
+
 TEST_F(WasmModuleVerifyTest, InvalidStructTypeDef) {
   WASM_FEATURE_SCOPE(reftypes);
   WASM_FEATURE_SCOPE(typed_funcref);