Reland "[torque] Don't generate k(?:Start|End)Of\w+FieldsOffset constants"

This is a reland of 7366f6e2

The test that failed after the initial commit was just flaky and has
been fixed; see https://bugs.chromium.org/p/v8/issues/detail?id=12341

Original change's description:
> [torque] Don't generate k(?:Start|End)Of\w+FieldsOffset constants
>
> Torque currently generates constants like kStartOfWeakFieldsOffset and
> kEndOfStrongFieldsOffset, which can be used when writing custom
> BodyDescriptors. However, these offsets have some potentially confusing
> behaviors:
>
> * They don't take inheritance into account and describe only the fields
>   defined by the current class itself, so there might be (for example)
>   strong fields before kStartOfStrongFieldsOffset if they were defined
>   by a superclass.
> * kStartOfWeakFieldsOffset points to the first field defined in Torque
>   using the keyword `weak`, which indicates fields with *custom*
>   weakness semantics (those that should be visited with
>   IterateCustomWeakPointers), not those that may contain standard weak
>   pointers (visited with IterateMaybeWeakPointers). (As a follow-up, I'd
>   like to also rename `weak` to `@customWeak`.)
>
> Given that these constants have very low usage and somewhat bizarre
> semantics, I propose that we remove them. This change does so, and
> updates the existing usages to either define the required constants
> directly in C++ or not use them. I know that defining these constants in
> C++ is more brittle, but I think that brittle and clear is better than
> automatic and incomprehensible.
>
> Bug: v8:7793
> Change-Id: I87f8c85ccae4027f61ac73d4e7e4e2820e92003b
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3199731
> Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
> Cr-Commit-Position: refs/heads/main@{#77411}

Bug: v8:7793
Change-Id: Iefdd4014ce4b85b48c19ead79a0316774a5ecd45
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3258082Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#77688}

src/diagnostics/objects-debug.cc

@@ -1653,8 +1653,7 @@ void WasmInstanceObject::WasmInstanceObjectVerify(Isolate* isolate) {
   // Just generically check all tagged fields. Don't check the untagged fields,
   // as some of them might still contain the "undefined" value if the
   // WasmInstanceObject is not fully set up yet.
-  for (int offset = kHeaderSize; offset < kEndOfStrongFieldsOffset;
-       offset += kTaggedSize) {
+  for (uint16_t offset : kTaggedFieldOffsets) {
     VerifyObjectField(isolate, offset);
   }
 }

src/heap/marking-visitor-inl.h

@@ -488,14 +488,14 @@ template <typename ConcreteVisitor, typename MarkingState>
 int MarkingVisitorBase<ConcreteVisitor, MarkingState>::VisitMap(Map meta_map,
                                                                 Map map) {
   if (!concrete_visitor()->ShouldVisit(map)) return 0;
-  int size = Map::BodyDescriptor::SizeOf(meta_map, map);
-  size += VisitDescriptorsForMap(map);
+  int map_size = Map::BodyDescriptor::SizeOf(meta_map, map);
+  int marked_size = map_size + VisitDescriptorsForMap(map);
 
   // Mark the pointer fields of the Map. If there is a transitions array, it has
   // been marked already, so it is fine that one of these fields contains a
   // pointer to it.
-  Map::BodyDescriptor::IterateBody(meta_map, map, size, this);
-  return size;
+  Map::BodyDescriptor::IterateBody(meta_map, map, map_size, this);
+  return marked_size;
 }
 
 template <typename ConcreteVisitor, typename MarkingState>

src/heap/objects-visiting.h

@@ -36,7 +36,6 @@ namespace internal {
   V(WeakCell)                           \
   V(JSWeakCollection)                   \
   V(JSWeakRef)                          \
-  V(Map)                                \
   V(NativeContext)                      \
   V(PreparseData)                       \
   V(PropertyArray)                      \

src/objects/contexts.h

@@ -472,6 +472,9 @@ class Context : public TorqueGeneratedContext<Context, HeapObject> {
   static const int kScopeInfoOffset = kElementsOffset;
   static const int kPreviousOffset = kScopeInfoOffset + kTaggedSize;
 
+  // All fields are tagged.
+  static const int kStartOfStrongFieldsOffset = HeapObject::kHeaderSize;
+
   /* Header size. */                                                  \
   /* TODO(ishell): use this as header size once MIN_CONTEXT_SLOTS */  \
   /* is removed in favour of offset-based access to common fields. */ \

src/objects/descriptor-array-inl.h

@@ -91,11 +91,10 @@ InternalIndex DescriptorArray::SearchWithCache(Isolate* isolate, Name name,
 }
 
