Add map-based read barrier to WeakCell

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index 3e6de5f0bc61f7ea0fc1ae51c437f4cdaadfbbaa..9efbcd2bc11a54ca95b2a800f0cfa01a1dda2655 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -2961,6 +2961,7 @@ void LCodeGen::DoLoadNamedField(LLoadNamedField* instr) {
   int offset = access.offset();
 
   if (access.IsExternalMemory()) {
+    DCHECK(!access.read_barrier());
     Register result = ToRegister(instr->result());
     MemOperand operand = instr->object()->IsConstantOperand()
         ? MemOperand::StaticVariable(ToExternalReference(
@@ -2972,6 +2973,7 @@ void LCodeGen::DoLoadNamedField(LLoadNamedField* instr) {
 
   Register object = ToRegister(instr->object());
   if (instr->hydrogen()->representation().IsDouble()) {
+    DCHECK(!access.read_barrier());
     XMMRegister result = ToDoubleRegister(instr->result());
     __ movsd(result, FieldOperand(object, offset));
     return;
@@ -2979,9 +2981,17 @@ void LCodeGen::DoLoadNamedField(LLoadNamedField* instr) {
 
   Register result = ToRegister(instr->result());
   if (!access.IsInobject()) {
+    DCHECK(!access.read_barrier());
     __ mov(result, FieldOperand(object, JSObject::kPropertiesOffset));
     object = result;
   }
+  if (access.read_barrier()) {
+    DCHECK(!result.is(object));
+    DCHECK(instr->hydrogen()->representation().IsTagged());
+    // Read barrier: Change the map to show we used the weak cell.
+    __ LoadHeapObject(result, factory()->used_weak_cell_map());
+    __ mov(FieldOperand(object, HeapObject::kMapOffset), result);
+  }
   __ Load(result, FieldOperand(object, offset), access.representation());
 }