Don't move the object start of objects on non-swept pages.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 5713537b5ab301076c51706d77f3eef5e5713970..a58b24c95d1c5af02d824dea16ae2f0857c9f1a4 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -3451,11 +3451,18 @@ bool Heap::CanMoveObjectStart(HeapObject* object) {
 
   if (lo_space()->Contains(object)) return false;
 
-  // We cannot move the object start if the given old space page is
-  // concurrently swept.
+  Page* page = Page::FromAddress(address);
+  // We can move the object start if:
+  // (1) the object is not in old pointer or old data space,
+  // (2) the page of the object was already swept,
+  // (3) the page was already concurrently swept. This case is an optimization
+  // for concurrent sweeping. The WasSwept predicate for concurrently swept
+  // pages is set after sweeping all pages.
   return (!is_in_old_pointer_space && !is_in_old_data_space) ||
-      Page::FromAddress(address)->parallel_sweeping() <=
-          MemoryChunk::PARALLEL_SWEEPING_FINALIZE;
+         page->WasSwept() ||
+         (mark_compact_collector()->AreSweeperThreadsActivated() &&
+              page->parallel_sweeping() <=
+                  MemoryChunk::PARALLEL_SWEEPING_FINALIZE);
 }