Provide a tagged allocation top pointer.

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
diff --git a/src/arm/macro-assembler-arm.cc b/src/arm/macro-assembler-arm.cc
index e741ff7ea6d53236a9a2394494e99e061bd66447..fe24f45ef6bff1b62611f0aab15288015f31227d 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -2037,7 +2037,7 @@ void MacroAssembler::Allocate(int object_size,
     // Align the next allocation. Storing the filler map without checking top is
     // safe in new-space because the limit of the heap is aligned there.
     STATIC_ASSERT(kPointerAlignment * 2 == kDoubleAlignment);
-    and_(result_end, result, Operand(kDoubleAlignmentMask), SetCC);
+    and_(result_end, result, Operand(kDoubleAlignmentMaskTagged), SetCC);
     Label aligned;
     b(eq, &aligned);
     if ((flags & PRETENURE) != 0) {
@@ -2045,7 +2045,8 @@ void MacroAssembler::Allocate(int object_size,
       b(hs, gc_required);
     }
     mov(result_end, Operand(isolate()->factory()->one_pointer_filler_map()));
-    str(result_end, MemOperand(result, kDoubleSize / 2, PostIndex));
+    str(result_end, MemOperand(result, -kHeapObjectTag));
+    add(result, result, Operand(kDoubleSize / 2));
     bind(&aligned);
   }
 
@@ -2078,9 +2079,6 @@ void MacroAssembler::Allocate(int object_size,
     // The top pointer is not updated for allocation folding dominators.
     str(result_end, MemOperand(top_address));
   }
-
-  // Tag object.
-  add(result, result, Operand(kHeapObjectTag));
 }
 
 
@@ -2143,7 +2141,7 @@ void MacroAssembler::Allocate(Register object_size, Register result,
     // Align the next allocation. Storing the filler map without checking top is
     // safe in new-space because the limit of the heap is aligned there.
     DCHECK(kPointerAlignment * 2 == kDoubleAlignment);
-    and_(result_end, result, Operand(kDoubleAlignmentMask), SetCC);
+    and_(result_end, result, Operand(kDoubleAlignmentMaskTagged), SetCC);
     Label aligned;
     b(eq, &aligned);
     if ((flags & PRETENURE) != 0) {
@@ -2151,7 +2149,8 @@ void MacroAssembler::Allocate(Register object_size, Register result,
       b(hs, gc_required);
     }
     mov(result_end, Operand(isolate()->factory()->one_pointer_filler_map()));
-    str(result_end, MemOperand(result, kDoubleSize / 2, PostIndex));
+    str(result_end, MemOperand(result, -kHeapObjectTag));
+    add(result, result, Operand(kDoubleSize / 2));
     bind(&aligned);
   }
 
@@ -2170,15 +2169,12 @@ void MacroAssembler::Allocate(Register object_size, Register result,
   // Update allocation top. result temporarily holds the new top.
   if (emit_debug_code()) {
     tst(result_end, Operand(kObjectAlignmentMask));
-    Check(eq, kUnalignedAllocationInNewSpace);
+    Check(ne, kUnalignedAllocationInNewSpace);
   }
   if ((flags & ALLOCATION_FOLDING_DOMINATOR) == 0) {
     // The top pointer is not updated for allocation folding dominators.
     str(result_end, MemOperand(top_address));
   }
-
-  // Tag object.
-  add(result, result, Operand(kHeapObjectTag));
 }
 
 void MacroAssembler::FastAllocate(Register object_size, Register result,
@@ -2201,11 +2197,12 @@ void MacroAssembler::FastAllocate(Register object_size, Register result,
     // Align the next allocation. Storing the filler map without checking top is
     // safe in new-space because the limit of the heap is aligned there.
     DCHECK(kPointerAlignment * 2 == kDoubleAlignment);
-    and_(result_end, result, Operand(kDoubleAlignmentMask), SetCC);
+    and_(result_end, result, Operand(kDoubleAlignmentMaskTagged), SetCC);
     Label aligned;
     b(eq, &aligned);
     mov(result_end, Operand(isolate()->factory()->one_pointer_filler_map()));
-    str(result_end, MemOperand(result, kDoubleSize / 2, PostIndex));
+    str(result_end, MemOperand(result, -kHeapObjectTag));
+    add(result, result, Operand(kDoubleSize / 2));
     bind(&aligned);
   }
 
@@ -2220,12 +2217,10 @@ void MacroAssembler::FastAllocate(Register object_size, Register result,
   // Update allocation top. result temporarily holds the new top.
   if (emit_debug_code()) {
     tst(result_end, Operand(kObjectAlignmentMask));
-    Check(eq, kUnalignedAllocationInNewSpace);
+    Check(ne, kUnalignedAllocationInNewSpace);
   }
   // The top pointer is not updated for allocation folding dominators.
   str(result_end, MemOperand(top_address));
-
-  add(result, result, Operand(kHeapObjectTag));
 }
 
 void MacroAssembler::FastAllocate(int object_size, Register result,
@@ -2253,11 +2248,12 @@ void MacroAssembler::FastAllocate(int object_size, Register result,
     // Align the next allocation. Storing the filler map without checking top is
     // safe in new-space because the limit of the heap is aligned there.
     STATIC_ASSERT(kPointerAlignment * 2 == kDoubleAlignment);
-    and_(result_end, result, Operand(kDoubleAlignmentMask), SetCC);
+    and_(result_end, result, Operand(kDoubleAlignmentMaskTagged), SetCC);
     Label aligned;
     b(eq, &aligned);
     mov(result_end, Operand(isolate()->factory()->one_pointer_filler_map()));
-    str(result_end, MemOperand(result, kDoubleSize / 2, PostIndex));
+    str(result_end, MemOperand(result, -kHeapObjectTag));
+    add(result, result, Operand(kDoubleSize / 2));
     bind(&aligned);
   }
 
@@ -2285,8 +2281,6 @@ void MacroAssembler::FastAllocate(int object_size, Register result,
 
   // The top pointer is not updated for allocation folding dominators.
   str(result_end, MemOperand(top_address));
-
-  add(result, result, Operand(kHeapObjectTag));
 }
 
 void MacroAssembler::AllocateTwoByteString(Register result,