what's the difference between __builtin_popcountll and_mm_popcnt_u64?

I was trying to how many 1 in 512MB memory and I found two possible methods, _mm_popcnt_u64() and __builtin_popcountll() in the gcc builtins.

_mm_popcnt_u64() is said to use the CPU introduction SSE4.2，which seems to be the fastest, and __builtin_popcountll() is excepted to use table lookup.

So, I think __builtin_popcountll() should be little slower than _mm_popcnt_u64().

However I got a result like this:

It took almost the same time for two methods. I highly doubt that they used the same way to work.

I also got this in popcntintrin.h

/* Calculate a number of bits set to 1. */
extern __inline int __attribute__((__gnu_inline__, __always_inline__, __artificial___))
_mm_popcnt_u32 (unsigned int __X)
{
  return __builtin_popcount (__X);
}

#ifdef __x86_64__
extern __inline long long __attribute__((__gnu_inline__, __always_inline__, __artificial__))
_mm_popcnt_u64 (unsigned long long __X)
{
  return __builtin_popcountll (__X);
}
#endif

So, I'm confused how __builtin_popcountll() works on earth