Preprocess structured stack trace on GC to get rid of code reference.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index e9379ac5658c9653cb21023f180d567a2364c068..d4038c78e52e5b284d2a0fc6e6d996afcf938014 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -706,6 +706,26 @@ void Heap::GarbageCollectionEpilogue() {
 }
 
 
+void Heap::PreprocessStackTraces() {
+  if (!weak_stack_trace_list()->IsWeakFixedArray()) return;
+  WeakFixedArray* array = WeakFixedArray::cast(weak_stack_trace_list());
+  int length = array->Length();
+  for (int i = 0; i < length; i++) {
+    if (array->IsEmptySlot(i)) continue;
+    FixedArray* elements = FixedArray::cast(array->Get(i));
+    for (int j = 1; j < elements->length(); j += 4) {
+      Code* code = Code::cast(elements->get(j + 2));
+      int offset = Smi::cast(elements->get(j + 3))->value();
+      Address pc = code->address() + offset;
+      int pos = code->SourcePosition(pc);
+      elements->set(j + 2, Smi::FromInt(pos));
+    }
+    array->Clear(i);
+  }
+  array->Compact();
+}
+
+
 void Heap::HandleGCRequest() {
   if (incremental_marking()->request_type() ==
       IncrementalMarking::COMPLETE_MARKING) {
@@ -1272,6 +1292,8 @@ void Heap::MarkCompactEpilogue() {
   isolate_->counters()->objs_since_last_full()->Set(0);
 
   incremental_marking()->Epilogue();
+
+  PreprocessStackTraces();
 }
 
 
@@ -3082,6 +3104,8 @@ void Heap::CreateInitialObjects() {
   cell->set_value(Smi::FromInt(Isolate::kArrayProtectorValid));
   set_array_protector(*cell);
 
+  set_weak_stack_trace_list(Smi::FromInt(0));
+
   set_allocation_sites_scratchpad(
       *factory->NewFixedArray(kAllocationSiteScratchpadSize, TENURED));
   InitializeAllocationSitesScratchpad();
@@ -3118,6 +3142,7 @@ bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
     case kDetachedContextsRootIndex:
     case kWeakObjectToCodeTableRootIndex:
     case kRetainedMapsRootIndex:
+    case kWeakStackTraceListRootIndex:
 // Smi values
 #define SMI_ENTRY(type, name, Name) case k##Name##RootIndex:
       SMI_ROOT_LIST(SMI_ENTRY)