What is Just-In-Time(JIT) Compiler in .NET

Just-In-Time compiler(JIT) is a part of Common Language Runtime (CLR) in .NET which is responsible for managing the execution of .NET programs regardless of any .NET programming language. A language-specific compiler converts the source code to the intermediate language. This intermediate language is then converted into the machine code by the Just-In-Time (JIT) compiler. This machine code is specific to the computer environment that the JIT compiler runs on. 

Working of JIT Compiler: The JIT compiler is required to speed up the code execution and provide support for multiple platforms. Its working is given as follows: 

The JIT compiler converts the Microsoft Intermediate Language(MSIL) or Common Intermediate Language(CIL) into the machine code. This is done before the MSIL or CIL can be executed. The MSIL is converted into machine code on a requirement basis i.e. the JIT compiler compiles the MSIL or CIL as required rather than the whole of it. The compiled MSIL or CIL is stored so that it is available for subsequent calls if required. 

Types of Just-In-Time Compiler: There are 3 types of JIT compilers which are as follows: 

• Pre-JIT Compiler: All the source code is compiled into the machine code at the same time in a single compilation cycle using the Pre-JIT Compiler. This compilation process is performed at application deployment time. And this compiler is always implemented in the Ngen.exe (Native Image Generator). 

• Normal JIT Compiler: The source code methods that are required at run-time are compiled into machine code the first time they are called by the Normal JIT Compiler. After that, they are stored in the cache and used whenever they are called again. 

• Econo JIT Compiler: The source code methods that are required at run-time are compiled into machine code by the Econo JIT Compiler. After these methods are not required anymore, they are removed. This JIT compiler is obsolete starting from dotnet 2.0

Advantages of JIT Compiler: 

• The JIT compiler requires less memory usage as only the methods that are required at run-time are compiled into machine code by the JIT Compiler.
• Page faults are reduced by using the JIT compiler as the methods required together are most probably in the same memory page.
• Code optimization based on statistical analysis can be performed by the JIT compiler while the code is running.

Disadvantages of JIT compiler:  

• The JIT compiler requires more startup time while the application is executed initially.
• The cache memory is heavily used by the JIT compiler to store the source code methods that are required at run-time.

Note: Much of the disadvantages of the JIT compiler can be handled using the Ahead-of-time (AOT) compilation. This involves compiling the MSIL into machine code so that runtime compilation is not required and the machine code file can be executed natively.