Three Address Code

Three Address Code

In computer science, three-address code (often abbreviated to TAC or 3AC) is a form of representing intermediate code used by compilers to aid in the implementation of code-improving transformations. Each instruction in three-address code can be described as a 4-tuple: (operator, operand1, operand2, result).

Each statement has the general form of:

such as:

where x, y and z are variables, constants or temporary variables generated by the compiler. op represents any operator, e.g. an arithmetic operator.

Expressions containing more than one fundamental operation, such as:

are not representable in three-address code as a single instruction. Instead, they are decomposed into an equivalent series of instructions, such as

The term three-address code is still used even if some instructions use more or fewer than two operands. The key features of three-address code are that every instruction implements exactly one fundamental operation, and that the source and destination may refer to any available register.

A refinement of three-address code is static single assignment form (SSA).