Add map-based read barrier to WeakCell

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index dd7bbfdb28460f825e7982f69aad9f618073bbfe..7a42356c04d91099574617779c075441eb83a3a5 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -6050,6 +6050,8 @@ class HObjectAccess FINAL {
     return ExistingInobjectPropertyField::decode(value_);
   }
 
+  inline bool read_barrier() const { return ReadBarrierField::decode(value_); }
+
   inline HObjectAccess WithRepresentation(Representation representation) {
     return HObjectAccess(portion(), offset(), representation, name(),
                          immutable(), existing_inobject_property());
@@ -6216,22 +6218,28 @@ class HObjectAccess FINAL {
                          Representation::UInteger16());
   }
 
-  static HObjectAccess ForPropertyCellValue() {
-    return HObjectAccess(kInobject, PropertyCell::kValueOffset);
-  }
-
-  static HObjectAccess ForCellValue() {
-    return HObjectAccess(kInobject, Cell::kValueOffset);
-  }
-
   static HObjectAccess ForWeakCellValue() {
-    return HObjectAccess(kInobject, WeakCell::kValueOffset);
+    return HObjectAccess(
+        kInobject, WeakCell::kValueOffsetDontForgetTheReadBarrier,
+        Representation::Tagged(), Handle<String>::null(), false, true, true);
   }
 
   static HObjectAccess ForWeakCellNext() {
     return HObjectAccess(kInobject, WeakCell::kNextOffset);
   }
 
+  // This version of the weak cell value does not trigger the read barrier.
+  // This means that for correctness you can only use the value for an
+  // immediate equality comparison, typically a map check. The GC is not
+  // allowed to see the value eg through a spill to the stack.
+  // TODO(erikcorry): When the SATB collector is actually running these
+  // need to be marked as untagged data when they are spilled to the stack.
+  static HObjectAccess ForWeakCellValueComparison() {
+    return HObjectAccess(
+        kInobject, WeakCell::kValueOffsetDontForgetTheReadBarrier,
+        Representation::Tagged(), Handle<String>::null(), false, true, false);
+  }
+
   static HObjectAccess ForAllocationMementoSite() {
     return HObjectAccess(kInobject, AllocationMemento::kAllocationSiteOffset);
   }
@@ -6403,15 +6411,16 @@ class HObjectAccess FINAL {
   HObjectAccess(Portion portion, int offset,
                 Representation representation = Representation::Tagged(),
                 Handle<String> name = Handle<String>::null(),
-                bool immutable = false,
-                bool existing_inobject_property = true)
-    : value_(PortionField::encode(portion) |
-             RepresentationField::encode(representation.kind()) |
-             ImmutableField::encode(immutable ? 1 : 0) |
-             ExistingInobjectPropertyField::encode(
-                 existing_inobject_property ? 1 : 0) |
-             OffsetField::encode(offset)),
-      name_(name) {
+                bool immutable = false, bool existing_inobject_property = true,
+                bool read_barrier = false)
+      : value_(PortionField::encode(portion) |
+               RepresentationField::encode(representation.kind()) |
+               ImmutableField::encode(immutable ? 1 : 0) |
+               ExistingInobjectPropertyField::encode(
+                   existing_inobject_property ? 1 : 0) |
+               ReadBarrierField::encode(read_barrier ? 1 : 0) |
+               OffsetField::encode(offset)),
+        name_(name) {
     // assert that the fields decode correctly
     DCHECK(this->offset() == offset);
     DCHECK(this->portion() == portion);
@@ -6425,7 +6434,8 @@ class HObjectAccess FINAL {
   class RepresentationField : public BitField<Representation::Kind, 3, 4> {};
   class ImmutableField : public BitField<bool, 7, 1> {};
   class ExistingInobjectPropertyField : public BitField<bool, 8, 1> {};
-  class OffsetField : public BitField<int, 9, 23> {};
+  class ReadBarrierField : public BitField<bool, 9, 1> {};
+  class OffsetField : public BitField<int, 10, 22> {};
 
   uint32_t value_;  // encodes portion, representation, immutable, and offset
   Handle<String> name_;