[wasm][interpreter] Remove the ReferenceStackScope

The reference stack was set by the scope, and reset when leaving the
scope, in order to avoid leaking objects via cycles in the reference
tree, involving global handles which are considered strong roots.
Since the interpreter cannot call out to JS any more, we cannot create
such cycles any more. Hence, the ReferenceStackScope is removed, and the
FixedArray for the reference stack is allocated as a global handle
instead.

This will unblock removing the WasmDebugInfo object, which was used by
the ReferenceStackScope before this CL.

R=ahaas@chromium.org

Bug: v8:10389
Change-Id: I2e3c6a03750846679eecd9e6a07042db962aad9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235542Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68257}

src/wasm/wasm-debug.cc

@@ -1017,11 +1017,9 @@ void DebugInfo::RemoveIsolate(Isolate* isolate) {
 Handle<WasmDebugInfo> WasmDebugInfo::New(Handle<WasmInstanceObject> instance) {
   DCHECK(!instance->has_debug_info());
   Factory* factory = instance->GetIsolate()->factory();
-  Handle<Cell> stack_cell = factory->NewCell(factory->empty_fixed_array());
   Handle<WasmDebugInfo> debug_info = Handle<WasmDebugInfo>::cast(
       factory->NewStruct(WASM_DEBUG_INFO_TYPE, AllocationType::kOld));
   debug_info->set_wasm_instance(*instance);
-  debug_info->set_interpreter_reference_stack(*stack_cell);
   instance->set_debug_info(*debug_info);
   return debug_info;
 }

src/wasm/wasm-interpreter.cc

@@ -1114,42 +1114,17 @@ class WasmInterpreterInternals {
         codemap_(module, module_bytes_.data(), zone),
         isolate_(instance_object->GetIsolate()),
         instance_object_(instance_object),
+        reference_stack_(isolate_->global_handles()->Create(
+            ReadOnlyRoots(isolate_).empty_fixed_array())),
         frames_(zone) {}
 
-  // The {ReferenceStackScope} sets up the reference stack in the interpreter.
-  // The handle to the reference stack has to be re-initialized everytime we
-  // call into the interpreter because there is no HandleScope that could
-  // contain that handle. A global handle is not an option because it can lead
-  // to a memory leak if a reference to the {WasmInstanceObject} is put onto the
-  // reference stack and thereby transitively keeps the interpreter alive.
-  class ReferenceStackScope {
-   public:
-    explicit ReferenceStackScope(WasmInterpreterInternals* impl) : impl_(impl) {
-      // The reference stack is already initialized, we don't have to do
-      // anything.
-      if (!impl_->reference_stack_cell_.is_null()) return;
-      impl_->reference_stack_cell_ = handle(
-          impl_->instance_object_->debug_info().interpreter_reference_stack(),
-          impl_->isolate_);
-      // We initialized the reference stack, so we also have to reset it later.
-      do_reset_stack_ = true;
-    }
-
-    ~ReferenceStackScope() {
-      if (do_reset_stack_) {
-        impl_->reference_stack_cell_ = Handle<Cell>();
-      }
-    }
-
-   private:
-    WasmInterpreterInternals* impl_;
-    bool do_reset_stack_ = false;
-  };
+  ~WasmInterpreterInternals() {
+    isolate_->global_handles()->Destroy(reference_stack_.location());
+  }
 
   WasmInterpreter::State state() { return state_; }
 
   void InitFrame(const WasmFunction* function, WasmValue* args) {
-    ReferenceStackScope stack_scope(this);
     DCHECK(frames_.empty());
     InterpreterCode* code = codemap_.GetCode(function);
     size_t num_params = function->sig->parameter_count();
@@ -1159,7 +1134,6 @@ class WasmInterpreterInternals {
   }
 
   WasmInterpreter::State Run(int num_steps = -1) {
-    ReferenceStackScope stack_scope(this);
     DCHECK(state_ == WasmInterpreter::STOPPED ||
            state_ == WasmInterpreter::PAUSED);
     DCHECK(num_steps == -1 || num_steps > 0);
@@ -1180,7 +1154,6 @@ class WasmInterpreterInternals {
   void Pause() { UNIMPLEMENTED(); }
 
   void Reset() {
-    ReferenceStackScope stack_scope(this);
     TRACE("----- RESET -----\n");
     ResetStack(0);
     frames_.clear();
@@ -1190,7 +1163,6 @@ class WasmInterpreterInternals {
   }
 
