Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 2309e6ca2aa3bb5c5b08cdf062cc4556206060b5..8a862bac50a2bdf1872e1148e45b18d277496f47 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -157,6 +157,7 @@ class ScavengerVisitor : public ObjectPointerVisitor {
           NOT_IN_PRODUCT(class_table->UpdateAllocatedOld(cid, size));
         } else {
           // Promotion did not succeed. Copy into the to space instead.
+          scavenger_->failed_to_promote_ = true;
           new_addr = scavenger_->TryAllocate(size);
           NOT_IN_PRODUCT(class_table->UpdateLiveNew(cid, size));
         }
@@ -337,7 +338,8 @@ Scavenger::Scavenger(Heap* heap,
       delayed_weak_properties_(NULL),
       gc_time_micros_(0),
       collections_(0),
-      external_size_(0) {
+      external_size_(0),
+      failed_to_promote_(false) {
   // Verify assumptions about the first word in objects which the scavenger is
   // going to use for forwarding pointers.
   ASSERT(Object::tags_offset() == 0);
@@ -787,6 +789,8 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
   ASSERT(!scavenging_);
   scavenging_ = true;
 
+  failed_to_promote_ = false;
+
   PageSpace* page_space = heap_->old_space();
   NoSafepointScope no_safepoints;
 
@@ -896,4 +900,14 @@ void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(external_size_ >= 0);
 }
 
+void Scavenger::Evacuate() {
+  SafepointOperationScope scope(Thread::Current());
+  Scavenge();

[rmacnak, 2017/06/19 19:29:00]

Can't we just update the promotion threshold and scavenge once?

Something like

survivor_end_ = top_;
Scavenge();

+  Scavenge();
+
+  // It is possible for objects to stay in the new space
+  // if the VM cannot create more pages for these objects.
+  ASSERT(UsedInWords() == 0 || failed_to_promote_);
+}
+
 }  // namespace dart