[cleanup] Remove unreachable code in PagedSpaces

counter_ could never be RO_SPACE. Make sure RO_SPACE and OLD_SPACE are
marked as unreachable.

Added tests for PagedSpaces and SpaceIterator.

Bug: v8:9183
Change-Id: I97bc2b4e0e5af37363a1c628ca7d69d2790a97b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1635696
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61943}

src/heap/heap.cc

@@ -5340,9 +5340,8 @@ void Heap::ClearRecordedSlotRange(Address start, Address end) {
 PagedSpace* PagedSpaces::next() {
   switch (counter_++) {
     case RO_SPACE:
-      // skip NEW_SPACE
-      counter_++;
-      return heap_->read_only_space();
+    case NEW_SPACE:
+      UNREACHABLE();
     case OLD_SPACE:
       return heap_->old_space();
     case CODE_SPACE:

src/heap/heap.h

@@ -2245,8 +2245,8 @@ class VerifySmisVisitor : public RootVisitor {
 };
 
 // Space iterator for iterating over all the paged spaces of the heap: Map
-// space, old space, code space and optionally read only space. Returns each
-// space in turn, and null when it is done.
+// space, old space and code space. Returns each space in turn, and null when it
+// is done.
 class V8_EXPORT_PRIVATE PagedSpaces {
  public:
   explicit PagedSpaces(Heap* heap) : heap_(heap), counter_(OLD_SPACE) {}
@@ -2257,8 +2257,7 @@ class V8_EXPORT_PRIVATE PagedSpaces {
   int counter_;
 };
 
-
-class SpaceIterator : public Malloced {
+class V8_EXPORT_PRIVATE SpaceIterator : public Malloced {
  public:
   explicit SpaceIterator(Heap* heap);
   virtual ~SpaceIterator();

test/cctest/heap/test-iterators.cc

@@ -96,6 +96,24 @@ TEST(CombinedHeapIterator) {
   CHECK(seen_sample_object);
 }
 
+TEST(PagedSpaces) {
+  Heap* const heap = CcTest::heap();
+  PagedSpaces iterator(heap);
+  CHECK_EQ(iterator.next(), reinterpret_cast<PagedSpace*>(heap->old_space()));
+  CHECK_EQ(iterator.next(), reinterpret_cast<PagedSpace*>(heap->code_space()));
+  CHECK_EQ(iterator.next(), reinterpret_cast<PagedSpace*>(heap->map_space()));
+  for (int i = 0; i < 20; i++) {
+    CHECK_NULL(iterator.next());
+  }
+}
+
+TEST(SpaceIterator) {
+  auto* const read_only_space = CcTest::read_only_heap()->read_only_space();
+  for (SpaceIterator it(CcTest::heap()); it.has_next();) {
+    CHECK_NE(it.next(), reinterpret_cast<Space*>(read_only_space));
+  }
+}
+
 }  // namespace heap
 }  // namespace internal
 }  // namespace v8