   WasmValue GetReturnValue(uint32_t index) {
-    ReferenceStackScope stack_scope(this);
     if (state_ == WasmInterpreter::TRAPPED) return WasmValue(0xDEADBEEF);
     DCHECK_EQ(WasmInterpreter::FINISHED, state_);
     return GetStackValue(index);
@@ -1212,11 +1184,8 @@ class WasmInterpreterInternals {
 
   uint64_t NumInterpretedCalls() { return num_interpreted_calls_; }
 
-  Handle<Cell> reference_stack_cell() const { return reference_stack_cell_; }
-
   WasmInterpreter::ExceptionHandlingResult RaiseException(
       Isolate* isolate, Handle<Object> exception) {
-    ReferenceStackScope stack_scope(this);
     DCHECK_EQ(WasmInterpreter::TRAPPED, state_);
     isolate->Throw(*exception);  // Will check that none is pending.
     if (HandleException(isolate) == WasmInterpreter::UNWOUND) {
@@ -1230,8 +1199,6 @@ class WasmInterpreterInternals {
   CodeMap* codemap() { return &codemap_; }
 
  private:
-  friend class ReferenceStackScope;
-
   // Handle a thrown exception. Returns whether the exception was handled inside
   // of wasm. Unwinds the interpreted stack accordingly.
   WasmInterpreter::ExceptionHandlingResult HandleException(Isolate* isolate) {
@@ -1287,7 +1254,7 @@ class WasmInterpreterInternals {
       if (IsReferenceValue()) {
         value_ = WasmValue(Handle<Object>::null());
         int ref_index = static_cast<int>(index);
-        impl->reference_stack().set(ref_index, *v.to_anyref());
+        impl->reference_stack_->set(ref_index, *v.to_anyref());
       }
     }
 
@@ -1296,7 +1263,7 @@ class WasmInterpreterInternals {
       DCHECK(value_.to_anyref().is_null());
       int ref_index = static_cast<int>(index);
       Isolate* isolate = impl->isolate_;
-      Handle<Object> ref(impl->reference_stack().get(ref_index), isolate);
+      Handle<Object> ref(impl->reference_stack_->get(ref_index), isolate);
       DCHECK(!ref->IsTheHole(isolate));
       return WasmValue(ref);
     }
@@ -1307,19 +1274,19 @@ class WasmInterpreterInternals {
       if (!IsReferenceValue()) return;
       int ref_index = static_cast<int>(index);
       Isolate* isolate = impl->isolate_;
-      impl->reference_stack().set_the_hole(isolate, ref_index);
+      impl->reference_stack_->set_the_hole(isolate, ref_index);
     }
 
     static void ClearValues(WasmInterpreterInternals* impl, sp_t index,
                             int count) {
       int ref_index = static_cast<int>(index);
-      impl->reference_stack().FillWithHoles(ref_index, ref_index + count);
+      impl->reference_stack_->FillWithHoles(ref_index, ref_index + count);
     }
 
     static bool IsClearedValue(WasmInterpreterInternals* impl, sp_t index) {
       int ref_index = static_cast<int>(index);
       Isolate* isolate = impl->isolate_;
-      return impl->reference_stack().is_the_hole(isolate, ref_index);
+      return impl->reference_stack_->is_the_hole(isolate, ref_index);
     }
 
    private:
@@ -1327,9 +1294,6 @@ class WasmInterpreterInternals {
   };
 
   const WasmModule* module() const { return codemap_.module(); }
-  FixedArray reference_stack() const {
-    return FixedArray::cast(reference_stack_cell_->value());
-  }
 
