Compilation Targets: Bytecode

OCaml can compile to bytecode, providing fast compilation, excellent portability, and predictable execution across different platforms.

What is OCaml Bytecode?

OCaml bytecode is a portable intermediate representation of OCaml programs that is executed by the OCaml bytecode interpreter. The bytecode system consists of:

• ocamlc - The bytecode compiler that compiles OCaml source files to bytecode
• ocamlrun - The bytecode interpreter that executes bytecode programs
• Runtime system - Includes the bytecode interpreter, garbage collector, and primitive C operations

Bytecode provides several advantages over native compilation:

• Fast compilation speed
• Portability across all platforms where OCaml is installed
• Smaller compiler footprint
• Predictable and consistent execution behaviour
• Easier debugging with built-in tools

The bytecode compiler can be built in either 32-bit or 64-bit mode, resulting in 31-bit or 63-bit integers, respectively (with 1 bit reserved for the garbage collector tag).

When to Use Bytecode

Use bytecode when you want fast compilation during development, need maximum portability, or are targeting platforms without a native code compiler.

Use native code (ocamlopt) when you need maximum runtime performance in production deployments.

Many OCaml developers use bytecode during development for its fast compile times, then switch to native code for production releases.

File Extensions

The bytecode compiler produces several types of files:

• .cmo - Compiled module object (bytecode)
• .cmi - Compiled module interface
• .cma - Bytecode library archive (collection of .cmo files)
• executable - Bytecode executable (often with no extension or .byte extension)

Learn More

• Dune Manual: Executable Linking Modes - How to specify native, bytecode, or other compilation modes in Dune
• OCaml Manual: Batch Compilation - Complete reference for ocamlc
• OCaml Manual: Runtime System - Details on ocamlrun and bytecode execution
• Compiler Backend Documentation - How the OCaml compiler works
• Compiling OCaml Projects - Getting started guide