Revert "[Context] Add a bit flag to indicate if extension might exist"

This reverts commit d7b67ce2.

Reason for revert: broke tsan https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8901789268326050304/+/steps/Check/0/logs/enumeration-order/0

Original change's description:
> [Context] Add a bit flag to indicate if extension might exist
> 
> Checking the bit flag instead of comparing pointers should improve performance.
> This will also allow us to remove the extension slot in Context and save memory.
> 
> Bug: v8:9744
> Change-Id: I7ab9feeadfb934955798d877d13bc0e1d78a191c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1814918
> Commit-Queue: Victor Gomes <victorgomes@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63906}

TBR=ulan@chromium.org,leszeks@chromium.org,victorgomes@google.com

Change-Id: I3d2261e24c9c7da5f5a1d49803361bc6f0770330
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:9744
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1816514Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63907}

src/codegen/code-stub-assembler.cc

@@ -2787,13 +2787,6 @@ TNode<Float64T> CodeStubAssembler::LoadDoubleWithHoleCheck(
   return UncheckedCast<Float64T>(Load(machine_type, base, offset));
 }
 
-TNode<BoolT> CodeStubAssembler::LoadContextHasExtensionField(
-    SloppyTNode<Context> context) {
-  TNode<IntPtrT> value =
-      LoadAndUntagObjectField(context, Context::kLengthOffset);
-  return IsSetWord<Context::HasExtensionField>(value);
-}
-
 TNode<Object> CodeStubAssembler::LoadContextElement(
     SloppyTNode<Context> context, int slot_index) {
   int offset = Context::SlotOffset(slot_index);

src/codegen/code-stub-assembler.h

@@ -1436,7 +1436,6 @@ class V8_EXPORT_PRIVATE CodeStubAssembler
                                           ElementsKind elements_kind);
 
   // Context manipulation
-  TNode<BoolT> LoadContextHasExtensionField(SloppyTNode<Context> context);
   TNode<Object> LoadContextElement(SloppyTNode<Context> context,
                                    int slot_index);
   TNode<Object> LoadContextElement(SloppyTNode<Context> context,

src/heap/factory.cc

@@ -1421,7 +1421,7 @@ Handle<Context> Factory::NewContext(RootIndex map_root_index, int size,
   Map map = Map::cast(isolate()->root(map_root_index));
   HeapObject result = AllocateRawWithImmortalMap(size, allocation, map);
   Handle<Context> context(Context::cast(result), isolate());
-  context->initialize_length_and_extension_bit(variadic_part_length);
+  context->set_length(variadic_part_length);
   DCHECK_EQ(context->SizeFromMap(map), size);
   if (size > Context::kTodoHeaderSize) {
     ObjectSlot start = context->RawField(Context::kTodoHeaderSize);

src/interpreter/interpreter-assembler.cc

@@ -205,32 +205,27 @@ void InterpreterAssembler::GotoIfHasContextExtensionUpToDepth(
   TVARIABLE(Uint32T, cur_depth, depth);
 
   Label context_search(this, {&cur_depth, &cur_context});
-  Label no_extension(this);
 
   // Loop until the depth is 0.
   Goto(&context_search);
   BIND(&context_search);
   {
-    // Check if context has an extension slot
-    TNode<BoolT> has_extension =
-        LoadContextHasExtensionField(cur_context.value());
-    GotoIfNot(has_extension, &no_extension);
+    // TODO(leszeks): We only need to do this check if the context had a sloppy
+    // eval, we could pass in a context chain bitmask to figure out which
+    // contexts actually need to be checked.
 
