Add mistagging-readbarrier to weak cell

src/ia32/macro-assembler-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_IA32
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
@@ skipping 2380 matching lines @@
     Handle<Cell> cell = isolate()->factory()->NewCell(object);
     push(Operand::ForCell(cell));
   } else {
     Push(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);
 }
 
 
 void MacroAssembler::Ret() {
   ret(0);
 }
 
 
 void MacroAssembler::Ret(int bytes_dropped, Register scratch) {
   if (is_uint16(bytes_dropped)) {
@@ skipping 845 matching lines @@
   if (mag.shift > 0) sar(edx, mag.shift);
   mov(eax, dividend);
   shr(eax, 31);
   add(edx, eax);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32