diff --git a/src/elements.cc b/src/elements.cc
index 6dd2a491950..6906fd47de7 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -2880,17 +2880,18 @@ void ElementsAccessor::TearDown() {
 
 Handle<JSArray> ElementsAccessor::Concat(Isolate* isolate, Arguments* args,
                                          uint32_t concat_size) {
-  int result_len = 0;
-  ElementsKind elements_kind = GetInitialFastElementsKind();
-  bool has_double = false;
+  uint32_t result_len = 0;
+  bool has_raw_doubles = false;
+  ElementsKind result_elements_kind = GetInitialFastElementsKind();
   {
     DisallowHeapAllocation no_gc;
+    bool is_holey = false;
     // Iterate through all the arguments performing checks
     // and calculating total length.
-    bool is_holey = false;
     for (uint32_t i = 0; i < concat_size; i++) {
-      Object* arg = (*args)[i];
-      int len = Smi::cast(JSArray::cast(arg)->length())->value();
+      JSArray* array = JSArray::cast((*args)[i]);
+      uint32_t len = 0;
+      array->length()->ToArrayLength(&len);
 
       // We shouldn't overflow when adding another len.
       const int kHalfOfMaxInt = 1 << (kBitsPerInt - 2);
@@ -2900,42 +2901,45 @@ Handle<JSArray> ElementsAccessor::Concat(Isolate* isolate, Arguments* args,
       DCHECK(0 <= result_len);
       DCHECK(result_len <= FixedDoubleArray::kMaxLength);
 
-      ElementsKind arg_kind = JSArray::cast(arg)->map()->elements_kind();
-      has_double = has_double || IsFastDoubleElementsKind(arg_kind);
+      ElementsKind arg_kind = array->GetElementsKind();
+      has_raw_doubles = has_raw_doubles || IsFastDoubleElementsKind(arg_kind);
       is_holey = is_holey || IsFastHoleyElementsKind(arg_kind);
-      elements_kind = GetMoreGeneralElementsKind(elements_kind, arg_kind);
+      result_elements_kind =
+          GetMoreGeneralElementsKind(result_elements_kind, arg_kind);
     }
     if (is_holey) {
-      elements_kind = GetHoleyElementsKind(elements_kind);
+      result_elements_kind = GetHoleyElementsKind(result_elements_kind);
     }
   }
 
   // If a double array is concatted into a fast elements array, the fast
   // elements array needs to be initialized to contain proper holes, since
   // boxing doubles may cause incremental marking.
-  ArrayStorageAllocationMode mode =
-      has_double && IsFastObjectElementsKind(elements_kind)
-          ? INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE
-          : DONT_INITIALIZE_ARRAY_ELEMENTS;
+  bool requires_double_boxing =
+      has_raw_doubles && !IsFastDoubleElementsKind(result_elements_kind);
+  ArrayStorageAllocationMode mode = requires_double_boxing
+                                        ? INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE
+                                        : DONT_INITIALIZE_ARRAY_ELEMENTS;
   Handle<JSArray> result_array = isolate->factory()->NewJSArray(
-      elements_kind, result_len, result_len, mode);
+      result_elements_kind, result_len, result_len, mode);
   if (result_len == 0) return result_array;
-  int j = 0;
+
+  uint32_t insertion_index = 0;
   Handle<FixedArrayBase> storage(result_array->elements(), isolate);
-  ElementsAccessor* accessor = ElementsAccessor::ForKind(elements_kind);
+  ElementsAccessor* accessor = ElementsAccessor::ForKind(result_elements_kind);
   for (uint32_t i = 0; i < concat_size; i++) {
     // It is crucial to keep |array| in a raw pointer form to avoid
     // performance degradation.
     JSArray* array = JSArray::cast((*args)[i]);
-    int len = Smi::cast(array->length())->value();
-    if (len > 0) {
-      ElementsKind from_kind = array->GetElementsKind();
-      accessor->CopyElements(array, 0, from_kind, storage, j, len);
-      j += len;
-    }
+    uint32_t len = 0;
+    array->length()->ToArrayLength(&len);
+    if (len == 0) continue;
+    ElementsKind from_kind = array->GetElementsKind();
+    accessor->CopyElements(array, 0, from_kind, storage, insertion_index, len);
+    insertion_index += len;
   }
 
-  DCHECK(j == result_len);
+  DCHECK_EQ(insertion_index, result_len);
   return result_array;
 }