Provide a tagged allocation top pointer.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 9653fa00916f8d54824fa7e5416d37992486c93f..aef03bd1e5bf38157b7636ae0694b2a217c8b310 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -4816,7 +4816,7 @@ void MacroAssembler::MakeSureDoubleAlignedHelper(Register result,
                                                  AllocationFlags flags) {
   if (kPointerSize == kDoubleSize) {
     if (FLAG_debug_code) {
-      testl(result, Immediate(kDoubleAlignmentMask));
+      testl(result, Immediate(kDoubleAlignmentMaskTagged));
       Check(zero, kAllocationIsNotDoubleAligned);
     }
   } else {
@@ -4828,7 +4828,7 @@ void MacroAssembler::MakeSureDoubleAlignedHelper(Register result,
     // used in UpdateAllocationTopHelper later.
     DCHECK(!scratch.is(kScratchRegister));
     Label aligned;
-    testl(result, Immediate(kDoubleAlignmentMask));
+    testl(result, Immediate(kDoubleAlignmentMaskTagged));
     j(zero, &aligned, Label::kNear);
     if (((flags & ALLOCATION_FOLDED) == 0) && ((flags & PRETENURE) != 0)) {
       ExternalReference allocation_limit =
@@ -4837,7 +4837,7 @@ void MacroAssembler::MakeSureDoubleAlignedHelper(Register result,
       j(above_equal, gc_required);
     }
     LoadRoot(kScratchRegister, Heap::kOnePointerFillerMapRootIndex);
-    movp(Operand(result, 0), kScratchRegister);
+    movp(Operand(result, -kHeapObjectTag), kScratchRegister);
     addp(result, Immediate(kDoubleSize / 2));
     bind(&aligned);
   }
@@ -4849,7 +4849,7 @@ void MacroAssembler::UpdateAllocationTopHelper(Register result_end,
                                                AllocationFlags flags) {
   if (emit_debug_code()) {
     testp(result_end, Immediate(kObjectAlignmentMask));
-    Check(zero, kUnalignedAllocationInNewSpace);
+    Check(not_zero, kUnalignedAllocationInNewSpace);
   }
 
   ExternalReference allocation_top =
@@ -4917,11 +4917,7 @@ void MacroAssembler::Allocate(int object_size,
   }
 
   if (top_reg.is(result)) {
-    subp(result, Immediate(object_size - kHeapObjectTag));
-  } else {
-    // Tag the result.
-    DCHECK(kHeapObjectTag == 1);
-    incp(result);
+    subp(result, Immediate(object_size));
   }
 }
 
@@ -4986,9 +4982,6 @@ void MacroAssembler::Allocate(Register object_size,
     // The top pointer is not updated for allocation folding dominators.
     UpdateAllocationTopHelper(result_end, scratch, flags);
   }
-
-  // Tag the result.
-  addp(result, Immediate(kHeapObjectTag));
 }
 
 void MacroAssembler::FastAllocate(int object_size, Register result,
@@ -5004,8 +4997,6 @@ void MacroAssembler::FastAllocate(int object_size, Register result,
   leap(result_end, Operand(result, object_size));
 
   UpdateAllocationTopHelper(result_end, no_reg, flags);
-
-  addp(result, Immediate(kHeapObjectTag));
 }
 
 void MacroAssembler::FastAllocate(Register object_size, Register result,
@@ -5021,8 +5012,6 @@ void MacroAssembler::FastAllocate(Register object_size, Register result,
   leap(result_end, Operand(result, object_size, times_1, 0));
 
   UpdateAllocationTopHelper(result_end, no_reg, flags);
-
-  addp(result, Immediate(kHeapObjectTag));
 }
 
 void MacroAssembler::AllocateHeapNumber(Register result,