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I was studying the inner workings of V8 and came across the term JIT compiler. Initially, I read in this article https://www.quora.com/How-does-the-JIT-compiler-work-in-JS that JIT compiler in V8 is called "Ignition" which is interpreter. Then, I came to conclusion that JIT compiler is just interpreter. But later I found another article https://blog.logrocket.com/how-javascript-works-optimizing-the-v8-compiler-for-efficiency/ describing jit-compilation as the combination of both interpreter and compiler. Guys, is JIT compiler a really a combination of interpreter and compiler? or Is JIT compiler is just interpreter only?
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Unlike other languages, The V8 engine uses both a compiler and an interpreter and follows Just in Time(JIT) Compilation for improved performance. Just in Time(JIT) Compilation: The V8 engine initially uses an interpreter, to interpret the code.
A Just-In-Time (JIT) compiler is a feature of the run-time interpreter, that instead of interpreting bytecode every time a method is invoked, will compile the bytecode into the machine code instructions of the running machine, and then invoke this object code instead.
As an example of optimizing based on program execution feedback, the **inline cache** allows V8 to optimize repeated calls to a function with the same type arguments. Specifically, the inline cache stores the types of input to a function.
JIT is the tool which can transform bytecode to the binary code. javac is the tool which can transform code to the Java bytecode.
In computing, just-in-time ( JIT) compilation (also dynamic translation or run-time compilations) is a way of executing computer code that involves compilation during execution of a program (at run time) rather than before execution.
However, because JIT compilation usually only has to be ran the first time a function is invoked, commonly used functions will only really see a performance hit on the first invocation. You can test this in C# with StopWatches—they pick up .NET “background noise,” including the time spent doing JIT compilation on the first run of a function.
Dynamic compilation was first described in a paper by J. McCarthy on LISP in 1960. Just In Time Compilation, JIT, or Dynamic Translation, is compilation that is being done during the execution of a program. Meaning, at run time, as opposed to prior to execution. What happens is the translation to machine code.
Code optimization based on statistical analysis can be performed by the JIT compiler while the code is running. The JIT compiler requires more startup time while the application is executed initially. The cache memory is heavily used by the JIT compiler to store the source code methods that are required at run-time.
V8 developer here. Just to clarify and expand what commenters have been pointing out already:
"JIT" means "just in time", and means that some execution environment dynamically (i.e. at runtime) decides to produce something (typically machine code -- colloquially, "JIT" tends to mean "just-in-time compilation", although if you decide to prepare a meal exactly when you're hungry and then eat it right away when it's done, then that's technically also "JIT" preparation.) The canonical opposite would be a language like C/C++, which is compiled by the developer, long before being delivered to and executed by the user. Another "opposite" in a different direction is an execution environment that executes something without producing machine code dynamically. Such environments are typically called "interpreters".
In the past, V8 used to always produce machine code. It simply had no way to execute JavaScript that did not involve compiling it to machine code first. Obviously this happened on the client, so it was a textbook example of a just-in-time compiler (or, more accurately, a set of several compilers... oh well, details!).
In recent years, V8 has had an interpreter as its first execution tier. Now usage of terms gets complicated, because this interpreter "compiles" JavaScript "just in time" to bytecode (which is then interpreted), but when someone says "JIT compiler", they usually mean that it's not an interpreter.
V8 also has an optimizing compiler that produces machine code. It runs at runtime (when a function is considered hot), so it's a just-in-time compiler.
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