[wasm] Make WasmDebugInfo::interpreted_functions a FixedArray

Store it as FixedArray instead of Object. Use the empty array as
uninitialized value instead of undefined.

R=titzer@chromium.org

Bug: v8:8015
Change-Id: I53275dc5fd2c3a0db1e021ee4da0fc0694e2053e
Reviewed-on: https://chromium-review.googlesource.com/1224372Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55867}

src/wasm/wasm-debug.cc

@@ -555,16 +555,17 @@ wasm::InterpreterHandle* GetInterpreterHandleOrNull(WasmDebugInfo* debug_info) {
 
 Handle<FixedArray> GetOrCreateInterpretedFunctions(
     Isolate* isolate, Handle<WasmDebugInfo> debug_info) {
-  Handle<Object> obj(debug_info->interpreted_functions(), isolate);
-  if (!obj->IsUndefined(isolate)) return Handle<FixedArray>::cast(obj);
-
+  Handle<FixedArray> arr(debug_info->interpreted_functions(), isolate);
   int num_functions = debug_info->wasm_instance()
                           ->module_object()
                           ->native_module()
                           ->num_functions();
-  Handle<FixedArray> new_arr = isolate->factory()->NewFixedArray(num_functions);
-  debug_info->set_interpreted_functions(*new_arr);
-  return new_arr;
+  if (arr->length() == 0 && num_functions > 0) {
+    arr = isolate->factory()->NewFixedArray(num_functions);
+    debug_info->set_interpreted_functions(*arr);
+  }
+  DCHECK_EQ(num_functions, arr->length());
+  return arr;
 }
 
 }  // namespace
@@ -575,6 +576,7 @@ Handle<WasmDebugInfo> WasmDebugInfo::New(Handle<WasmInstanceObject> instance) {
   Handle<WasmDebugInfo> debug_info = Handle<WasmDebugInfo>::cast(
       factory->NewStruct(WASM_DEBUG_INFO_TYPE, TENURED));
   debug_info->set_wasm_instance(*instance);
+  debug_info->set_interpreted_functions(*factory->empty_fixed_array());
   instance->set_debug_info(*debug_info);
   return debug_info;
 }

src/wasm/wasm-objects-inl.h

@@ -225,7 +225,7 @@ SMI_ACCESSORS(WasmExportedFunctionData, function_index, kFunctionIndexOffset)
 // WasmDebugInfo
 ACCESSORS(WasmDebugInfo, wasm_instance, WasmInstanceObject, kInstanceOffset)
 ACCESSORS(WasmDebugInfo, interpreter_handle, Object, kInterpreterHandleOffset)
-ACCESSORS(WasmDebugInfo, interpreted_functions, Object,
+ACCESSORS(WasmDebugInfo, interpreted_functions, FixedArray,
           kInterpretedFunctionsOffset)
 OPTIONAL_ACCESSORS(WasmDebugInfo, locals_names, FixedArray, kLocalsNamesOffset)
 OPTIONAL_ACCESSORS(WasmDebugInfo, c_wasm_entries, FixedArray,

src/wasm/wasm-objects.h

@@ -544,7 +544,7 @@ class WasmDebugInfo : public Struct, public NeverReadOnlySpaceObject {
  public:
   DECL_ACCESSORS(wasm_instance, WasmInstanceObject)
   DECL_ACCESSORS(interpreter_handle, Object);
-  DECL_ACCESSORS(interpreted_functions, Object);
+  DECL_ACCESSORS(interpreted_functions, FixedArray);
   DECL_OPTIONAL_ACCESSORS(locals_names, FixedArray)
   DECL_OPTIONAL_ACCESSORS(c_wasm_entries, FixedArray)
   DECL_OPTIONAL_ACCESSORS(c_wasm_entry_map, Managed<wasm::SignatureMap>)