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:

$x\; :=\; y\; ,\; operatorname\{op\}\; ,\; z,$

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:

$p\; :=\; x\; +\; y\; imes\; z,$

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

$t\_1\; :=\; y\; imes\; z,$

$p\; :=\; x\; +\; t\_1,$

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).

Example

int main(void) { int i; int b [10] ; for (i = 0; i < 10; ++i) { b [i] = i*i; } }The preceding C program, translated into three-address code, might look something like the following: i := 0 ; assignment L1: if i < 10 goto L2 ; conditional jump goto L3 ; unconditional jump L2: t0 := i*i t1 := &b ; address-of operation t2 := t1 + i ; t2 holds the address of b [i] *t2 := t0 ; store through pointer i := i + 1 goto L1 L3:

ee also

*Intermediate language
*Reduced instruction set computer

External links

* [http://cs.wwc.edu/~aabyan/464/Quads.html Three-Address Code and Register Allocation]
* [http://www.cs.arizona.edu/classes/cs453/fall07/DOCS/intcode.html CSc 453: A Three-Address Intermediate Code Instruction Set for C--]