Add mistagging-readbarrier to weak cell

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 27f8f35b66ff277f2d340c581e1c05e51f894f42..bb902da535fbc9c89e465190c201069c9c3d8023 100644
--- a/src/ia32/macro-assembler-ia32.cc
+++ b/src/ia32/macro-assembler-ia32.cc
@@ -2398,16 +2398,31 @@ void MacroAssembler::PushHeapObject(Handle<HeapObject> object) {
 
 void MacroAssembler::CmpWeakValue(Register value, Handle<WeakCell> cell,
                                   Register scratch) {
+  // TODO(erikcorry): Perhaps we can use a special weak cell that does not have
+  // a read barrier, since we are not using it here.  But then we have to be
+  // sure we don't use the weak cell for normal loads that need a read barrier.
   mov(scratch, cell);
-  cmp(value, FieldOperand(scratch, WeakCell::kValueOffset));
+  mov(scratch,
+      FieldOperand(scratch, WeakCell::kValueOffsetDontForgetTheReadBarrier));
+  and_(scratch, Immediate(~kReadBarrierTag));
+  cmp(value, scratch);
 }
 
 
-void MacroAssembler::LoadWeakValue(Register value, Handle<WeakCell> cell,
-                                   Label* miss) {
-  mov(value, cell);
-  mov(value, FieldOperand(value, WeakCell::kValueOffset));
+void MacroAssembler::LoadWeakValue(Register value, Register scratch,
+                                   Handle<WeakCell> cell, Label* miss) {
+  DCHECK(!value.is(scratch));
+  mov(scratch, cell);
+  mov(value,
+      FieldOperand(scratch, WeakCell::kValueOffsetDontForgetTheReadBarrier));
   JumpIfSmi(value, miss);
+  Label done;
+  test(value, Immediate(kReadBarrierTag));
+  j(not_zero, &done);

[Hannes Payer (out of office), 2015/02/04 13:03:44]

Do we need the test? Maybe the cpu is more happy with just untagging. WDYT?

+  and_(value, Immediate(~kReadBarrierTag));
+  mov(FieldOperand(scratch, WeakCell::kValueOffsetDontForgetTheReadBarrier),
+      value);
+  bind(&done);
 }