Ensure the word after top is cleared in newspace if top < high.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 81de035b0af7a6ae67de51f738b30ecbdbcb2da5..fc9f16627e423910358d2cbdec0707f3fd44fe9f 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -3007,6 +3007,20 @@ void MarkCompactCollector::EvacuateNewSpace() {
   new_space->Flip();
   new_space->ResetAllocationInfo();
 
+  // UpdateAllocationSiteFeedback expects that only objects at the end of
+  // newspace are not guaranteed to have the next word clear. It relies on
+  // FromSpacePageHigh to check whether an object is at the end of newspace.
+  // However, it is possible that newspace is being evacuated without it being
+  // full, e.g. to make the heap iterable, hence top will not equal high. In
+  // that case, fill up newspace with a filler to ensure the next word is
+  // cleared.
+  if (FLAG_allocation_site_pretenuring &&
+      from_top < new_space->FromSpacePageHigh()) {
+    Address limit = NewSpacePage::FromLimit(from_top)->area_end();
+    int remaining_in_page = static_cast<int>(limit - from_top);
+    heap()->CreateFillerObjectAt(from_top, remaining_in_page);
+  }
+
   int survivors_size = 0;
 
   // First pass: traverse all objects in inactive semispace, remove marks,