 ObjectSlot DescriptorArray::GetFirstPointerSlot() {
-  static_assert(kEndOfStrongFieldsOffset == kStartOfWeakFieldsOffset,
-                "Weak and strong fields are continuous.");
-  static_assert(kEndOfWeakFieldsOffset == kHeaderSize,
-                "Weak fields extend up to the end of the header.");
-  return RawField(DescriptorArray::kStartOfStrongFieldsOffset);
+  static_assert(kEnumCacheOffset + kTaggedSize == kHeaderSize,
+                "The strong field for enum_cache is contiguous with the "
+                "variable-size portion.");
+  return RawField(DescriptorArray::kEnumCacheOffset);
 }
 
 ObjectSlot DescriptorArray::GetDescriptorSlot(int descriptor) {

src/objects/descriptor-array.h

@@ -144,7 +144,6 @@ class DescriptorArray
   // Constant for denoting key was not found.
   static const int kNotFound = -1;
 
-  STATIC_ASSERT(IsAligned(kStartOfWeakFieldsOffset, kTaggedSize));
   STATIC_ASSERT(IsAligned(kHeaderSize, kTaggedSize));
 
   // Garbage collection support.
@@ -164,10 +163,6 @@ class DescriptorArray
   inline ObjectSlot GetFirstPointerSlot();
   inline ObjectSlot GetDescriptorSlot(int descriptor);
 
-  static_assert(kEndOfStrongFieldsOffset == kStartOfWeakFieldsOffset,
-                "Weak fields follow strong fields.");
-  static_assert(kEndOfWeakFieldsOffset == kHeaderSize,
-                "Weak fields extend up to the end of the header.");
   static_assert(kDescriptorsOffset == kHeaderSize,
                 "Variable-size array follows header.");
   class BodyDescriptor;

src/objects/map.cc

@@ -170,9 +170,6 @@ VisitorId Map::GetVisitorId(Map map) {
     case FEEDBACK_METADATA_TYPE:
       return kVisitFeedbackMetadata;
 
-    case MAP_TYPE:
-      return kVisitMap;
-
     case CODE_TYPE:
       return kVisitCode;
 

src/objects/map.h

@@ -52,7 +52,6 @@ enum InstanceType : uint16_t;
   V(JSTypedArray)                       \
   V(JSWeakRef)                          \
   V(JSWeakCollection)                   \
-  V(Map)                                \
   V(NativeContext)                      \
   V(PreparseData)                       \
   V(PropertyArray)                      \

src/objects/map.tq

@@ -34,6 +34,7 @@ bitfield struct MapBitFields3 extends uint32 {
   construction_counter: int32: 3 bit;
 }
 
+@generateBodyDescriptor
 extern class Map extends HeapObject {
   macro PrototypeInfo(): PrototypeInfo labels HasNoPrototypeInfo {
     typeswitch (this.transitions_or_prototype_info) {
@@ -73,8 +74,8 @@ extern class Map extends HeapObject {
   instance_descriptors: DescriptorArray;
   dependent_code: DependentCode;
   prototype_validity_cell: Smi|Cell;
-  weak transitions_or_prototype_info: Map|Weak<Map>|TransitionArray|
-      PrototypeInfo|Smi;
+  transitions_or_prototype_info: Map|Weak<Map>|TransitionArray|PrototypeInfo|
+      Smi;
 }
 
 @export

src/objects/objects-body-descriptors-inl.h

@@ -862,28 +862,6 @@ class Code::BodyDescriptor final : public BodyDescriptorBase {
   }
 };
 
