Static vector internal data layout - `union` vs `std::aligned_storage_t` - huge performance difference

Question

Assume that you have to implement a static_vector<T, N> class, which is a fixed capacity container that entirely lives on the stack and never allocates, and exposes an std::vector-like interface. (Boost provides boost::static_vector.)

Considering that we must have uninitialized storage for maximum N instances of T, there are multiple choices that can be made when designing the internal data layout:

• Single-member union:

  union U { T _x; };
  std::array<U, N> _data;
  

• Single std::aligned_storage_t:

  std::aligned_storage_t<sizeof(T) * N, alignof(T)> _data;
  

• Array of std::aligned_storage_t:

  using storage = std::aligned_storage_t<sizeof(T), alignof(T)>;
  std::array<storage, N> _data;
  

Regardless of the choice, creating the members will require the use of "placement new" and accessing them will require something along the lines of reinterpret_cast.

---

Now assume that we have two very minimal implementations of static_vector<T, N>:

• with_union: implemented using the "single-member union" approach;

• with_storage: implemented using the "single std::aligned_storage_t" approach.

Let's perform the following benchmark using both g++ and clang++ with -O3. I used quick-bench.com for this task:

void escape(void* p) { asm volatile("" : : "g"(p) : "memory"); }
void clobber()       { asm volatile("" : :        : "memory"); }

template <typename Vector>
void test()
{
    for(std::size_t j = 0; j < 10; ++j)
    {
        clobber();
        Vector v;
        for(int i = 0; i < 123456; ++i) v.emplace_back(i);
        escape(&v);
    }
}

(escape and clobber are taken from Chandler Carruth's CppCon 2015 talk: "Tuning C++: Benchmarks, and CPUs, and Compilers! Oh My!")

• Results for g++ 7.2 (live here):

• Results for clang++ 5.0 (live here):

---

As you can see from the results, g++ seems to be able to aggressively optimize (vectorization) the implementation that uses the "single std::aligned_storage_t" approach, but not the implementation using the union.

My questions are:

• Is there anything in the Standard that prevents the implementation using union from being aggressively optimized? (I.e. does the Standard grant more freedom to the compiler when using std::aligned_storage_t - if so, why?)

• Is this purely a "quality of implementation" issue?

Answer 1

xskxzr is right, this is the same issue as in this question. Fundamentally, gcc is missing an optimization opportunity in forgetting that std::array's data is aligned. John Zwinck helpfully reported bug 80561.

You can verify this in your benchmark by making one of two changes to with_union:

1. Change _data from a std::array<U, N> to simply a U[N]. Performance becomes identical

2. Remind gcc that _data is actually aligned by changing the implementation of emplace_back() to:

   template <typename... Ts> 
   T& emplace_back(Ts&&... xs)
   {
       U* data = static_cast<U*>(__builtin_assume_aligned(_data.data(), alignof(U)));
       T* ptr = &data[_size++]._x;
       return *(new (ptr) T{std::forward<Ts>(xs)...});
   }
   

Either of those changes with the rest of your benchmark gets me comparable results between WithUnion and WithAlignedStorage.