Add map-based read barrier to WeakCell

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 2381 matching lines @@
     push(Operand::ForCell(cell));
   } else {
     Push(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);
 }
 
 
 void MacroAssembler::Ret() {
   ret(0);
 }
 
 
 void MacroAssembler::Ret(int bytes_dropped, Register scratch) {
@@ skipping 858 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