Compiler vs Interpreter vs JIT: Differences Explained

There are three main ways to run source code. A compiler (ahead-of-time, or AOT) translates the whole program into machine code before it runs. An interpreter reads and executes the program as it runs, without producing a separate machine-code file. A just-in-time (JIT) compiler starts by interpreting and then compiles the frequently used parts into machine code while the program is running. Many languages mix these approaches.

AspectCompiler (AOT)InterpreterJIT compiler
When code is translatedBefore running: the whole program becomes machine codeWhile running: the program is read and executed step by stepWhile running: frequently used code is compiled into machine code
Start-upFast to start (but needs a build step first)Fast to start, with no build stepSlower at first, until the code is warmed up
Execution speedFastest and most predictableSlowest, often many times slower than compiled codeClose to compiled speed after warm-up, sometimes faster by using run-time information
PortabilityA binary for each platformPortable: runs wherever the interpreter existsPortable bytecode, with a platform-specific runtime
Error detectionMany errors are caught at compile timeErrors appear when the faulty line runsDepends on the language
Memory useLow run-time overheadThe interpreter itself stays in memoryExtra memory for the compiler and compiled-code cache
Development cycleEdit, build, runEdit and run immediatelyUsually edit and run, or build to bytecode
Example languagesC, C++, Rust, GoClassic Python (CPython), Ruby, shell scriptsJava (HotSpot), JavaScript (V8), C# (.NET), PyPy

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Compiler
When performance, small size and predictable behavior matter: operating systems, games, embedded firmware.
Interpreter
For quick experiments, scripting and learning, where fast development matters more than speed.
JIT
For long-running programs on a portable platform that need speed, such as web browsers and server applications.

FAQ

Is Python compiled or interpreted?

The standard implementation, CPython, compiles source code to bytecode and then interprets it. Other implementations such as PyPy add a JIT compiler for more speed.

Is Java compiled or interpreted?

Both. Java is compiled ahead of time to bytecode, which the JVM then interprets and JIT-compiles to machine code while the program runs.

Why is compiled code faster?

The whole program is translated into optimized machine code before it runs, so nothing needs to be interpreted at run time.

What is the warm-up of a JIT compiler?

At first the program runs in an interpreter, and the JIT compiles the hot code once it has seen it run often. Until then the speed is lower.

Check your understanding

Which correctly distinguishes a compiler from an interpreter?

  1. AA compiler translates before running; an interpreter executes while reading
  2. BA compiler translates line by line while running; an interpreter translates everything before running
  3. CA compiler handles only numeric computation, and an interpreter only text processing
Show answer and explanation

Answer: A. A compiler translates before running; an interpreter executes while reading

A compiler translates an entire program to machine code or similar before execution and then runs the result. Translation takes time, but execution is faster and optimization can be thorough. An interpreter reads and executes the program as it goes, so it is quick to try but tends to run slower. In practice many systems combine the two, translating to bytecode, interpreting it and JIT-compiling hot parts. They are not distinguished by the kinds of tasks they handle.

Which describes a runtime that uses JIT compilation?

  1. AThe entire program is translated to machine code before execution and then distributed
  2. BHot code is compiled during execution, using run-time information
  3. CBytecode is interpreted one instruction at a time forever, never using machine code
Show answer and explanation

Answer: B. Hot code is compiled during execution, using run-time information

With JIT compilation, bytecode is first interpreted, and when frequently executed parts (hot spots) are found, they are compiled to machine code while the program runs. Run-time information, such as which branches are taken and which types are passed, enables aggressive optimizations hard to achieve ahead of time, with a fallback to interpretation if assumptions fail. Translating everything beforehand is ahead-of-time (AOT) compilation, and only interpreting is a pure interpreter.

What is the main advantage of using an intermediate representation (IR), independent of both source language and CPU, inside a compiler?

  1. AUsing an IR lets code be compiled without reading the source
  2. BThe CPU can execute the IR directly, so machine code generation is unnecessary
  3. CLanguage-specific front ends and CPU-specific back ends can be separated, and optimizations shared
Show answer and explanation

Answer: C. Language-specific front ends and CPU-specific back ends can be separated, and optimizations shared

Supporting M languages and N CPUs with a compiler per combination needs M × N compilers, but with a common IR it takes only M front ends (language to IR) and N back ends (IR to CPU). Most optimizations, written once on the IR, then serve every language and CPU. The IR cannot be executed directly by the CPU and must finally be converted to machine code.

  1. What is the role of lexical analysis in a compiler?
  2. What is done during syntax analysis (parsing) in a compiler?

Practice Compilers questions

Key terms explained

JIT compilation
Compiling frequently executed parts of a program to machine code while it runs, enabling optimizations based on run-time information. Used in runtimes for Java, JavaScript and others.

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