哪个是比较两位数最高效的方法？

Question

I need to compare a lot of integer values to equality.

Which method is the fastest?

A)

int a1 = 12345;
int a2 = 54321;
if(a1 == a2)
{
   //do something.
}

B)

int a1 = 12345;
int a2 = 54321;
if( (a1 ^ a2) == 0 )
{
   //do something.
}

Answer 1

I suspect first is actually faster anyway. It's not like comparing two integers for equality is an uncommon operation, so I imagine it's pretty well supported :) However, far more importantly it's much, much more readable .

It seems very unlikely to me that comparing integers for equality will be your bottleneck. If you really feel it is, then by all means benchmark what you've got in as realistic a situation as you can, then try various changes (perhaps XOR, perhaps subtraction etc) and remeasure. But do make sure you've actually got something you need to fix before spending time moving away from the most obvious, simple, readable code.

Answer 2

The only way to answer a question like this is to measure it, in your specific application and environment.

But do the first and don't worry about it. There's nothing you're likely to do in C# where this is going to be an issue.

If you're in some bizarre corner case of performance where you really do have better knowledge about how to compare integers than the MS tools team, then you should be writing in assembler.

Personally, I cannot imagine a scenario where the comparison of two integers in a tight loop will dominate your timing - the loop and branching overhead is going to be similar or greater than the comparison cost, and even then that assumes that this great block of data has magically appeared in CPU cache without any paging or memory I/O cost.

Rococo substitutions for operations like compares (and set-to-zero and multiply-by-constant powers of two) haven't been appropriate with proper commercial tool chains (craptastic embedded C compilers might still need them) for the best part of 20 years. It's time to stop thinking about them.

Answer 3

I have wrote some code to verify this.

static void Main( string[] args )
{
  int a1 = 123456;
  int a2 = 654321;
  int a3;

  var t = new System.Diagnostics.Stopwatch();
  t.Start();
  for ( int i = 0; i < int.MaxValue; i++ )
  {
    if ( a1 == a2 )
    {
      a3 = a1 + a2;
    }
  }
  t.Stop();

  Console.WriteLine( t.ElapsedMilliseconds );

  t.Reset();

  t.Start();
  for ( int i = 0; i < int.MaxValue; i++ )
  {
    if ( ( a1 ^ a2 ) == 0 )
    {
      a3 = a1 + a2;
    }
  }
  t.Stop();

  Console.WriteLine( t.ElapsedMilliseconds );

  Console.ReadKey();
}

The result is the next:
For the first 6275 milliseconds
For the second 6277 milliseconds

It looks that these methods works identical.

Thanks for your answers.

Answer 4

The first one. Because both of them do a comparing operation (==), but the second one uses additional XOR operation.