   void DoTrap(TrapReason trap, pc_t pc) {
     TRACE("TRAP: %s\n", WasmOpcodes::TrapReasonMessage(trap));
@@ -1533,7 +1497,7 @@ class WasmInterpreterInternals {
       StackValue* stack = stack_.get();
       memmove(stack + dest, stack + src, arity * sizeof(StackValue));
       // Also move elements on the reference stack accordingly.
-      reference_stack().MoveElements(
+      reference_stack_->MoveElements(
           isolate_, static_cast<int>(dest), static_cast<int>(src),
           static_cast<int>(arity), UPDATE_WRITE_BARRIER);
     }
@@ -3672,12 +3636,12 @@ class WasmInterpreterInternals {
     // Also resize the reference stack to the same size.
     int grow_by = static_cast<int>(new_size - old_size);
     HandleScope handle_scope(isolate_);  // Avoid leaking handles.
-    Handle<FixedArray> old_ref_stack(reference_stack(), isolate_);
     Handle<FixedArray> new_ref_stack =
-        isolate_->factory()->CopyFixedArrayAndGrow(old_ref_stack, grow_by);
+        isolate_->factory()->CopyFixedArrayAndGrow(reference_stack_, grow_by);
     new_ref_stack->FillWithHoles(static_cast<int>(old_size),
                                  static_cast<int>(new_size));
-    reference_stack_cell_->set_value(*new_ref_stack);
+    isolate_->global_handles()->Destroy(reference_stack_.location());
+    reference_stack_ = isolate_->global_handles()->Create(*new_ref_stack);
   }
 
   sp_t StackHeight() { return sp_ - stack_.get(); }
@@ -3813,10 +3777,8 @@ class WasmInterpreterInternals {
   std::unique_ptr<StackValue[]> stack_;
   StackValue* stack_limit_ = nullptr;  // End of allocated stack space.
   StackValue* sp_ = nullptr;           // Current stack pointer.
-  // The reference stack is pointed to by a {Cell} to be able to replace the
-  // underlying {FixedArray} when growing the stack. This avoids having to
-  // recreate or update the global handle keeping this object alive.
-  Handle<Cell> reference_stack_cell_;  // References are on an on-heap stack.
+  // References are on an on-heap stack.
+  Handle<FixedArray> reference_stack_;
   ZoneVector<Frame> frames_;
   WasmInterpreter::State state_ = WasmInterpreter::STOPPED;
   TrapReason trap_reason_ = kTrapCount;

src/wasm/wasm-objects-inl.h

@@ -394,8 +394,6 @@ ACCESSORS(WasmIndirectFunctionTable, refs, FixedArray, kRefsOffset)
 // WasmDebugInfo
 ACCESSORS(WasmDebugInfo, wasm_instance, WasmInstanceObject, kInstanceOffset)
 ACCESSORS(WasmDebugInfo, interpreter_handle, Object, kInterpreterHandleOffset)
-ACCESSORS(WasmDebugInfo, interpreter_reference_stack, Cell,
-          kInterpreterReferenceStackOffset)
 OPTIONAL_ACCESSORS(WasmDebugInfo, c_wasm_entries, FixedArray,
                    kCWasmEntriesOffset)
 OPTIONAL_ACCESSORS(WasmDebugInfo, c_wasm_entry_map, Managed<wasm::SignatureMap>,

src/wasm/wasm-objects.h

@@ -819,12 +819,12 @@ class WasmJSFunctionData : public Struct {
 
 // Debug info used for wasm debugging in the interpreter. For Liftoff debugging,
 // all information is held off-heap in {wasm::DebugInfo}.
+// TODO(clemensb): Remove this object.
 class WasmDebugInfo : public Struct {
  public:
   NEVER_READ_ONLY_SPACE
   DECL_ACCESSORS(wasm_instance, WasmInstanceObject)
   DECL_ACCESSORS(interpreter_handle, Object)  // Foreign or undefined
-  DECL_ACCESSORS(interpreter_reference_stack, Cell)
   DECL_OPTIONAL_ACCESSORS(c_wasm_entries, FixedArray)
   DECL_OPTIONAL_ACCESSORS(c_wasm_entry_map, Managed<wasm::SignatureMap>)
 

src/wasm/wasm-objects.tq

@@ -44,7 +44,6 @@ extern class WasmIndirectFunctionTable extends Struct {
 extern class WasmDebugInfo extends Struct {
   instance: WasmInstanceObject;
   interpreter_handle: Foreign|Undefined;
-  interpreter_reference_stack: Cell;
   c_wasm_entries: FixedArray|Undefined;
   c_wasm_entry_map: Foreign|Undefined;  // Managed<wasm::SignatureMap>
 }