Add a bit to a virtual frame element telling if it's been copied. Set

it when a copy is made, clear it when the element is next written.
Review URL: http://codereview.chromium.org/42324

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1529 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/virtual-frame-ia32.cc

@@ -550,6 +550,7 @@ FrameElement VirtualFrame::AdjustCopies(int index) {
     return copy;
   }
 
+  elements_[index].clear_copied();
   return FrameElement::InvalidElement();
 }
 
@@ -569,7 +570,9 @@ void VirtualFrame::TakeFrameSlotAt(int index) {
       // push that register on top of the frame.  If it is copied,
       // make the first copy the backing store and push a fresh copy
       // on top of the frame.
-      FrameElement copy = AdjustCopies(index);
+      FrameElement copy = original.is_copied()
+                          ? AdjustCopies(index)
+                          : FrameElement::InvalidElement();
       if (copy.is_valid()) {
         // The original element was a copy.  Push the copy of the new
         // backing store.
@@ -593,7 +596,9 @@ void VirtualFrame::TakeFrameSlotAt(int index) {
       // If the element is not copied, push it on top of the frame.
       // If it is copied, make the first copy be the new backing store
       // and push a fresh copy on top of the frame.
-      FrameElement copy = AdjustCopies(index);
+      FrameElement copy = original.is_copied()
+                          ? AdjustCopies(index)
+                          : FrameElement::InvalidElement();
       if (copy.is_valid()) {
         // The original element was a copy.  Push the copy of the new
         // backing store.
@@ -634,7 +639,8 @@ void VirtualFrame::StoreToFrameSlotAt(int index) {
   FrameElement original = elements_[index];
   // If the stored-to slot may be copied, adjust to preserve the
   // copy-on-write semantics of copied elements.
-  if (original.is_register() || original.is_memory()) {
+  if (original.is_copied() &&
+      (original.is_register() || original.is_memory())) {
     FrameElement ignored = AdjustCopies(index);
   }
 

src/virtual-frame.cc

@@ -93,9 +93,11 @@ FrameElement VirtualFrame::CopyElementAt(int index) {
     case FrameElement::REGISTER:
       // All copies are backed by memory or register locations.
       result.type_ =
-          FrameElement::TypeField::encode(FrameElement::COPY) |
-          FrameElement::SyncField::encode(FrameElement::NOT_SYNCED);
+          FrameElement::TypeField::encode(FrameElement::COPY)
+          | FrameElement::IsCopiedField::encode(false)
+          | FrameElement::SyncField::encode(FrameElement::NOT_SYNCED);
       result.data_.index_ = index;
+      elements_[index].set_copied();
       break;
 
     case FrameElement::INVALID:
@@ -367,7 +369,8 @@ void VirtualFrame::SetElementAt(int index, Result* value) {
 
   // If the original may be a copy, adjust to preserve the copy-on-write
   // semantics of copied elements.
-  if (original.is_register() || original.is_memory()) {
+  if (original.is_copied() &&
+      (original.is_register() || original.is_memory())) {
     FrameElement ignored = AdjustCopies(frame_index);
   }
 

src/virtual-frame.h

@@ -101,6 +101,16 @@ class FrameElement BASE_EMBEDDED {
   bool is_constant() const { return type() == CONSTANT; }
   bool is_copy() const { return type() == COPY; }
 
+  bool is_copied() const { return IsCopiedField::decode(type_); }
+
+  void set_copied() {
+    type_ = (type_ & ~IsCopiedField::mask()) | IsCopiedField::encode(true);
+  }
+
+  void clear_copied() {
+    type_ = (type_ & ~IsCopiedField::mask()) | IsCopiedField::encode(false);
+  }
+
   Register reg() const {
     ASSERT(is_register());
     return data_.reg_;
@@ -129,7 +139,8 @@ class FrameElement BASE_EMBEDDED {
 
   // BitField is <type, shift, size>.
   class SyncField : public BitField<SyncFlag, 0, 1> {};
-  class TypeField : public BitField<Type, 1, 32 - 1> {};
+  class IsCopiedField : public BitField<bool, 1, 1> {};
+  class TypeField : public BitField<Type, 2, 32 - 2> {};
 
   Type type() const { return TypeField::decode(type_); }
 
@@ -144,10 +155,6 @@ class FrameElement BASE_EMBEDDED {
     int index_;
   } data_;
 
-  // The index of the next element in a list of copies, or the frame's
-  // illegal index if there is no next element.
-  int next_;
-
   // Used to construct memory and register elements.
   FrameElement(Type type, Register reg, SyncFlag is_synced) {
     Initialize(type, reg, is_synced);
@@ -175,16 +182,18 @@ class FrameElement BASE_EMBEDDED {
 namespace v8 { namespace internal {
 
 FrameElement::FrameElement(Handle<Object> value, SyncFlag is_synced) {
-  type_ = TypeField::encode(CONSTANT) | SyncField::encode(is_synced);
+  type_ = TypeField::encode(CONSTANT)
+          | IsCopiedField::encode(false)
+          | SyncField::encode(is_synced);
   data_.handle_ = value.location();
-  next_ = VirtualFrame::kIllegalIndex;
 }
 
 
 void FrameElement::Initialize(Type type, Register reg, SyncFlag is_synced) {
-  type_ = TypeField::encode(type) | SyncField::encode(is_synced);
+  type_ = TypeField::encode(type)
+          | IsCopiedField::encode(false)
+          | SyncField::encode(is_synced);
   data_.reg_ = reg;
-  next_ = VirtualFrame::kIllegalIndex;
 }