Fix external size accounting for prologue weak persistent handles.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
===================================================================
--- runtime/vm/scavenger.cc	(revision 33504)
+++ runtime/vm/scavenger.cc	(working copy)
@@ -271,25 +271,28 @@
 
 class ScavengerWeakVisitor : public HandleVisitor {
  public:
-  explicit ScavengerWeakVisitor(Scavenger* scavenger)
+  explicit ScavengerWeakVisitor(Scavenger* scavenger,
+                                bool prologue_weak_were_strong)
       :  HandleVisitor(Isolate::Current()),
-         scavenger_(scavenger) {
+         scavenger_(scavenger),
+         prologue_weak_were_strong_(prologue_weak_were_strong) {
   }
 
   void VisitHandle(uword addr, bool is_prologue_weak) {
     FinalizablePersistentHandle* handle =
       reinterpret_cast<FinalizablePersistentHandle*>(addr);
     RawObject** p = handle->raw_addr();
-    Heap::Space before = handle->SpaceForExternal();
     if (scavenger_->IsUnreachable(p)) {
+      ASSERT(!is_prologue_weak || prologue_weak_were_strong_);
       handle->UpdateUnreachable(isolate(), is_prologue_weak);
     } else {
-      handle->UpdateRelocated(before, isolate());
+      handle->UpdateRelocated(isolate());
     }
   }
 
  private:
   Scavenger* scavenger_;
+  bool prologue_weak_were_strong_;
 
   DISALLOW_COPY_AND_ASSIGN(ScavengerWeakVisitor);
 };
@@ -704,8 +707,8 @@
   // Done with promoted stack; restore external allocation.
   ASSERT(!PromotedStackHasMore());
   AllocateExternal(saved_external);
-  ScavengerWeakVisitor weak_visitor(this);
-  IterateWeakRoots(isolate, &weak_visitor, invoke_api_callbacks);
+  ScavengerWeakVisitor weak_visitor(this, !invoke_api_callbacks);
+  IterateWeakRoots(isolate, &weak_visitor, true);

[siva, 2014/03/11 16:35:01]

Why did you set the parameter 'visit_prologue_weak_persistent_handles' to true
unconditionally in this call to IterateWeakRoots?

[koda, 2014/03/11 17:35:45]

Because we must visit the prologue weak persistent handles to detect any
promotions. That was the core issue behind the performance degradation: that
UpdateRelocated was not being called for pwphs, causing incorrect accounting and
scavenge on every alloc.

The change to store a bit in the handle is a consequence, since at this point,
the ScavengerVisitor has already updated the raw_ pointers of the prologue
weaks, i.e., we are not observing the promotion as it happens (IsUnreachable),
as we could do for the non-prologue ones.

I added a brief comment, and named the flag, for clarity. Let me know if you
think it needs more.

   visitor.Finalize();
   ProcessWeakTables();
   int64_t end = OS::GetCurrentTimeMicros();