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Just-In-Time Compilation ○｜Definition｜1st｜20251119205401-00-⌔

Just-in-time compilation - Wikipedia

Just-in-time compilation

Just-in-time (JIT) compilation (also dynamic translation or run-time compilations)1 is compilation of computer code during execution of a program at run time rather than before execution.2 This may consist of source code translation but is more commonly bytecode translation to machine code, which is then executed directly. A system implementing a JIT compiler typically continuously analyses the code being executed and identifies parts of the code where the speedup gained from compilation or recompilation would outweigh the overhead of compiling that code.

JIT compilation is a combination of the two traditional approaches to translation to machine code: ahead-of-time compilation (AOT), and interpretation, which combines some advantages and drawbacks of both.2 Roughly, JIT compilation combines the speed of compiled code with the flexibility of interpretation, with the overhead of an interpreter and the additional overhead of compiling and linking (not just interpreting). JIT compilation is a form of dynamic compilation, and allows adaptive optimization such as dynamic recompilation and microarchitecture -specific speedups.34 Interpretation and JIT compilation are particularly suited for dynamic programming languages, as the runtime system can handle late-bound data types and enforce security guarantees.

Printed 2026-06-28.

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Footnotes

  1. Languages, Compilers, and Runtime Systems, University of Michigan, Computer Science and Engineering, archived from the original on March 26, 2018, retrieved March 15, 2018 ↩

  2. Aycock 2003. ↩ ↩2

  3. Ahead-of-Time compilers can target specific microarchitectures as well, but the difference between AOT and JIT in that matter is one of portability. A JIT can render code tailored to the currently running CPU at runtime, whereas an AOT, in lieu of optimizing for a generalized subset of uarches, must know the target CPU in advance: such code may not only be not performant on other CPU types but may be outright unstable. ↩

  4. “Does the JIT take advantage of my CPU?”. David Notario’s WebLog. Retrieved 2018-12-03. ↩

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