[heap] Full MC/Minor MC: Add support for ThinString

This reverts commit b4d2f3eb.

Bug: chromium:651354, chromium:788113
Change-Id: I04b7541e7eb69f737d45e90bbfec591cf8f8190f
Reviewed-on: https://chromium-review.googlesource.com/800552Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49765}

src/heap/mark-compact.cc

@@ -1452,9 +1452,11 @@ class EvacuateNewSpaceVisitor final : public EvacuateVisitorBase {
         buffer_(LocalAllocationBuffer::InvalidBuffer()),
         promoted_size_(0),
         semispace_copied_size_(0),
-        local_pretenuring_feedback_(local_pretenuring_feedback) {}
+        local_pretenuring_feedback_(local_pretenuring_feedback),
+        is_incremental_marking_(heap->incremental_marking()->IsMarking()) {}
 
   inline bool Visit(HeapObject* object, int size) override {
+    if (TryEvacuateWithoutCopy(object)) return true;
     HeapObject* target_object = nullptr;
     if (heap_->ShouldBePromoted(object->address()) &&
         TryEvacuateObject(OLD_SPACE, object, size, &target_object)) {
@@ -1474,6 +1476,26 @@ class EvacuateNewSpaceVisitor final : public EvacuateVisitorBase {
   intptr_t semispace_copied_size() { return semispace_copied_size_; }
 
  private:
+  inline bool TryEvacuateWithoutCopy(HeapObject* object) {
+    if (is_incremental_marking_) return false;
+
+    Map* map = object->map();
+
+    // Some objects can be evacuated without creating a copy.
+    if (map->visitor_id() == kVisitThinString) {
+      HeapObject* actual = ThinString::cast(object)->unchecked_actual();
+      if (MarkCompactCollector::IsOnEvacuationCandidate(actual)) return false;
+      base::Relaxed_Store(
+          reinterpret_cast<base::AtomicWord*>(object->address()),
+          reinterpret_cast<base::AtomicWord>(
+              MapWord::FromForwardingAddress(actual).ToMap()));
+      return true;
+    }
+    // TODO(mlippautz): Handle ConsString.
+
+    return false;
+  }
+
   inline AllocationSpace AllocateTargetObject(HeapObject* old_object, int size,
                                               HeapObject** target_object) {
     AllocationAlignment alignment = old_object->RequiredAlignment();
@@ -1505,6 +1527,7 @@ class EvacuateNewSpaceVisitor final : public EvacuateVisitorBase {
   intptr_t promoted_size_;
   intptr_t semispace_copied_size_;
   Heap::PretenuringFeedbackMap* local_pretenuring_feedback_;
+  bool is_incremental_marking_;
 };
 
 template <PageEvacuationMode mode>

src/objects/string-inl.h

@@ -525,6 +525,10 @@ void ConsString::set_second(String* value, WriteBarrierMode mode) {
 
 ACCESSORS(ThinString, actual, String, kActualOffset);
 
+HeapObject* ThinString::unchecked_actual() const {
+  return reinterpret_cast<HeapObject*>(READ_FIELD(this, kActualOffset));
+}
+
 bool ExternalString::is_short() {
   InstanceType type = map()->instance_type();
   return (type & kShortExternalStringMask) == kShortExternalStringTag;

src/objects/string.h

@@ -631,6 +631,7 @@ class ThinString : public String {
  public:
   // Actual string that this ThinString refers to.
   inline String* actual() const;
+  inline HeapObject* unchecked_actual() const;
   inline void set_actual(String* s,
                          WriteBarrierMode mode = UPDATE_WRITE_BARRIER);