C# byte[] comparison without bound checks

Question

I am looking for performance efficient ways to compare two byte[] for equality. Sizes are above 1 MB, so the overhead for each array element should be minimized.

I aim to beat the speeds of SequenceEqual or a hand-coded for-loop over every item, by avoiding the repetitive bound checks for both arrays. In the same way that Array.Copy could lead to fast memcpy, what will lead to a memcmp?

Answer 1

You can use unsafe code to do pointer operations. You can compare the bytes four at a time as integers:

public static bool ArrayCompare(byte[] a, byte[] b) {
  if (a.Length != b.Length) return false;
  int len = a.Length;
  unsafe {
    fixed(byte* ap = a, bp = b) {
      int* aip = (int*)ap, bip = (int*)bp;
      for (;len >= 4;len-=4) {
        if (*aip != *bip) return false;
        aip++;
        bip++;
      }
      byte* ap2 = (byte*)aip, bp2 = (byte*)bip;
      for (;len>0;len--) {
        if (*ap2 != *bp2) return false;
        ap2++;
        bp2++;
      }
    }
  }
  return true;
}

A tested this against a simple loop, and it's about six times faster.

As suggested by Josh Einstein, long could be used on a 64 bit system. Actually it seems to be almost twice as fast both on 32 and 64 bit systems:

public static bool ArrayCompare64(byte[] a, byte[] b) {
  if (a.Length != b.Length) return false;
  int len = a.Length;
  unsafe {
    fixed (byte* ap = a, bp = b) {
      long* alp = (long*)ap, blp = (long*)bp;
      for (; len >= 8; len -= 8) {
        if (*alp != *blp) return false;
        alp++;
        blp++;
      }
      byte* ap2 = (byte*)alp, bp2 = (byte*)blp;
      for (; len > 0; len--) {
        if (*ap2 != *bp2) return false;
        ap2++;
        bp2++;
      }
    }
  }
  return true;
}

Answer 2

If performance really matters then the fastest way to do it is by using the CRT library included with every version of Windows. This code takes ~51 msec on my poky laptop, works on 64-bit machines too:

using System;
using System.Runtime.InteropServices;
using System.Diagnostics;

class Program {
  static void Main(string[] args) {
    byte[] arr1 = new byte[50 * 1024 * 1024];
    byte[] arr2 = new byte[50 * 1024 * 1024];
    var sw = Stopwatch.StartNew();
    bool equal = memcmp(arr1, arr2, arr1.Length) == 0;
    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
    Console.ReadLine();
  }
  [DllImport("msvcrt.dll")]
  private static extern int memcmp(byte[] arr1, byte[] arr2, int cnt);
}

Answer 3

From: http://www.pinvoke.net/default.aspx/msvcrt.memcmp : Belowmentioned signature (by Saar) of memcmp is a x64 only signature. Using the the x64 only signatures on an x86 machine will result in a PInvoke stack imbalance. For x86 and x64 platform compatibility make sure you use a signature which specifies the Cdecl calling convention and uses the UIntPtr type to correctly marshall the size_t count argument:

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

    static bool doImagesMatch(byte[] b1, byte[] b2) 
    {     
        return b1.Length == b2.Length && memcmp(b1, b2, new UIntPtr((uint)b1.Length)) == 0;
    } 

I use this code with success but I didn't have time to measure performance (yet). I'm using small array's of roughly 600 bytes. I have to use x86-compatible code because the vast majority of the computers in our non-profit organisation is x86.

Obviously you need a fast algorhythm to convert bitmap to byte[].