-class Map::BodyDescriptor final : public BodyDescriptorBase {
- public:
-  static bool IsValidSlot(Map map, HeapObject obj, int offset) {
-    static_assert(
-        Map::kEndOfStrongFieldsOffset == Map::kStartOfWeakFieldsOffset,
-        "Leverage that weak fields directly follow strong fields for the "
-        "check below");
-    return offset >= Map::kStartOfStrongFieldsOffset &&
-           offset < Map::kEndOfWeakFieldsOffset;
-  }
-
-  template <typename ObjectVisitor>
-  static inline void IterateBody(Map map, HeapObject obj, int object_size,
-                                 ObjectVisitor* v) {
-    IteratePointers(obj, Map::kStartOfStrongFieldsOffset,
-                    Map::kEndOfStrongFieldsOffset, v);
-    IterateMaybeWeakPointer(obj, kTransitionsOrPrototypeInfoOffset, v);
-  }
-
-  static inline int SizeOf(Map map, HeapObject obj) { return Map::kSize; }
-};
-
 class DataHandler::BodyDescriptor final : public BodyDescriptorBase {
  public:
   static bool IsValidSlot(Map map, HeapObject obj, int offset) {
@@ -1193,8 +1171,6 @@ ReturnType BodyDescriptorApply(InstanceType type, T1 p1, T2 p2, T3 p3, T4 p4) {
       return Op::template apply<JSProxy::BodyDescriptor>(p1, p2, p3, p4);
     case FOREIGN_TYPE:
       return Op::template apply<Foreign::BodyDescriptor>(p1, p2, p3, p4);
-    case MAP_TYPE:
-      return Op::template apply<Map::BodyDescriptor>(p1, p2, p3, p4);
     case CODE_TYPE:
       return Op::template apply<Code::BodyDescriptor>(p1, p2, p3, p4);
     case CELL_TYPE:

src/torque/implementation-visitor.cc

@@ -3767,13 +3767,10 @@ class FieldOffsetsGenerator {
   }
   void Begin(FieldSectionType type) {
     DCHECK(type != FieldSectionType::kNoSection);
-    if (!IsPointerSection(type)) return;
-    WriteMarker("kStartOf" + ToString(type) + "Offset");
   }
   void End(FieldSectionType type) {
     if (!IsPointerSection(type)) return;
     completed_sections_ |= type;
-    WriteMarker("kEndOf" + ToString(type) + "Offset");
   }
 
   FieldSectionType current_section_ = FieldSectionType::kNoSection;

src/torque/types.cc

@@ -603,6 +603,7 @@ void ComputeSlotKindsHelper(std::vector<ObjectSlotKind>* slots,
   size_t offset = start_offset;
   for (const Field& field : fields) {
     size_t field_size = std::get<0>(field.GetFieldSizeInformation());
+    if (field_size == 0) continue;
     size_t slot_index = offset / TargetArchitecture::TaggedSize();
     // Rounding-up division to find the number of slots occupied by all the
     // fields up to and including the current one.

src/wasm/wasm-objects.h

@@ -697,7 +697,8 @@ class WasmIndirectFunctionTable
 
   DECL_PRINTER(WasmIndirectFunctionTable)
 
-  STATIC_ASSERT(kStartOfStrongFieldsOffset == kManagedNativeAllocationsOffset);
+  static constexpr int kStartOfStrongFieldsOffset =
+      kManagedNativeAllocationsOffset;
   using BodyDescriptor = FlexibleBodyDescriptor<kStartOfStrongFieldsOffset>;
 
   TQ_OBJECT_CONSTRUCTORS(WasmIndirectFunctionTable)
@@ -711,6 +712,9 @@ class WasmFunctionData
 
   DECL_PRINTER(WasmFunctionData)
 
+  // All fields after those inherited from Foreign are tagged.
+  static constexpr int kStartOfStrongFieldsOffset = Foreign::kHeaderSize;
+
   TQ_OBJECT_CONSTRUCTORS(WasmFunctionData)
 };
 
@@ -729,6 +733,8 @@ class WasmExportedFunctionData
 
   class BodyDescriptor;
 
+  static constexpr int kEndOfStrongFieldsOffset = kHeaderSize;
+
   TQ_OBJECT_CONSTRUCTORS(WasmExportedFunctionData)
 };
 
@@ -740,6 +746,9 @@ class WasmApiFunctionRef
 
   class BodyDescriptor;
 
+  static constexpr int kStartOfStrongFieldsOffset = Foreign::kHeaderSize;
+  static constexpr int kEndOfStrongFieldsOffset = kHeaderSize;
+
   TQ_OBJECT_CONSTRUCTORS(WasmApiFunctionRef)
 };
 
@@ -757,6 +766,8 @@ class WasmJSFunctionData
 
   class BodyDescriptor;
 
+  static constexpr int kEndOfStrongFieldsOffset = kHeaderSize;
+
  private:
   DECL_ACCESSORS(raw_wasm_to_js_wrapper_code, CodeT)
 
@@ -771,6 +782,8 @@ class WasmCapiFunctionData
 
   class BodyDescriptor;
 
+  static constexpr int kEndOfStrongFieldsOffset = kHeaderSize;
+
   TQ_OBJECT_CONSTRUCTORS(WasmCapiFunctionData)
 };