Fix object initialization when slack tracking for it's map is still enabled.

src/crankshaft/arm64/lithium-codegen-arm64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/crankshaft/arm64/lithium-codegen-arm64.h"
 
 #include "src/arm64/frames-arm64.h"
 #include "src/base/bits.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
@@ skipping 1537 matching lines @@
     __ Allocate(size, result, temp1, temp2, deferred->entry(), flags);
   } else {
     Register size = ToRegister32(instr->size());
     __ Sxtw(size.X(), size);
     __ Allocate(size.X(), result, temp1, temp2, deferred->entry(), flags);
   }
 
   __ Bind(deferred->exit());
 
   if (instr->hydrogen()->MustPrefillWithFiller()) {
-    Register filler_count = temp1;
-    Register filler = temp2;
-    Register untagged_result = ToRegister(instr->temp3());
+    Register start = temp1;
+    Register end = temp2;
+    Register filler = ToRegister(instr->temp3());
+
+    __ Sub(start, result, kHeapObjectTag);
 
     if (instr->size()->IsConstantOperand()) {
       int32_t size = ToInteger32(LConstantOperand::cast(instr->size()));
-      __ Mov(filler_count, size / kPointerSize);
+      __ Add(end, start, size);
     } else {
-      __ Lsr(filler_count.W(), ToRegister32(instr->size()), kPointerSizeLog2);
+      __ Add(end, start, ToRegister(instr->size()));
     }
-
-    __ Sub(untagged_result, result, kHeapObjectTag);
-    __ Mov(filler, Operand(isolate()->factory()->one_pointer_filler_map()));
-    __ FillFields(untagged_result, filler_count, filler);
+    __ LoadRoot(filler, Heap::kOnePointerFillerMapRootIndex);
+    __ InitializeFieldsWithFiller(start, end, filler);
   } else {
     DCHECK(instr->temp3() == NULL);
   }
 }
 
 
 void LCodeGen::DoDeferredAllocate(LAllocate* instr) {
   // TODO(3095996): Get rid of this. For now, we need to make the
   // result register contain a valid pointer because it is already
   // contained in the register pointer map.
@@ skipping 4368 matching lines @@
   Handle<ScopeInfo> scope_info = instr->scope_info();
   __ Push(scope_info);
   __ Push(ToRegister(instr->function()));
   CallRuntime(Runtime::kPushBlockContext, 2, instr);
   RecordSafepoint(Safepoint::kNoLazyDeopt);
 }
 
 
 }  // namespace internal
 }  // namespace v8