The V8 JavaScript engine compiles JavaScript code using a multi-stage pipeline that includes parsing, compilation, and execution phases, leveraging Just-In-Time (JIT) compilation for efficient performance. Here's a detailed breakdown of each stage:
1. Parsing Phase
Initially, when JavaScript code is loaded into V8, the engine parses the source code to convert it into an Abstract Syntax Tree (AST). The AST represents the structure of the code in a tree format, where each node denotes a part of the code (like variables, operators, etc.). This process involves breaking down the code into tokens and constructing the tree based on the syntax rules of JavaScript[2][4][19].
2. Compilation Phase
After parsing, the AST is passed to the Ignition interpreter. Ignition is V8's interpreter that processes the AST to generate bytecode. Bytecode is a lower-level, simpler set of instructions that is closer to machine code but is not directly executable by the CPU[2][4][15][19].
3. Execution Phase
The bytecode generated by Ignition is then executed. Initially, this execution is not highly optimized. V8 monitors the execution to gather type information and runtime statistics, which helps in identifying "hot" parts of the code that are executed frequently[2][4][15].
4. Optimization with TurboFan
For frequently executed code, V8 employs its optimizing compiler, TurboFan. TurboFan takes the bytecode and the collected type information to generate highly optimized machine code. This machine code is directly executable by the CPU, leading to faster execution of JavaScript code[2][4][15][16].
5. Deoptimization
If, during execution, the assumptions made during the optimization phase are invalidated (for example, due to unexpected types of inputs), V8 can deoptimize the code. This means reverting back to the less optimized bytecode and recompiling it with updated assumptions, ensuring that the execution continues correctly and efficiently[4][15].
6. Inline Caching and Hidden Classes
V8 also uses techniques like inline caching and hidden classes to optimize property access in objects. Inline caching speeds up property access by remembering the locations of previously accessed prope...
