Code Generator Oddity

The following is a exerpted example of my implementation of a well-known permutation algorithm. Note the code generation oddity flagged by the comment. This occurs in .NET 3.5 and Visual C# 2008. I leave the solution to the mystery to readers who like to disassemble and read MSIL, and think about what the JIT compiler may do to it.

protected static void Permute <TDatum> (
  int              beginIn,
  int              endIn,
  TDatum[]         dataIn,
  Action<TDatum[]> dataActionIn)
where TDatum : IComparable
{
  TDatum xDatumBegin;
      
  if ((endIn-beginIn)<=1)
  {
    dataActionIn(dataIn);
  }
  else
  {
    Permute(
      beginIn+1,
      endIn,
      dataIn,
      dataActionIn); 
        
    for (int i=beginIn+1; i<endIn; i++)
    {
      xDatumBegin = dataIn[beginIn];
          
      dataIn[beginIn] = dataIn[i];
      dataIn[i]       = xDatumBegin;
          
      Permute(
        beginIn+1,
        endIn,
        dataIn,
        dataActionIn); 
          
      // Believe it or not, the compiler/loader 
      // will optimize the first block below 
      // to be almost 10% faster than the second.
          
      #if thisIsFasterInDotNet
        xDatumBegin     = dataIn[beginIn];
        dataIn[beginIn] = dataIn[i];
        dataIn[i]       = xDatumBegin;
      #else  
        dataIn[i]       = dataIn[beginIn];
        dataIn[beginIn] = xDatumBegin;
      #endif
    }
  }
}