Revert "[heap] Correctly check for black allocated objects in concurrent marker."

This reverts commit 2690e2fc.

Reason for revert: this is not needed because objects in the worklist
are guaranteed to be not black allocated.

Original change's description:
> [heap] Correctly check for black allocated objects in concurrent marker.
> 
> The markbit check should be performed before using the map of the
> object.
> 
> Change-Id: Ia19e48fd4660387d239e1e330368808727359c7f
> Reviewed-on: https://chromium-review.googlesource.com/c/1301496
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57040}

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org

Change-Id: I4f188197620c511060fda4f60c80a3c389007054
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1301993Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57043}

src/heap/concurrent-marking.cc

@@ -105,8 +105,6 @@ class ConcurrentMarkingVisitor final
   }
 
   bool ShouldVisit(HeapObject* object) {
-    // TODO(ulan): It is sufficient to flip the second markbit without
-    // doing checking for the first bit.
     return marking_state_.GreyToBlack(object);
   }
 
@@ -408,8 +406,6 @@ class ConcurrentMarkingVisitor final
     }
   }
 
-  ConcurrentMarkingState* marking_state() { return &marking_state_; }
-
  private:
   // Helper class for collecting in-object slot addresses and values.
   class SlotSnapshottingVisitor final : public ObjectVisitor {
@@ -624,21 +620,7 @@ void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
         Address addr = object->address();
         if (new_space_top <= addr && addr < new_space_limit) {
           on_hold_->Push(task_id, object);
-        } else if (!visitor.marking_state()->IsBlackAssumingNotWhite(object)) {
-          // We need to ensure that the object is fully initialized.
-          // The new_space_top check above takes care of the newly allocated
-          // objects in the young generation. Now we need to rule out the black
-          // allocated objects in the old generation.
-          //
-          // Black allocation performs the following operations:
-          // 1) Set marking bitmap range to black using relaxed stores.
-          // 2) Issue base::SeqCst_MemoryFence().
-          // 3) Allocate, initialize, and publish an object.
-          //
-          // Since we loaded the object from the worklist, it must have
-          // been published. Since IsBlackAssumingNotWhite was false,
-          // was grey after it was published, so it is not black allocated.
-          // Thus the following operation loads a valid map pointer.
+        } else {
           Map* map = object->synchronized_map();
           current_marked_bytes += visitor.Visit(map, object);
         }

src/heap/mark-compact.h

@@ -64,10 +64,6 @@ class MarkingStateBase {
     return Marking::IsBlackOrGrey<access_mode>(MarkBitFrom(obj));
   }
 
-  V8_INLINE bool IsBlackAssumingNotWhite(HeapObject* obj) {
-    return Marking::IsBlackAssumingNotWhite<access_mode>(MarkBitFrom(obj));
-  }
-
   V8_INLINE bool WhiteToGrey(HeapObject* obj);
   V8_INLINE bool WhiteToBlack(HeapObject* obj);
   V8_INLINE bool GreyToBlack(HeapObject* obj);

src/heap/marking.h

@@ -248,13 +248,6 @@ class Marking : public AllStatic {
     return mark_bit.Get<mode>();
   }
 
-  // IsBlackAssumingNotWhite assumes that the color known to be
-  // black or grey. It only checks the second bit.
-  template <AccessMode mode = AccessMode::NON_ATOMIC>
-  V8_INLINE static bool IsBlackAssumingNotWhite(MarkBit mark_bit) {
-    return mark_bit.Next().Get<mode>();
-  }
-
   template <AccessMode mode = AccessMode::NON_ATOMIC>
   V8_INLINE static void MarkWhite(MarkBit markbit) {
     STATIC_ASSERT(mode == AccessMode::NON_ATOMIC);