-    // Jump to the target if the extension slot is not a hole.
     TNode<Object> extension_slot =
         LoadContextElement(cur_context.value(), Context::EXTENSION_INDEX);
-    Branch(TaggedNotEqual(extension_slot, TheHoleConstant()), target,
-           &no_extension);
 
-    BIND(&no_extension);
-    {
-      cur_depth = Unsigned(Int32Sub(cur_depth.value(), Int32Constant(1)));
-      cur_context = CAST(
-          LoadContextElement(cur_context.value(), Context::PREVIOUS_INDEX));
+    // Jump to the target if the extension slot is not a hole.
+    GotoIf(TaggedNotEqual(extension_slot, TheHoleConstant()), target);
 
-      GotoIf(Word32NotEqual(cur_depth.value(), Int32Constant(0)),
-             &context_search);
-    }
+    cur_depth = Unsigned(Int32Sub(cur_depth.value(), Int32Constant(1)));
+    cur_context =
+        CAST(LoadContextElement(cur_context.value(), Context::PREVIOUS_INDEX));
+
+    GotoIf(Word32NotEqual(cur_depth.value(), Int32Constant(0)),
+           &context_search);
   }
 }
 

src/objects/contexts-inl.h

@@ -47,23 +47,10 @@ Context ScriptContextTable::get_context(int i) const {
 OBJECT_CONSTRUCTORS_IMPL(Context, HeapObject)
 NEVER_READ_ONLY_SPACE_IMPL(Context)
 CAST_ACCESSOR(Context)
-SMI_ACCESSORS(Context, length_and_extension_flag, kLengthOffset)
+SMI_ACCESSORS(Context, length, kLengthOffset)
 
 CAST_ACCESSOR(NativeContext)
 
-int Context::length() const {
-  return LengthField::decode(length_and_extension_flag());
-}
-
-void Context::initialize_length_and_extension_bit(int len,
-                                                  Context::HasExtension flag) {
-  DCHECK(LengthField::is_valid(len));
-  int value = 0;
-  value = LengthField::update(value, len);
-  value = HasExtensionField::update(value, flag == Context::HasExtension::kYes);
-  set_length_and_extension_flag(value);
-}
-
 Object Context::get(int index) const {
   Isolate* isolate = GetIsolateForPtrCompr(*this);
   return get(isolate, index);
@@ -107,20 +94,11 @@ void Context::set_previous(Context context) { set(PREVIOUS_INDEX, context); }
 
 Object Context::next_context_link() { return get(Context::NEXT_CONTEXT_LINK); }
 
-bool Context::has_extension() {
-  return static_cast<bool>(
-             HasExtensionField::decode(length_and_extension_flag())) &&
-         !extension().IsTheHole();
-}
-
+bool Context::has_extension() { return !extension().IsTheHole(); }
 HeapObject Context::extension() {
   return HeapObject::cast(get(EXTENSION_INDEX));
 }
-void Context::set_extension(HeapObject object) {
-  set(EXTENSION_INDEX, object);
-  set_length_and_extension_flag(
-      HasExtensionField::update(length_and_extension_flag(), true));
-}
+void Context::set_extension(HeapObject object) { set(EXTENSION_INDEX, object); }
 
 NativeContext Context::native_context() const {
   Object result = get(NATIVE_CONTEXT_INDEX);

src/objects/contexts.h

@@ -453,15 +453,9 @@ class Context : public HeapObject {
 
   DECL_CAST(Context)
 
-  enum class HasExtension { kYes, kNo };
-
   // [length]: length of the context.
   V8_INLINE int length() const;
-  V8_INLINE void initialize_length_and_extension_bit(
-      int len, HasExtension flag = HasExtension::kNo);
-
-  using LengthField = BitField<int, 0, kSmiValueSize - 2>;
-  using HasExtensionField = BitField<int, kSmiValueSize - 2, 1>;
+  V8_INLINE void set_length(int value);
 
   // Setter and getter for elements.
   V8_INLINE Object get(int index) const;
@@ -668,7 +662,6 @@ class Context : public HeapObject {
 #endif
 
   OBJECT_CONSTRUCTORS(Context, HeapObject);
-  DECL_INT_ACCESSORS(length_and_extension_flag)
 };
 
 class NativeContext : public Context {