JIT Compilation

the C# source code was compiled into IL by C# compiler, and at runtime, the JIT compiler will compile the IL into Native code, the JIT compiler compiles the code on a method-by-method basis, if the method has been JITed, it will not be JITed again.

JIT compilation can hurt startup times. so here comes NGen.

NGen can precompile IL to native code, that can be done before run time

Microsoft is now working on a replacement JIT called RyuJIT

Inlining

JIT compiler will replace method call with method body in the call site, for example

int add(int x, int y) {
  return x + y;
}

int z = add(4,5);

will be optimized to 

int z = 4 + 5

this will get rid of method call overhead

when JIT will inline a method

1. Don’t contain exception handling (try…catch)
2. Are not virtual
3. Are not recursive

[MethodImpl(MethodImplOptions.NoInlining)] tells JIT never inline this method
void Method1()
[MethodImpl(MethodImplOptions.AggressiveInlining)] tells JIT always inline this method when possible
void Method2()