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A just-in-time (JIT) compiler is a compiler that compiles code during program execution, rather than ahead of time. Many traditional compilers compiled code, transitioning between code input and machine language, well before runtime. A JIT compiler is a way to compile in real time or on the fly as the program executes.
Just-in-time compilation is also known as dynamic translation.
Ahead-of-time (AOT) compilers go through all of the code before the program is ever run. This allows for the allocation of more resources to the compile process without slowing down initial program execution. JIT compilers can be slower, because they have to balance compile resources with runtime concerns. However, one of the benefits of a JIT compiler is that the on-the-fly compile can allow for dynamic changes according to the platform.
In general, JIT compilers offer a key choice for developers and engineers, the ability to only compile when the program is run, rather than splitting up compile and initial execution into two separate stages.