Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/scavenger.h"
 
 #include "vm/dart.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
@@ skipping 139 matching lines @@
         new_addr =
             page_space_->TryAllocatePromoLocked(size, PageSpace::kForceGrowth);
         if (new_addr != 0) {
           // If promotion succeeded then we need to remember it so that it can
           // be traversed later.
           scavenger_->PushToPromotedStack(new_addr);
           bytes_promoted_ += size;
           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));
         }
       }
       // During a scavenge we always succeed to at least copy all of the
       // current objects to the to space.
       ASSERT(new_addr != 0);
       // Copy the object to the new location.
       memmove(reinterpret_cast<void*>(new_addr),
               reinterpret_cast<void*>(raw_addr), size);
@@ skipping 160 matching lines @@
 Scavenger::Scavenger(Heap* heap,
                      intptr_t max_semi_capacity_in_words,
                      uword object_alignment)
     : heap_(heap),
       max_semi_capacity_in_words_(max_semi_capacity_in_words),
       object_alignment_(object_alignment),
       scavenging_(false),
       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);
 
   // Set initial size resulting in a total of three different levels.
   const intptr_t initial_semi_capacity_in_words =
       max_semi_capacity_in_words /
       (FLAG_new_gen_growth_factor * FLAG_new_gen_growth_factor);
   to_ = SemiSpace::New(initial_semi_capacity_in_words);
   if (to_ == NULL) {
@@ skipping 429 matching lines @@
 
   int64_t pre_safe_point = OS::GetCurrentMonotonicMicros();
 
   Thread* thread = Thread::Current();
   SafepointOperationScope safepoint_scope(thread);
 
   // Scavenging is not reentrant. Make sure that is the case.
   ASSERT(!scavenging_);
   scavenging_ = true;
 
+  failed_to_promote_ = false;
+
   PageSpace* page_space = heap_->old_space();
   NoSafepointScope no_safepoints;
 
   int64_t post_safe_point = OS::GetCurrentMonotonicMicros();
   heap_->RecordTime(kSafePoint, post_safe_point - pre_safe_point);
 
   // TODO(koda): Make verification more compatible with concurrent sweep.
   if (FLAG_verify_before_gc && !FLAG_concurrent_sweep) {
     OS::PrintErr("Verifying before Scavenge...");
     heap_->Verify(kForbidMarked);
@@ skipping 89 matching lines @@
   external_size_ += size;
 }
 
 
 void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ -= 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