Add specialized sweeper for cell and map space.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 6d14463c00953867f4dc2f4f605244279b2c6752..70b36809d920a792f46d3cacedb1ef7329744301 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -2661,6 +2661,50 @@ static void SweepPrecisely(PagedSpace* space, Page* p, SweepingMode mode) {
 }
 
 
+// Sweep a page of a space which contains objects of fixed size.  This is used
+// for map space and cell space, it always sweeps those spaces precisely.
+template<int size>
+static void SweepSpecialized(PagedSpace* space, Page* p) {
+  ASSERT(!p->IsEvacuationCandidate() && !p->WasSwept());
+  MarkBit::CellType* cells = p->markbits()->cells();
+  p->MarkSweptPrecisely();
+
+  int last_cell_index =
+      Bitmap::IndexToCell(
+          Bitmap::CellAlignIndex(
+              p->AddressToMarkbitIndex(p->ObjectAreaEnd())));
+
+  int cell_index = Page::kFirstUsedCell;
+  Address free_start = p->ObjectAreaStart();
+  Address free_end = p->ObjectAreaStart();
+  uint32_t mark_mask = 0;
+  uint32_t mark_bits;
+
+  ASSERT(size % kPointerSize == 0);
+  int bits_per_object = size / kPointerSize;
+
+  for (cell_index = Page::kFirstUsedCell;
+       cell_index < last_cell_index || mark_mask != 0;
+       free_end += size, mark_mask <<= bits_per_object) {
+    if (mark_mask == 0) {
+      mark_mask = 1 << (p->AddressToMarkbitIndex(free_end) % 32);
+      mark_bits = cells[cell_index];
+      cells[cell_index] = 0;
+      cell_index++;
+    }
+    if ((mark_bits & mark_mask) != 0) {
+      if (free_end != free_start) {
+        space->Free(free_start, static_cast<int>(free_end - free_start));
+      }
+      free_start = free_end + size;
+    }
+  }
+  if (free_start != p->ObjectAreaEnd()) {
+    space->Free(free_start, static_cast<int>(p->ObjectAreaEnd() - free_start));
+  }
+}
+
+
 void MarkCompactCollector::EvacuateNewSpaceAndCandidates() {
   EvacuateNewSpace();
   EvacuatePages();
@@ -3241,6 +3285,14 @@ void MarkCompactCollector::SweepSpace(PagedSpace* space,
         SweepPrecisely(space, p, SWEEP_ONLY);
         break;
       }
+      case PRECISE_CELLS: {
+        SweepSpecialized<JSGlobalPropertyCell::kSize>(space, p);
+        break;
+      }
+      case PRECISE_MAPS: {
+        SweepSpecialized<Map::kSize>(space, p);
+        break;
+      }
       default: {
         UNREACHABLE();
       }
@@ -3265,14 +3317,14 @@ void MarkCompactCollector::SweepSpaces() {
   SweepSpace(heap()->old_pointer_space(), how_to_sweep);
   SweepSpace(heap()->old_data_space(), how_to_sweep);
   SweepSpace(heap()->code_space(), PRECISE);
-  SweepSpace(heap()->cell_space(), PRECISE);
+  SweepSpace(heap()->cell_space(), PRECISE_CELLS);
   { GCTracer::Scope gc_scope(tracer_, GCTracer::Scope::MC_SWEEP_NEWSPACE);
     EvacuateNewSpaceAndCandidates();
   }
   // ClearNonLiveTransitions depends on precise sweeping of map space to
   // detect whether unmarked map became dead in this collection or in one
   // of the previous ones.
-  SweepSpace(heap()->map_space(), PRECISE);
+  SweepSpace(heap()->map_space(), PRECISE_MAPS);
 
   ASSERT(live_map_objects_size_ <= heap()->map_space()->Size());