Lazy peer API.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 49eafb53e51ebb6a28ceaf9eaaa483cf6d3e139e..246343b8f1eacfc13b29d0c95cc11c52bb007d1f 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -511,6 +511,30 @@ uword Scavenger::ProcessWeakProperty(RawWeakProperty* raw_weak,
 }
 
 
+void Scavenger::ProcessPeerReferents() {
+  std::map<RawObject*, void*>::iterator it = peer_.begin();
+  while (it != peer_.end()) {
+    RawObject* raw_obj = it->first;
+    ASSERT(raw_obj->IsHeapObject());
+    uword raw_addr = RawObject::ToAddr(raw_obj);
+    uword header = *reinterpret_cast<uword*>(raw_addr);
+    if (IsForwarding(header)) {
+      // The object has survived.  If it has been copied to new space,
+      // update the table for the new location.  If it has been copied
+      // to old space, move it to the old space table.
+      uword new_addr = ForwardedAddr(header);
+      raw_obj = RawObject::FromAddr(new_addr);
+      void* peer = it->second;
+      peer_.erase(it++);
+      heap_->SetPeer(raw_obj, peer);

[Anton Muhin, 2012/09/13 06:27:07]

That looks sketchy, but I am not sure I remember STL semantics correctly.

My concern is you modify the map while iterating over it.  So are there any
chances that you'll iterate over just added objects too?  That looks possible to
me, esp. if to-spaces resides in higher addresses.

And if all above is possible, won't that make peers disappear?

[cshapiro, 2012/09/15 01:23:41]

Modifying the map while traversing it should be okay as far as the STL is
concerned.  However, as you have observed, this is not correct as far as the
algorithm is concerned.

Using a separate table has some additional advantages.  I will make that change.

+    } else {
+      // The object has become garbage.  Remove its record.
+      peer_.erase(it++);
+    }
+  }
+}
+
+
 void Scavenger::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
   uword cur = FirstObjectStart();
   while (cur < top_) {
@@ -565,6 +589,7 @@ void Scavenger::Scavenge(bool invoke_api_callbacks, const char* gc_reason) {
   ScavengerWeakVisitor weak_visitor(this);
   IterateWeakRoots(isolate, &weak_visitor, invoke_api_callbacks);
   visitor.Finalize();
+  ProcessPeerReferents();
   Epilogue(isolate, invoke_api_callbacks);
   timer.Stop();
   if (FLAG_verbose_gc) {
@@ -591,4 +616,24 @@ void Scavenger::WriteProtect(bool read_only) {
       read_only ? VirtualMemory::kReadOnly : VirtualMemory::kReadWrite);
 }
 
+
+void Scavenger::SetPeer(RawObject* raw_obj, void* peer) {

[Anton Muhin, 2012/09/13 06:27:07]

you've written this code once, maybe abstract away into PeerMap?

+  if (peer == NULL) {
+    peer_.erase(raw_obj);
+  } else {
+    peer_[raw_obj] = peer;
+  }
+}
+
+
+void* Scavenger::GetPeer(RawObject* raw_obj) {
+  std::map<RawObject*, void*>::iterator it = peer_.find(raw_obj);
+  return (it == peer_.end()) ? NULL : it->second;
+}
+
+
+int64_t Scavenger::PeerCount() const {
+  return static_cast<int64_t>(peer_.size());
+}
+
 }  // namespace dart