[heap] Do not skip LAB in PagedSpaceObjectIterator

Instead of skipping LAB in PagedSpaceObjectIterator, make the space
iterable by inserting a filler object into the LAB.

Bug: v8:10315
Change-Id: I6d79c309b7b8180b2a173ebd5ebdf8a893e88c4d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2210234Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67928}

src/heap/paged-spaces-inl.h

@@ -27,10 +27,6 @@ HeapObject PagedSpaceObjectIterator::Next() {
 
 HeapObject PagedSpaceObjectIterator::FromCurrentPage() {
   while (cur_addr_ != cur_end_) {
-    if (cur_addr_ == space_->top() && cur_addr_ != space_->limit()) {
-      cur_addr_ = space_->limit();
-      continue;
-    }
     HeapObject obj = HeapObject::FromAddress(cur_addr_);
     const int obj_size = obj.Size();
     cur_addr_ += obj_size;

src/heap/paged-spaces.cc

@@ -32,6 +32,7 @@ PagedSpaceObjectIterator::PagedSpaceObjectIterator(Heap* heap,
       space_(space),
       page_range_(space->first_page(), nullptr),
       current_page_(page_range_.begin()) {
+  space_->MakeLinearAllocationAreaIterable();
   heap->mark_compact_collector()->EnsureSweepingCompleted();
 }
 
@@ -43,6 +44,7 @@ PagedSpaceObjectIterator::PagedSpaceObjectIterator(Heap* heap,
       space_(space),
       page_range_(page),
       current_page_(page_range_.begin()) {
+  space_->MakeLinearAllocationAreaIterable();
   heap->mark_compact_collector()->EnsureSweepingCompleted();
 #ifdef DEBUG
   AllocationSpace owner = page->owner_identity();
@@ -56,7 +58,9 @@ PagedSpaceObjectIterator::PagedSpaceObjectIterator(OffThreadSpace* space)
       cur_end_(kNullAddress),
       space_(space),
       page_range_(space->first_page(), nullptr),
-      current_page_(page_range_.begin()) {}
+      current_page_(page_range_.begin()) {
+  space_->MakeLinearAllocationAreaIterable();
+}
 
 // We have hit the end of the page and should advance to the next block of
 // objects.  This happens at the end of the page.
@@ -394,6 +398,23 @@ void PagedSpace::UnmarkLinearAllocationArea() {
   }
 }
 
+void PagedSpace::MakeLinearAllocationAreaIterable() {
+  Address current_top = top();
+  Address current_limit = limit();
+  if (current_top != kNullAddress && current_top != current_limit) {
+    base::Optional<CodePageMemoryModificationScope> optional_scope;
+
+    if (identity() == CODE_SPACE) {
+      MemoryChunk* chunk = MemoryChunk::FromAddress(current_top);
+      optional_scope.emplace(chunk);
+    }
+
+    heap_->CreateFillerObjectAt(current_top,
+                                static_cast<int>(current_limit - current_top),
+                                ClearRecordedSlots::kNo);
+  }
+}
+
 void PagedSpace::FreeLinearAllocationArea() {
   // Mark the old linear allocation area with a free space map so it can be
   // skipped when scanning the heap.

src/heap/paged-spaces.h

@@ -189,6 +189,8 @@ class V8_EXPORT_PRIVATE PagedSpace
   // Empty space linear allocation area, returning unused area to free list.
   void FreeLinearAllocationArea();
 
+  void MakeLinearAllocationAreaIterable();
+
   void MarkLinearAllocationAreaBlack();
   void UnmarkLinearAllocationArea();