heap: make array buffer maps disjoint

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index f95b91ac4db80791a7cf2dc9ea408e9369a1286e..d431aa0dfa082eb791fb87d037e8865edb1f1155 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -4431,10 +4431,13 @@ void MarkCompactCollector::SweepSpaces() {
   // buffer entries are already filter out. We can just release the memory.
   heap()->FreeQueuedChunks();
 
-  heap()->FreeDeadArrayBuffers(false);
-
   EvacuateNewSpaceAndCandidates();
 
+  // NOTE: ArrayBuffers must be evacuated first, before freeing them. Otherwise

[Michael Lippautz, 2015/09/01 08:30:06]

Can you elaborate on this comment a bit more. Why are the not yet discovered? We
should've computed a transitive closure, marking ALL live objects, icluding new
space ones. This would move them into the live map of buffers.

[fedor.indutny, 2015/09/01 08:50:08]

I might not be understanding this correctly, but we are not really visiting the
new space objects until we are doing the Sweep phase. So we do erase ptrs in the
`not_yet_discovered` only on `EvacuateNewSpaceAndCandidates` call.

The marking phase do not register them, because it is backed by
`StaticMarkingVisitor`, and only `StaticNewSpaceVisitor` is capable of
registering.

[Michael Lippautz, 2015/09/01 09:01:36]

StaticMarkingVisitor is also registering live arrays and should do the right
thing. Mind investigating the root problem? It would be good to maintain the
invariant that we can free the dead buffers anywhere after marking.

+  // not yet discovered buffers for scavenge will have all of them, and they
+  // will be erroneously freed.
+  heap()->FreeDeadArrayBuffers(false);
+
   // Clear the marking state of live large objects.
   heap_->lo_space()->ClearMarkingStateOfLiveObjects();