As experienced programmers, in whatever language we favored before C#, we internalized several practices for developing more efficient code. Sometimes what worked in our previous environment is counterproductive in the .NET environment. This is very true when you try to hand-optimize algorithms for the C# compiler. Your actions often prevent the JIT compiler from more effective optimizations. Your extra work, in the name of performance, actually generates slower code. You're better off writing the clearest code you can create. Let the JIT compiler do the rest. One of the most common examples of premature optimizations causing problems is when you create longer, more complicated functions in the hopes of avoiding function calls. Practices such as hoisting function logic into the bodies of loops actually harm the performance of your .NET applications. It's counterintuitive, so let's go over all the details.
This chapter's introduction contains a simplified discussion of how the JIT compiler performs its work. The .NET runtime invokes the JIT compiler to translate the IL generated by the C# compiler into machine code. This task is amortized across the lifetime of your program's execution. Instead of JITing your entire application when it starts, the CLR invokes the JITer on a function-by-function basis. This minimizes the startup cost to a reasonable level, yet keeps the application from becoming unresponsive later when more code needs to be JITed. Functions that do not ever get called do not get JITed. You can minimize the amount of extraneous code that gets JITed by factoring code into more, smaller functions rather than fewer larger functions.
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