[MEMORY] Fix gcc 4.8 compact (#4461)

include/tvm/runtime/memory.h

@@ -149,7 +149,11 @@ class SimpleObjAllocator :
   template<typename ArrayType, typename ElemType>
   class ArrayHandler {
    public:
-    using StorageType = typename std::aligned_union<sizeof(ArrayType), ArrayType, ElemType>::type;
+    using StorageType = typename std::aligned_storage<sizeof(ArrayType), alignof(ArrayType)>::type;
+    // for now only support elements that aligns with array header.
+    static_assert(alignof(ArrayType) % alignof(ElemType) == 0 &&
+                  sizeof(ArrayType) % alignof(ElemType) == 0,
+                  "element alignment constraint");
 
     template<typename... Args>
     static ArrayType* New(SimpleObjAllocator*, size_t num_elems, Args&&... args) {
@@ -160,15 +164,15 @@ class SimpleObjAllocator :
       // In the case of an object pool, an allocator needs to create
       // a special chunk memory that hides reference to the allocator
       // and call allocator's release function in the deleter.
-
       // NOTE2: Use inplace new to allocate
       // This is used to get rid of warning when deleting a virtual
       // class with non-virtual destructor.
       // We are fine here as we captured the right deleter during construction.
       // This is also the right way to get storage type for an object pool.
-      size_t factor = sizeof(ArrayType) / sizeof(ElemType);
-      num_elems = (num_elems + factor - 1) / factor;
-      StorageType* data = new StorageType[num_elems+1];
+      size_t unit = sizeof(StorageType);
+      size_t requested_size = num_elems * sizeof(ElemType) + sizeof(ArrayType);
+      size_t num_storage_slots = (requested_size + unit - 1) / unit;
+      StorageType* data = new StorageType[num_storage_slots];
       new (data) ArrayType(std::forward<Args>(args)...);
       return reinterpret_cast<ArrayType*>(data);
     }