[heap] Do not verify slots in concurrent sweeper

Cannot verify whether slots are cleared from a concurrent sweeping task,
the OLD_TO_NEW remembered set is owned by the main thread.

Bug: v8:10562
Change-Id: I0c880f7f96350aa35b44d2966c9621576825374c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2215820
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68098}

src/heap/heap.cc

@@ -3032,6 +3032,13 @@ HeapObject Heap::CreateFillerObjectAt(ReadOnlyRoots roots, Address addr,
   return filler;
 }
 
+void Heap::CreateFillerObjectFromSweeper(
+    Address addr, int size, ClearFreedMemoryMode clear_memory_mode) {
+  CreateFillerObjectAtImpl(ReadOnlyRoots(this), addr, size, clear_memory_mode);
+  // Do not verify whether slots are cleared here: the concurrent sweeper is not
+  // allowed to access the main thread's remembered set.
+}
+
 HeapObject Heap::CreateFillerObjectAt(Address addr, int size,
                                       ClearRecordedSlots clear_slots_mode) {
   if (size == 0) return HeapObject();

src/heap/heap.h

@@ -474,6 +474,9 @@ class Heap {
   V8_EXPORT_PRIVATE HeapObject CreateFillerObjectAt(
       Address addr, int size, ClearRecordedSlots clear_slots_mode);
 
+  void CreateFillerObjectFromSweeper(Address addr, int size,
+                                     ClearFreedMemoryMode clear_memory_mode);
+
   template <typename T>
   void CreateFillerForArray(T object, int elements_to_trim, int bytes_to_trim);
 

src/heap/sweeper.cc

@@ -265,14 +265,15 @@ V8_INLINE size_t Sweeper::FreeAndProcessFreedMemory(
   if (free_space_mode == ZAP_FREE_SPACE) {
     ZapCode(free_start, size);
   }
+  ClearFreedMemoryMode clear_memory_mode =
+      (free_list_mode == REBUILD_FREE_LIST)
+          ? ClearFreedMemoryMode::kDontClearFreedMemory
+          : ClearFreedMemoryMode::kClearFreedMemory;
+  page->heap()->CreateFillerObjectFromSweeper(
+      free_start, static_cast<int>(size), clear_memory_mode);
   if (free_list_mode == REBUILD_FREE_LIST) {
-    freed_bytes = reinterpret_cast<PagedSpace*>(space)->Free(
-        free_start, size, SpaceAccountingMode::kSpaceUnaccounted);
-
-  } else {
-    Heap::CreateFillerObjectAt(ReadOnlyRoots(page->heap()), free_start,
-                               static_cast<int>(size),
-                               ClearFreedMemoryMode::kClearFreedMemory);
+    freed_bytes =
+        reinterpret_cast<PagedSpace*>(space)->UnaccountedFree(free_start, size);
   }
   if (should_reduce_memory_) page->DiscardUnusedMemory(free_start, size);