Provide a tagged allocation top pointer.

src/code-stub-assembler.cc

Index: src/code-stub-assembler.cc
diff --git a/src/code-stub-assembler.cc b/src/code-stub-assembler.cc
index be6bf6e2001f1779548aafd923fe26d288f7a483..a4bcceb329958f606518d67f69b5029144ea9eb7 100644
--- a/src/code-stub-assembler.cc
+++ b/src/code-stub-assembler.cc
@@ -378,9 +378,7 @@ Node* CodeStubAssembler::AllocateRawUnaligned(Node* size_in_bytes,
   Node* no_runtime_result = top;
   StoreNoWriteBarrier(MachineType::PointerRepresentation(), top_address,
                       new_top);
-  no_runtime_result = BitcastWordToTagged(
-      IntPtrAdd(no_runtime_result, IntPtrConstant(kHeapObjectTag)));
-  result.Bind(no_runtime_result);
+  result.Bind(BitcastWordToTagged(no_runtime_result));
   Goto(&merge_runtime);
 
   Bind(&merge_runtime);
@@ -398,8 +396,9 @@ Node* CodeStubAssembler::AllocateRawAligned(Node* size_in_bytes,
   if (flags & kDoubleAlignment) {
     // TODO(epertoso): Simd128 alignment.
     Label aligned(this), not_aligned(this), merge(this, &adjusted_size);
-    Branch(WordAnd(top, IntPtrConstant(kDoubleAlignmentMask)), &not_aligned,
-           &aligned);
+    Branch(WordAnd(IntPtrSub(top, IntPtrConstant(kHeapObjectTag)),
+                   IntPtrConstant(kDoubleAlignmentMask)),
+           &not_aligned, &aligned);
 
     Bind(&not_aligned);
     Node* not_aligned_size =
@@ -425,7 +424,8 @@ Node* CodeStubAssembler::AllocateRawAligned(Node* size_in_bytes,
   // Store a filler and increase the address by kPointerSize.
   // TODO(epertoso): this code assumes that we only align to kDoubleSize. Change
   // it when Simd128 alignment is supported.
-  StoreNoWriteBarrier(MachineType::PointerRepresentation(), top,
+  StoreNoWriteBarrier(MachineType::PointerRepresentation(),
+                      IntPtrSub(top, IntPtrConstant(1)),
                       LoadRoot(Heap::kOnePointerFillerMapRootIndex));
   address.Bind(BitcastWordToTagged(
       IntPtrAdd(address.value(), IntPtrConstant(kPointerSize))));