Prepares allocation for proper sync with mutator and bg threads.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 336f9c9cc156b2ab953259ed43561f9e988f8419..bf07ea22962bee8113f14db284c4d9f5972bb796 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -324,7 +324,8 @@ Scavenger::Scavenger(Heap* heap,
       gc_time_micros_(0),
       collections_(0),
       external_size_(0),
-      failed_to_promote_(false) {
+      failed_to_promote_(false),
+      space_lock_(new Mutex()) {
   // Verify assumptions about the first word in objects which the scavenger is
   // going to use for forwarding pointers.
   ASSERT(Object::tags_offset() == 0);
@@ -355,6 +356,7 @@ Scavenger::Scavenger(Heap* heap,
 Scavenger::~Scavenger() {
   ASSERT(!scavenging_);
   to_->Delete();
+  delete space_lock_;
 }
 
 intptr_t Scavenger::NewSizeInWords(intptr_t old_size_in_words) const {
@@ -715,16 +717,28 @@ void Scavenger::ProcessWeakReferences() {
   }
 }
 
+uword Scavenger::FindTopOfSpace(Isolate* isolate) const {
+  Thread* current = heap_->isolate()->thread_registry()->active_list();
+  uword furthest_addr = 0;

[rmacnak, 2017/08/01 21:51:25]

If we make all outstanding TLABs walkable, everything from Scavenger::base_ to
Scavenger::top_ should be walkable, right? Then there is no need to find the
highest top among the TLABs or to copy from a TLAB to the Scavenger and back.

[danunez, 2017/08/01 23:25:34]

That make sense. However, I am going to split this into two functions.
1) MakeALLTLABsIterable: Fills all TLABS to make the new space iterable.
2) FindTopOfSpace: Returns the address of the last object in the new space.

2) is specifically for UsedInWords() so we do not have an incorrect size
reported in Observatory (https://github.com/dart-lang/sdk/issues/30311).

+  while (current != NULL) {
+    if (current->HasActiveTLAB() && current->top() > furthest_addr) {
+      furthest_addr = current->top();
+      heap_->FillRemainingTLAB(current);
+    }
+    current = current->next();
+  }
+  return furthest_addr;
+}
+
 uword Scavenger::FlushTLS() const {
   ASSERT(heap_ != NULL);

[rmacnak, 2017/08/01 21:51:25]

Per comment above, I think this could be replaced with MakeAllTLABsIterable...

[danunez, 2017/08/01 23:25:34]

Done.

   uword saved_top = top_;
-  if (heap_->isolate()->IsMutatorThreadScheduled() && !scavenging_) {
+  if (!scavenging_) {
     Thread* mutator_thread = heap_->isolate()->mutator_thread();
-    saved_top = mutator_thread->heap()->new_space()->top();
-    if (mutator_thread->HasActiveTLAB()) {
-      ASSERT(mutator_thread->top() <=
-             mutator_thread->heap()->new_space()->top());
-      mutator_thread->heap()->new_space()->set_top(mutator_thread->top());
+    uword furthest_addr = FindTopOfSpace(heap_->isolate());
+    if (furthest_addr > 0) {
+      ASSERT(furthest_addr <= mutator_thread->heap()->new_space()->top());
+      mutator_thread->heap()->new_space()->set_top(furthest_addr);
     }
   }
   return saved_top;
@@ -739,6 +753,14 @@ void Scavenger::UnflushTLS(uword value) const {
   }
 }
 
+void Scavenger::AbandonAllTLABs() {
+  Thread* current = heap_->isolate()->thread_registry()->active_list();
+  while (current != NULL) {
+    heap_->AbandonRemainingTLAB(current);
+    current = current->next();
+  }
+}
+
 void Scavenger::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
   uword saved_top = FlushTLS();
   uword cur = FirstObjectStart();
@@ -815,12 +837,9 @@ void Scavenger::Scavenge(bool invoke_api_callbacks) {
   int64_t post_safe_point = OS::GetCurrentMonotonicMicros();
   heap_->RecordTime(kSafePoint, post_safe_point - pre_safe_point);
 
-  if (isolate->IsMutatorThreadScheduled()) {
-    Thread* mutator_thread = isolate->mutator_thread();
-    if (mutator_thread->HasActiveTLAB()) {
-      heap_->AbandonRemainingTLAB(mutator_thread);
-    }
-  }
+  // TODO(danunez): Abandon all threads' TLABs. Every thread should be stopped
+  // here anyway.
+  AbandonAllTLABs();
 
   // TODO(koda): Make verification more compatible with concurrent sweep.
   if (FLAG_verify_before_gc && !FLAG_concurrent_sweep) {