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

This is a reland of d7b67ce206

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}

Bug: v8:9744
Change-Id: Ic4725ad5730a8f8fff6288d6af2205c230aff79d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1815256
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@google.com>
Cr-Commit-Position: refs/heads/master@{#63993}

src/codegen/code-stub-assembler.cc

@@ -2717,6 +2717,13 @@ 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

@@ -1433,6 +1433,7 @@ class V8_EXPORT_PRIVATE CodeStubAssembler
   TNode<BigInt> BigIntFromUint32Pair(TNode<UintPtrT> low, TNode<UintPtrT> high);
 
   // 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

@@ -1422,7 +1422,7 @@ 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->set_length(variadic_part_length);
+  context->initialize_length_and_extension_bit(variadic_part_length);
   DCHECK_EQ(context->SizeFromMap(map), size);
   if (size > Context::kTodoHeaderSize) {
     ObjectSlot start = context->RawField(Context::kTodoHeaderSize);

src/interpreter/interpreter-assembler.cc

@@ -205,27 +205,32 @@ 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);
   {
-    // 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.
-
-    TNode<Object> extension_slot =
-        LoadContextElement(cur_context.value(), Context::EXTENSION_INDEX);
+    // Check if context has an extension slot
+    TNode<BoolT> has_extension =
+        LoadContextHasExtensionField(cur_context.value());
+    GotoIfNot(has_extension, &no_extension);
 
     // Jump to the target if the extension slot is not a hole.
-    GotoIf(TaggedNotEqual(extension_slot, TheHoleConstant()), target);
+    TNode<Object> extension_slot =
+        LoadContextElement(cur_context.value(), Context::EXTENSION_INDEX);
+    Branch(TaggedNotEqual(extension_slot, TheHoleConstant()), target,
+           &no_extension);
 
-    cur_depth = Unsigned(Int32Sub(cur_depth.value(), Int32Constant(1)));
-    cur_context =
-        CAST(LoadContextElement(cur_context.value(), Context::PREVIOUS_INDEX));
+    BIND(&no_extension);
+    {
+      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);
+      GotoIf(Word32NotEqual(cur_depth.value(), Int32Constant(0)),
+             &context_search);
+    }
   }
 }
 

src/objects/contexts-inl.h

@@ -47,10 +47,29 @@ 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, kLengthOffset)
+
+SMI_ACCESSORS(Context, length_and_extension_flag, kLengthOffset)
+SYNCHRONIZED_SMI_ACCESSORS(Context, length_and_extension_flag, kLengthOffset)
 
 CAST_ACCESSOR(NativeContext)
 
+int Context::length() const {
+  return LengthField::decode(length_and_extension_flag());
+}
+
+int Context::synchronized_length() const {
+  return LengthField::decode(synchronized_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);
@@ -94,11 +113,20 @@ 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 !extension().IsTheHole(); }
+bool Context::has_extension() {
+  return static_cast<bool>(
+             HasExtensionField::decode(length_and_extension_flag())) &&
+         !extension().IsTheHole();
+}
+
 HeapObject Context::extension() {
   return HeapObject::cast(get(EXTENSION_INDEX));
 }
-void Context::set_extension(HeapObject object) { set(EXTENSION_INDEX, object); }
+void Context::set_extension(HeapObject object) {
+  set(EXTENSION_INDEX, object);
+  synchronized_set_length_and_extension_flag(
+      HasExtensionField::update(length_and_extension_flag(), true));
+}
 
 NativeContext Context::native_context() const {
   Object result = get(NATIVE_CONTEXT_INDEX);

src/objects/contexts.h

@@ -453,9 +453,19 @@ 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 set_length(int value);
+  V8_INLINE int synchronized_length() const;
+  V8_INLINE void initialize_length_and_extension_bit(
+      int len, HasExtension flag = HasExtension::kNo);
+
+  // We use the 30th bit. Otherwise if we set the 31st bit,
+  // the number would be pottentially bigger than an SMI.
+  // Any DCHECK(Smi::IsValue(...)) would fail.
+  using LengthField = BitField<int, 0, kSmiValueSize - 2>;
+  using HasExtensionField = BitField<int, kSmiValueSize - 2, 1>;
 
   // Setter and getter for elements.
   V8_INLINE Object get(int index) const;
@@ -662,6 +672,8 @@ class Context : public HeapObject {
 #endif
 
   OBJECT_CONSTRUCTORS(Context, HeapObject);
+  DECL_INT_ACCESSORS(length_and_extension_flag)
+  DECL_SYNCHRONIZED_INT_ACCESSORS(length_and_extension_flag)
 };
 
 class NativeContext : public Context {

src/objects/objects.cc

@@ -2188,7 +2188,8 @@ int HeapObject::SizeFromMap(Map map) const {
   }
   if (IsInRange(instance_type, FIRST_CONTEXT_TYPE, LAST_CONTEXT_TYPE)) {
     if (instance_type == NATIVE_CONTEXT_TYPE) return NativeContext::kSize;
-    return Context::SizeFor(Context::unchecked_cast(*this).length());
+    return Context::SizeFor(
+        Context::unchecked_cast(*this).synchronized_length());
   }
   if (instance_type == ONE_BYTE_STRING_TYPE ||
       instance_type == ONE_BYTE_INTERNALIZED_STRING_TYPE) {