Why Intermediate Representation

• Machine and Language Independent version of the original source code
• The use of intermediate representation provides
  • Increased abstraction
  • Cleaner separation between the frontend and backend
  • Adds possibilities for re-targeting/cross-compilation.
  • Supports compiler optimizations.

Types of Intermediate Representation

• Trees
  • Includes parse trees and abstract syntax trees
• Linear Representation
  • Three-Address Code (TAC)
• Intermediate Representations are categorized according to where they fall between a high-level language and machine code.
  • IRs close to high-level language are called ****high-level IRs
    • It preserves information like
      • Loop-structure, array subscripts, if-then-else statements, etc.
      • They tend to reflect the source language they are compiling more than low-level IRs
  • IRs close to assembly are called ****low-level IRs
    • Converts the array subscripts into explicit addresses and offsets.
    • Tend to reflect the target architecture very closely, often machine-dependent.
• Different Optimizations are possible at different IR levels.

Directed Acyclic Graph (DAG)

A syntax tree with more than one path from the start symbol to the terminal is called DAG.

Three Address Code (TAC)

A three-address code is a linearized representation of a syntax tree or a DAG in which explicit names correspond to the interior nodes of the graph.

A DAG and its corresponding three-address code

Three-Address Code Various Statements

Implementation of Three Address Code

IR is implemented in a data structure as objects with fields for their operators and operands.

Commonly used data structures are