Just In Time (JIT) Compilers - Computerphile

TL;DR

Just-in-time compilation makes programs faster by observing them as they run and converting frequently used or predictable code into optimized machine code. Unlike static compilation, a JIT compiler can use runtime details, such as a function repeatedly receiving integers, though the program initially runs more slowly while it warms up. Read on to compare JITs with compilers and interpreters and see how the technique applies to Java, JavaScript, and Python.

Transcript

we're going to be looking at uh just in time compilation which is a technique for making programming languages run faster so we all want faster programming languages because then we get more speed and there are various ways we can do it so we can statically compile things that's typically done for languages like C or we can just in time compile the... Read More

Key Insights

• ⌛ Just-in-time compilation optimizes programming languages at runtime based on observed program behavior.
• ❓ Static compilation, interpretation, and JIT compilation are all valid approaches for implementing programming languages.
• 💁 JIT compilation can be more effective than static compilation due to having more information at runtime.
• ❓ JIT compilers have been used for languages like Java, JavaScript, and Python to improve performance.
• ❓ The effectiveness of JIT compilation depends on the nature of the program, not just its size.
• 😒 JIT compilation has been in use since the 1980s and has evolved to be used in various modern programming languages.
• 😤 JIT compilation is considered expensive and complex to implement, requiring teams and years of development.

Questions & Answers

Q: What is just-in-time (JIT) compilation?

Just-in-time compilation is a technique for making programming languages run faster by analyzing a program while it is running. A JIT compiler observes behavior such as frequent function calls or consistent parameter types, then produces optimized machine code for that part of the program.

Q: How does JIT compilation differ from static compilation?

A static compiler examines source code and converts it into machine code before the program runs. A JIT compiler begins with the running program, observes its actual behavior, and creates optimized machine code at runtime using information that was unavailable during static compilation.

Q: How is a JIT compiler different from an interpreter?

An interpreter executes a program largely as written, often after converting it into another representation, without producing optimized machine code in the same way. A JIT compiler nearly always starts by running the program through an interpreter, then compiles selected parts after observing how they behave.

Q: Why can JIT compilation be faster than static compilation?

A JIT compiler has runtime information about what the program actually does, while a static compiler sees only the code and must make assumptions. The JIT can therefore specialize frequently used code, for example, a function whose parameters are consistently integers, and produce a higher-quality optimization.

Q: What does JIT compiler warm-up mean?

A JIT-compiled program may start slowly because the compiler must first watch and analyze its execution. As optimized versions of frequently used code are produced, the program may get faster; this process is called warming up, though the transcript notes that it does not always behave as expected.

Q: Which programming languages and runtimes use JIT compilers?

The transcript identifies Java and JavaScript as languages often implemented with JIT compilation. It also names the Java Virtual Machine, V8 in Chrome, and SpiderMonkey in Firefox as JIT-based runtimes, and demonstrates a JIT approach for Python.

Q: Does using a browser involve JIT compilation?

Yes, according to the transcript, using a browser means using a JavaScript just-in-time compiler. Two examples given are V8 in Chrome and SpiderMonkey in Firefox.

Q: Are programming languages inherently compiled or interpreted?

No. The transcript explains that a given programming language can be implemented with a compiler, an interpreter, or a just-in-time compiler; C interpreters and static JavaScript compilers can exist even though those languages are commonly associated with other implementation approaches.

Summary & Key Takeaways

• Just-in-time (JIT) compilation is a technique used to optimize programming languages by observing the program while it's running and optimizing it based on that information.

• JIT compilation can be more effective than static compilation because it has more information about the program at runtime.

• JIT compilers are commonly used in languages like Java, JavaScript, and Python to improve performance.