Intermediate Representations: Three-Address Code, SSA & Forms

An Intermediate Representation (IR) serves as the universal pivot of a modern retargetable compiler: an M-language front-end targeting N architectures requires M + N components rather than M × N monolithic compilers.

Three-Address Code (TAC): Linear low-level representation where each instruction has at most one operator on the right-hand side. Complex nested expressions like x = a + b c are linearized into: t1 = b c; t2 = a + t1; x = t2.

Representation Formats:

1. Quadruples: Stores op, arg1, arg2, result. Example: (*, b, c, t1). Facilitates easy optimization because instructions can be moved freely without breaking operand linkages.

2. Triples: Stores op, arg1, arg2. Example: (0) (*, b, c); (1) (+, a, (0)). Saves memory but reordering instructions requires rewriting all subsequent reference indices.

3. Indirect Triples: Maintains an array of pointers pointing to triple entries. Reordering optimizations simply permute the pointer array.

Static Single Assignment (SSA):

In SSA form, each variable is assigned exactly once, and each use refers to a unique definition. When control flow paths merge (e.g., after an if-else construct), a special \phi-function (phi-node) is placed at the merge point to select the appropriate definition based on the predecessor block.