Remove the --collect-maps flag. Maps should be always collected.

Reasons:
- It is not used.
- It complicates code in GC.

BUG=chromium:499713
LOG=NO

Review URL: https://codereview.chromium.org/1184723002

Cr-Commit-Position: refs/heads/master@{#29027}

src/flag-definitions.h

@@ -613,8 +613,6 @@ DEFINE_INT(trace_allocation_stack_interval, -1,
 DEFINE_BOOL(trace_fragmentation, false, "report fragmentation for old space")
 DEFINE_BOOL(trace_fragmentation_verbose, false,
             "report fragmentation for old space (detailed)")
-DEFINE_BOOL(collect_maps, true,
-            "garbage collect maps from which no objects can be reached")
 DEFINE_BOOL(weak_embedded_maps_in_optimized_code, true,
             "make maps embedded in optimized code weak")
 DEFINE_BOOL(weak_embedded_objects_in_optimized_code, true,

src/heap-snapshot-generator.cc

@@ -1294,7 +1294,7 @@ void V8HeapExplorer::ExtractMapReferences(int entry, Map* map) {
         TransitionArray::cast(raw_transitions_or_prototype_info);
     int transitions_entry = GetEntry(transitions)->index();
 
-    if (FLAG_collect_maps && map->CanTransition()) {
+    if (map->CanTransition()) {
       if (transitions->HasPrototypeTransitions()) {
         FixedArray* prototype_transitions =
             transitions->GetPrototypeTransitions();

src/heap/mark-compact.cc

@@ -345,7 +345,7 @@ void MarkCompactCollector::CollectGarbage() {
   // arrays are cleared or contain only live code objects.
   ProcessAndClearWeakCells();
 
-  if (FLAG_collect_maps) ClearNonLiveReferences();
+  ClearNonLiveReferences();
 
   ClearWeakCollections();
 
@@ -369,7 +369,7 @@ void MarkCompactCollector::CollectGarbage() {
 
 #ifdef VERIFY_HEAP
   VerifyWeakEmbeddedObjectsInCode();
-  if (FLAG_collect_maps && FLAG_omit_map_checks_for_leaf_maps) {
+  if (FLAG_omit_map_checks_for_leaf_maps) {
     VerifyOmittedMapChecks();
   }
 #endif

src/heap/objects-visiting-inl.h

@@ -302,7 +302,7 @@ void StaticMarkingVisitor<StaticVisitor>::VisitMap(Map* map,
 
   // When map collection is enabled we have to mark through map's transitions
   // and back pointers in a special way to make these links weak.
-  if (FLAG_collect_maps && map_object->CanTransition()) {
+  if (map_object->CanTransition()) {
     MarkMapContents(heap, map_object);
   } else {
     StaticVisitor::VisitPointers(

src/objects-inl.h

@@ -4867,7 +4867,6 @@ Object* Code::GetObjectFromEntryAddress(Address location_of_address) {
 
 
 bool Code::IsWeakObjectInOptimizedCode(Object* object) {
-  if (!FLAG_collect_maps) return false;
   if (object->IsMap()) {
     return Map::cast(object)->CanTransition() &&
            FLAG_weak_embedded_maps_in_optimized_code;

test/cctest/test-heap.cc

@@ -3407,7 +3407,6 @@ TEST(TransitionArraySimpleToFull) {
 
 
 TEST(Regress2143a) {
-  i::FLAG_collect_maps = true;
   i::FLAG_incremental_marking = true;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
@@ -3447,7 +3446,6 @@ TEST(Regress2143a) {
 
 
 TEST(Regress2143b) {
-  i::FLAG_collect_maps = true;
   i::FLAG_incremental_marking = true;
   i::FLAG_allow_natives_syntax = true;
   CcTest::InitializeVM();