The Just-in-Time compiler

...
Unlike a traditional off-line C++ compilation process, the time spent in the JIT compiler is a "wall clock time" delay, in each user's face, so the JIT compiler does not have the luxury of exhaustive optimization passes. Even so, the list of optimizations the JIT compiler performs is impressive:

Constant folding
Constant and copy propagation
Common subexpression elimination
Code motion of loop invariants
Dead store and dead code elimination
Register allocation
Method inlining
Loop unrolling (small loops with small bodies)

ArrayList ar = new ArrayList();
for(int i = 0; i < ar.Count; i++)
{
   ar[i] = ...;
}

ArrayList ar = new ArrayList();
int count = ar.Count;
for(int i = 0; i < count; i++)
{
   ar[i] = ...;
}

class C
{
  private ArrayList myList;
  public C(ArrayList list) {myList = list;}
  public void foo() {myList.Add(1);}
}

......
ArrayList list = new ArrayList (collection);
C c = new C(list);
for (int i=0; i<list.Count(); i++)
  c.foo();

X>class C
X>{
X>  private ArrayList myList;
X>  public C(ArrayList list) {myList = list;}
X>  public void foo() {myList.Add(1);}
X>}

X>......
X>ArrayList list = new ArrayList (collection);
X>C c = new C(list);
X>for (int i=0; i<list.Count(); i++)
X>  c.foo();
X>

class C {
  int Count() const;
  void foo() const; 
}

void Foo( C &c ) {
  
  for( int i=0; i<c.Count(); ++i ) {
    c.foo();
  }
}

int []a = new int[100];
for( int i=0; i<a.Length; ++i ) {
  a[i] = i;
}

int []a = new int[100];
int nItems = a.Length;
for( int i=0; i<nItems; ++i ) {
  a[i] = i;
}

            Console.WriteLine("Framework Version: " + Environment.Version);

            const int count = 10000;
            List<int> array = new List<int>(count);

            for (int i = 0; i < count; i++)
                array.Add(i % 20);

            Console.WriteLine("Start");

            int start = Environment.TickCount;

            int sum = 0;

            int len = array.Count;

            for (int k = 0; k < 100000; k++)
                for (int i = 0; i < len; i++)
                    sum += array[i];

            Console.WriteLine("Time is {0}, sum = {1}", 
                Environment.TickCount - start, sum);

Framework Version: 2.0.50215.44
Start
Time is 2719, sum = 910065408

            Console.WriteLine("Framework Version: " + Environment.Version);

            const int count = 10000;
            List<int> array = new List<int>(count);

            for (int i = 0; i < count; i++)
                array.Add(i % 20);

            Console.WriteLine("Start");

            int start = Environment.TickCount;

            int sum = 0;

            for (int k = 0; k < 100000; k++)
                for (int i = 0; i < array.Count; i++)
                    sum += array[i];

            Console.WriteLine("Time is {0}, sum = {1}", 
                Environment.TickCount - start, sum);

Framework Version: 2.0.50215.44
Start
Time is 2250, sum = 910065408

            Console.WriteLine("Framework Version: " + Environment.Version);

            const int count = 10000;
            List<int> array1 = new List<int>(count);

            for (int i = 0; i < count; i++)
                array1.Add(i % 20);

            Console.WriteLine("Start");

            int start = Environment.TickCount;

            int sum = 0;

            int len = array1.Count;
            int[] array = array1.ToArray();

            for (int k = 0; k < 100000; k++)
                for (int i = 0; i < array.Length; i++)
                    sum += array[i];

            Console.WriteLine("Time is {0}, sum = {1}", 
                Environment.TickCount - start, sum);

Framework Version: 2.0.50215.44
Start
Time is 1359, sum = 910065408

            Console.WriteLine("Framework Version: " + Environment.Version);

            const int count = 10000;
            List<int> array1 = new List<int>(count);

            for (int i = 0; i < count; i++)
                array1.Add(i % 20);

            Console.WriteLine("Start");

            int start = Environment.TickCount;

            int sum = 0;

            int len = array1.Count;
            int[] array = array1.ToArray();

            for (int k = 0; k < 100000; k++)
                for (int i = 0; i < len; i++)
                    sum += array[i];

            Console.WriteLine("Time is {0}, sum = {1}", 
                Environment.TickCount - start, sum);

Framework Version: 2.0.50215.44
Start
Time is 1359, sum = 910065408

            Console.WriteLine("Framework Version: " + Environment.Version);

            const int count = 10000;
            int [] array = new int[count];

            for (int i = 0; i < count; i++)
                array[i] = i % 20;

            Console.WriteLine("Start");

            int start = Environment.TickCount;

            int sum = 0;

            int len = array.Length;

            for (int k = 0; k < 100000; k++)
                for (int i = 0; i < len; i++)
                    sum += array[i];

            Console.WriteLine("Time is {0}, sum = {1}", 
                Environment.TickCount - start, sum);

Framework Version: 2.0.50215.44
Start
Time is 1344, sum = 910065408

            Console.WriteLine("Framework Version: " + Environment.Version);

            const int count = 10000;
            int [] array = new int[count];

            for (int i = 0; i < count; i++)
                array[i] = i % 20;

            Console.WriteLine("Start");

            int start = Environment.TickCount;

            int sum = 0;

            int len = array.Length;

            for (int k = 0; k < 100000; k++)
                for (int i = 0; i < array.Length; i++)
                    sum += array[i];

            Console.WriteLine("Time is {0}, sum = {1}", 
                Environment.TickCount - start, sum);

Framework Version: 2.0.50215.44
Start
Time is 906, sum = 910065408