In computing, just-in-time compilation (JIT), also known as dynamic translation, is a technique for improving the runtime performance of a computer program. JIT builds upon two earlier ideas in run-time environments: bytecode compilation and dynamic compilation. It converts code at runtime prior to executing it natively, for example bytecode into native machine code. The performance improvement over interpreters originates from caching the results of translating blocks of code, and not simply reevaluating each line or operand each time it is met.
The high level programming languages that need to be compiled require a runtime, so that the architecture on which the language runs is provided with details on how to execute its code. All the programming languages use its corresponding runtime to run the application. For example, to run an application developed using Visual Basic, the computer on which the application will be run must be installed with the Visual Basic runtime. The Visual Basic runtime can run only the applications developed with Visual Basic and not the ones developed with any other programming language like Java.
In the .NET Framework, all the Microsoft .NET languages use a common language runtime, which solves the problem of installing separate runtime for each of the programming languages. Microsoft .NET Common Language Runtime installed on a computer can run any language that is Microsoft .NET compatible.
The main advantage of the .NET Framework is the interoperability between different languages. As all the Microsoft .NET languages share the same common runtime language, they all work well together. For example, you can use an object written in C# from Visual Basic.NET. The same applies for all the other Microsoft .NET languages.
When you compile a Microsoft.NET language, the complier generates code written in the Microsoft Intermediate Language (MSIL). MSIL is a set of instructions that can quickly be translated into native code.
A Microsoft.NET application can be run only after the MSIL code is translated into native machine code. In .NET Framework, the intermediate language is complied "just in time" (JIT) into native code when the application or component is run instead of compiling the application at development time. The Microsoft.NET runtime consists of two JIT compilers. They are standard JIT compiler and the EconoJIT compiler. The EconoJIT compiler compiles faster than the standard JIT compiler, but the code it produces is not as optimized as the code obtained from the standard JIT compiler.
JIT code generally offers far better performance than interpreters. In addition, it can in some or many cases offer better performance than static compilation, as many optimizations are only feasible at run-time:
- The compilation can be optimized to the targeted CPU and the operating system model where the application runs. For example JIT can choose SSE2 CPU instructions when it detects that the CPU supports them. With a static compiler one must write two versions of the code, possibly using inline assembly.
- The system is able to collect statistics about how the program is actually running in the environment it is in, and it can rearrange and recompile for optimum performance. However, some static compilers can also take profile information as input.
- The system can do global code optimizations without losing the advantages of dynamic linking and without the overheads inherent to static compilers and linkers. Specifically, when doing global inline substitutions, a static compiler must insert run-time checks and ensure that a virtual call would occur if the actual class of the object overrides the inlined method.
- Although this is possible with statically compiled garbage collected languages, a bytecode system can more easily rearrange memory for better cache utilization.