VietMX Staff asked 3 years ago
V8 has two compilers:
- A “Full” Compiler that can generate good code for any JavaScript: good but not great JIT code. The goal of this compiler is to generate code quickly. To achieve its goal, it doesn’t do any type analysis and doesn’t know anything about types. Instead, it uses an Inline Caches or “IC” strategy to refine knowledge about types while the program runs. IC is very efficient and brings about 20 times speed improvment.
- An Optimizing Compiler that produces great code for most of the JavaScript language. It comes later and re-compiles hot functions. The optimizing compiler takes types from the Inline Cache and make decisions about how to optimize the code better. However, some language features are not supported yet like try/catch blocks for instance. (The workaround for try/catch blocks is to write the “non stable” code in a function and call the function in the try block)
V8 also supports de-optimization: the optimizing compiler makes optimistic assumptions from the Inline Cache about the different types, de-optimization comes if these assumptions are invalid. For example, if a hidden class generated was not the one expected, V8 throws away the optimized code and comes back to the Full Compiler to get types again from the Inline Cache. This process is slow and should be avoided by trying to not change functions after they are optimized.
