Remove 'length' field from ScopeInfo

This change relands the last part of https://crrev.com/c/2601880 .

ScopeInfo has a vestigial 'length' field from when it used to be a
FixedArray. This change removes that field, which saves some memory.

More specifically:

- Make ScopeInfo inherit from HeapObject, not FixedArrayBase which
  supplied the 'length' field.
- Change FactoryBase::NewScopeInfo to allocate the updated object shape.
  It maintains the existing behavior of filling the newly-allocated
  object with undefined, even though that's not a valid ScopeInfo and
  further initialization is required.
- Change a few length computations to use HeapObject::kHeaderSize rather
  than FixedArray::kHeaderSize.
- Remove an unnecessary heap verifier function.

Change-Id: I9b3980157568fdb0402fa31660949966b401fd31
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2733037Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#73278}

src/diagnostics/objects-debug.cc

@@ -583,20 +583,6 @@ void Context::ContextVerify(Isolate* isolate) {
   }
 }
 
-void ScopeInfo::ScopeInfoVerify(Isolate* isolate) {
-  TorqueGeneratedClassVerifiers::ScopeInfoVerify(*this, isolate);
-
-  // Make sure that the FixedArray-style length matches the length that we would
-  // compute based on the Torque indexed fields.
-  CHECK_EQ(FixedArray::SizeFor(length()), AllocatedSize());
-
-  // Code that treats ScopeInfo like a FixedArray expects all values to be
-  // tagged.
-  for (int i = 0; i < length(); ++i) {
-    Object::VerifyPointer(isolate, get(isolate, i));
-  }
-}
-
 void NativeContext::NativeContextVerify(Isolate* isolate) {
   ContextVerify(isolate);
   CHECK_EQ(length(), NativeContext::NATIVE_CONTEXT_SLOTS);

src/heap/factory-base.cc

@@ -703,11 +703,13 @@ template <typename Impl>
 Handle<ScopeInfo> FactoryBase<Impl>::NewScopeInfo(int length,
                                                   AllocationType type) {
   DCHECK(type == AllocationType::kOld || type == AllocationType::kReadOnly);
-  Handle<HeapObject> result =
-      Handle<HeapObject>::cast(NewFixedArray(length, type));
-  result->set_map_after_allocation(*read_only_roots().scope_info_map_handle(),
-                                   SKIP_WRITE_BARRIER);
-  return Handle<ScopeInfo>::cast(result);
+  int size = ScopeInfo::SizeFor(length);
+  HeapObject obj = AllocateRawWithImmortalMap(
+      size, type, read_only_roots().scope_info_map());
+  ScopeInfo scope_info = ScopeInfo::cast(obj);
+  MemsetTagged(scope_info.data_start(), read_only_roots().undefined_value(),
+               length);
+  return handle(scope_info, isolate());
 }
 
 template <typename Impl>

src/heap/setup-heap-internal.cc

@@ -510,11 +510,10 @@ bool Heap::CreateInitialMaps() {
 
   {
     AllocationResult alloc =
-        AllocateRaw(FixedArray::SizeFor(ScopeInfo::kVariablePartIndex),
+        AllocateRaw(ScopeInfo::SizeFor(ScopeInfo::kVariablePartIndex),
                     AllocationType::kReadOnly);
     if (!alloc.To(&obj)) return false;
     obj.set_map_after_allocation(roots.scope_info_map(), SKIP_WRITE_BARRIER);
-    ScopeInfo::cast(obj).set_length(ScopeInfo::kVariablePartIndex);
     int flags = ScopeInfo::IsEmptyBit::encode(true);
     DCHECK_EQ(ScopeInfo::LanguageModeBit::decode(flags), LanguageMode::kSloppy);
     DCHECK_EQ(ScopeInfo::ReceiverVariableBits::decode(flags),

src/objects/scope-info.cc

@@ -616,7 +616,7 @@ int ScopeInfo::length() const {
   // AllocatedSize() is generated by Torque and represents the size in bytes of
   // the object, as computed from flags, context_local_count, and possibly
   // module_variable_count. Convert that size into a number of slots.
-  return (AllocatedSize() - FixedArray::kHeaderSize) / kTaggedSize;
+  return (AllocatedSize() - HeapObject::kHeaderSize) / kTaggedSize;
 }
 
 // static

src/objects/scope-info.h

@@ -38,13 +38,11 @@ class Zone;
 
 // This object provides quick access to scope info details for runtime
 // routines.
-class ScopeInfo : public TorqueGeneratedScopeInfo<ScopeInfo, FixedArrayBase> {
+class ScopeInfo : public TorqueGeneratedScopeInfo<ScopeInfo, HeapObject> {
  public:
   DEFINE_TORQUE_GENERATED_SCOPE_FLAGS()
 
   DECL_PRINTER(ScopeInfo)
-  DECL_VERIFIER(ScopeInfo)
-
   class BodyDescriptor;
 
   // Return the type of this scope.
@@ -310,10 +308,10 @@ class ScopeInfo : public TorqueGeneratedScopeInfo<ScopeInfo, FixedArrayBase> {
   // index (number of tagged-pointer-sized slots starting after the standard
   // HeapObject header).
   static constexpr int OffsetOfElementAt(int index) {
-    return FixedArray::kHeaderSize + index * kTaggedSize;
+    return HeapObject::kHeaderSize + index * kTaggedSize;
   }
   static constexpr int ConvertOffsetToIndex(int offset) {
-    int index = (offset - FixedArray::kHeaderSize) / kTaggedSize;
+    int index = (offset - HeapObject::kHeaderSize) / kTaggedSize;
     CONSTEXPR_DCHECK(OffsetOfElementAt(index) == offset);
     return index;
   }

src/objects/scope-info.tq

@@ -99,7 +99,7 @@ struct ModuleVariable {
 
 @generateCppClass
 @generateBodyDescriptor
-extern class ScopeInfo extends FixedArrayBase {
+extern class ScopeInfo extends HeapObject {
   const flags: SmiTagged<ScopeFlags>;
 
   // The number of parameters. For non-function scopes this is 0.