.Net，最有效的比较

Question

I have 2 arrays of N objects, xs and ys. The objects are boxed ints. There are no null elements in either arrays. 1 >= N < 50

I can compare them with the following code:

for (var i = 0; i < N; i++)
{
    if (!xs[i].Equals(ys[i]))
    {
        return false;
    }
}

return true;

My question is: using .Net JIT or CLR tricks or some transformation, can I optimize this algorithm further?

Answer 1

If these aren't boxed you have a few more options, also this would basically only start to pay with larger arrays.

Also as mentioned in the comments, you could use parallel. But plinq and TPL probably won't give you any positive benefit with such small arrays, and will most definitely give a lot more overhead.

public static unsafe bool UnsafeCompare(int[] ary1, int[] ary2)
{
   fixed (int* pAry1 = ary1, pAry2 = ary2)
   {
      var pLen = pAry1 + ary1.Length;
      for (int* p1 = pAry1, p2 = pAry2; p1 < pLen; p1++, p2++)
         if (*p1 != *p2)
            return false;
   }
   return true;
}

or

[DllImport("msvcrt.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int memcmp(IntPtr b1, IntPtr b2, IntPtr count);

public static unsafe bool UnsafeCompare2(int[] ary1, int[] ary2)
{
   fixed (int* pAry1 = ary1, pAry2 = ary2)
   {
      return memcmp((IntPtr)pAry1, (IntPtr)pAry2, (IntPtr)(ary2.Length * sizeof(int))) == 0;
   }
}

Anyway, if you really are after performance, I'd be getting out your benchmarker and seeing what works for you.

Just an interesting note, this is the source for memcmp (or close enough):

memcmp (const PTR str1, const PTR str2, size_t count)
{
  register const unsigned char *s1 = (const unsigned char*)str1;
  register const unsigned char *s2 = (const unsigned char*)str2;

  while (count-- > 0)
    {
      if (*s1++ != *s2++)
      return s1[-1] < s2[-1] ? -1 : 1;
    }
  return 0;
}

Benchmarks

Take this with a grain of salt, however these tests were run a massive 100,000 times to try to tease out any subtle differences.

   ----------------------------------------------------------------------------
Mode             : Release (64Bit)
Test Framework   : .NET Framework 4.7.1 (CLR 4.0.30319.42000)
----------------------------------------------------------------------------
Operating System : Microsoft Windows 10 Pro
Version          : 10.0.17134
----------------------------------------------------------------------------
CPU Name         : Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Description      : Intel64 Family 6 Model 42 Stepping 7
Cores (Threads)  : 4 (8)      : Architecture  : x64
Clock Speed      : 3401 MHz   : Bus Speed     : 100 MHz
L2Cache          : 1 MB       : L3Cache       : 8 MB
----------------------------------------------------------------------------

Results

--- Standard input ------------------------------------------------------------
| Value    |    Average |    Fastest |    Cycles | Garbage | Test |      Gain |
--- Scale 5 ---------------------------------------------------- Time 0.475 ---
| Unsafe   |  54.271 ns |   0.000 ns |   1.850 K | 0.000 B | Pass |   16.95 % |
| Original |  65.349 ns |   0.000 ns |   1.820 K | 0.000 B | Base |    0.00 % |
| Memcmp   | 143.527 ns |   0.000 ns |   1.977 K | 0.000 B | Pass | -119.63 % |
--- Scale 50 --------------------------------------------------- Time 0.483 ---
| Unsafe   |  78.542 ns |   0.000 ns |   1.936 K | 0.000 B | Pass |   36.69 % |
| Original | 124.064 ns |   0.000 ns |   2.038 K | 0.000 B | Base |    0.00 % |
| Memcmp   | 181.076 ns |   0.000 ns |   2.066 K | 0.000 B | Pass |  -45.95 % |
--- Scale 500 -------------------------------------------------- Time 0.620 ---
| Memcmp   | 445.434 ns | 300.000 ns |   3.044 K | 0.000 B | Pass |   44.14 % |
| Unsafe   | 501.585 ns | 300.000 ns |   3.341 K | 0.000 B | Pass |   37.10 % |
| Original | 797.435 ns | 300.000 ns |   4.291 K | 0.000 B | Base |    0.00 % |
--- Scale 5,000 ------------------------------------------------ Time 2.172 ---
| Memcmp   |   3.519 µs |   2.701 µs |  13.625 K | 0.000 B | Pass |   46.95 % |
| Unsafe   |   5.110 µs |   4.502 µs |  19.084 K | 0.000 B | Pass |   22.96 % |
| Original |   6.633 µs |   5.703 µs |  24.364 K | 0.000 B | Base |    0.00 % |
--- Scale 50,000 ---------------------------------------------- Time 25.561 ---
| Memcmp   |  52.378 µs |  35.422 µs | 180.681 K | 0.000 B | Pass |   34.55 % |
| Unsafe   |  74.634 µs |  49.832 µs | 257.216 K | 0.000 B | Pass |    6.74 % |
| Original |  80.031 µs |  62.740 µs | 274.704 K | 0.000 B | Base |    0.00 % |
--- Scale 500,000 --------------------------------------------- Time 38.306 ---
| Memcmp   | 505.916 µs | 447.289 µs |   1.726 M | 0.000 B | Pass |   37.20 % |
| Unsafe   | 662.644 µs | 590.781 µs |   2.262 M | 0.000 B | Pass |   17.75 % |
| Original | 805.634 µs | 675.736 µs |   2.748 M | 0.000 B | Base |    0.00 % |
-------------------------------------------------------------------------------