What is jit?

Just-In-Time (JIT) Compilation

Just-In-Time (JIT) compilation is a program execution technique that involves compiling code during runtime, rather than before execution. It's a hybrid approach combining the advantages of both interpreters and traditional ahead-of-time (AOT) compilers.

Key Concepts:

• Runtime Compilation: JIT compilers translate bytecode or intermediate representation (IR) into native machine code while the program is running. This contrasts with AOT compilers that perform the translation before execution.
• Dynamic Optimization: JIT compilers can analyze the program's behavior during execution and optimize the generated machine code based on observed characteristics. This is known as <a href="https://www.wikiwhat.page/kavramlar/Dynamic%20Optimization">dynamic optimization</a>. For example, it can inline frequently called methods, optimize loops, and specialize code for specific data types.
• Adaptive Compilation: Modern JIT compilers often employ adaptive compilation techniques. This means they initially compile code using a fast, less optimized approach, and then recompile frequently executed parts with more aggressive optimizations (often referred to as "hotspots"). This is known as <a href="https://www.wikiwhat.page/kavramlar/Hotspot%20Detection">hotspot detection</a>.
• Bytecode/Intermediate Representation (IR): Many JIT compilers operate on an intermediate representation of the code, rather than directly compiling the source code. This bytecode or IR is often platform-independent, allowing the same code to be executed on different architectures. A prominent example is the Java Virtual Machine (JVM) which uses <a href="https://www.wikiwhat.page/kavramlar/JVM%20Bytecode">JVM bytecode</a>.
• Trade-offs: JIT compilation offers performance benefits compared to interpreters because compiled code executes much faster. However, it incurs a compilation overhead at runtime. This overhead can be significant when the JIT compiler needs to perform extensive analysis and optimization. Therefore, JIT compilers make trade-offs between compilation time and the quality of the generated code. <a href="https://www.wikiwhat.page/kavramlar/Compilation%20Overhead">Compilation Overhead</a> is a crucial factor in the overall JIT efficiency.

Advantages of JIT Compilation:

• Performance Improvements: Significant performance gains over interpreters as code is translated to native machine code.
• Platform Independence: Facilitates platform independence if combined with bytecode/IR. The bytecode can run on any platform with a compatible JIT compiler.
• Dynamic Optimization: Enables optimizations that are not possible at compile time, based on runtime behavior.
• Reduced Startup Time (compared to AOT in some cases): Only compiles necessary code during runtime, potentially leading to faster startup times.

Disadvantages of JIT Compilation:

• Runtime Overhead: Compilation at runtime consumes CPU resources, potentially impacting performance, especially during initial execution.
• Increased Memory Footprint: Both the bytecode/IR and the generated machine code reside in memory.
• Security Considerations: JIT compilers are complex pieces of software and can be susceptible to security vulnerabilities.

Examples of Languages/Platforms using JIT Compilation:

• Java (JVM)
• .NET (CLR)
• JavaScript (V8, SpiderMonkey, JavaScriptCore)
• Python (PyPy)