Add map-based read barrier to WeakCell

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
diff --git a/src/ia32/macro-assembler-ia32.cc b/src/ia32/macro-assembler-ia32.cc
index 53ffa39357c97a47aa53df3f934a712fb30e9380..973bdd5874ea3b59c99e59adfe4f96324a970f51 100644
--- a/src/ia32/macro-assembler-ia32.cc
+++ b/src/ia32/macro-assembler-ia32.cc
@@ -2399,19 +2399,46 @@ void MacroAssembler::PushHeapObject(Handle<HeapObject> object) {
 void MacroAssembler::CmpWeakValue(Register value, Handle<WeakCell> cell,
                                   Register scratch) {
   mov(scratch, cell);
-  cmp(value, FieldOperand(scratch, WeakCell::kValueOffset));
+  cmp(value,
+      FieldOperand(scratch, WeakCell::kValueOffsetDontForgetTheReadBarrier));
 }
 
 
-void MacroAssembler::GetWeakValue(Register value, Handle<WeakCell> cell) {
+void MacroAssembler::GetWeakValueNoReadBarrier(Register value,
+                                               Handle<WeakCell> cell) {
   mov(value, cell);
-  mov(value, FieldOperand(value, WeakCell::kValueOffset));
+  mov(value,
+      FieldOperand(value, WeakCell::kValueOffsetDontForgetTheReadBarrier));
+}
+
+
+void MacroAssembler::LoadWeakValueNoReadBarrier(Register value,
+                                                Handle<WeakCell> cell,
+                                                Label* miss) {
+  GetWeakValueNoReadBarrier(value, cell);
+  JumpIfSmi(value, miss);
+}
+
+
+void MacroAssembler::GetWeakValue(Register value, Register scratch,
+                                  Handle<WeakCell> cell) {
+  DCHECK(!value.is(scratch));
+  mov(value, cell);
+
+  // The read barrier means we have to overwrite the cell map with the one that
+  // indicates the cell has been read from.

[Hannes Payer (out of office), 2015/02/04 12:45:59]

This seems to be more expensive than just clearing a bit. The advantage is, that
we do not slow down the case when we can go directly to the value (omit read
barrier). I guess we have to measure the performance impact.

+  Handle<Map> weak_cell_map(isolate()->heap()->used_weak_cell_map());
+  mov(scratch, weak_cell_map);
+  mov(scratch, FieldOperand(value, HeapObject::kMapOffset));

[ulan, 2015/02/04 09:35:04]

mov(FieldOperand(value, HeapObject::kMapOffset), scratch); ?

+
+  mov(value,
+      FieldOperand(value, WeakCell::kValueOffsetDontForgetTheReadBarrier));
 }
 
 
 void MacroAssembler::LoadWeakValue(Register value, Handle<WeakCell> cell,
-                                   Label* miss) {
-  GetWeakValue(value, cell);
+                                   Register scratch, Label* miss) {
+  GetWeakValue(value, scratch, cell);
   JumpIfSmi(value, miss);
 }