Three address code

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In computer science, three address code (TAC) is a form of representing intermediate code used by compilers to aid in the implementation of code-improving transformations. Each TAC can be described as a quadruple (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 \times z\,

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

t_1 := y \times z\,

p := x + t_1\,

The term three address code is still used even if some instructions use more or less 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.

The three address code table:

Assignment Statement x := y op z x, y, z are names, constants, or compiler generated temporaries; op is any binary operator
x := op y op is a unary operator
Indexed Assignment x := y[i]

x[i] := y

 x, y, i are data objects
Copy Statement x := y x, y are data objects
Unconditional jump goto L L is the label of the next statement to be executed
Conditional jump if x relop y goto L x, y are data objects

relop is a binary operator that compares x to y and indicates true/false

L is the label of the next statement to be executed if relop indicates true
Procedure calls and returns param x x is a parameter
call p, n p is the procedure, n is the number of parameters
return y y is an optional return value
Address and pointer assignments

x := &y

x := *y

*x := y

 x is set to the location of y

x is set to the value at the location y

the value pointed to by x is set to the r-value of y

Take this example of an C-program

  main ( )
  {
     int i;
     int b [12];
     i=0;
     while(i<12){
        b[i]=i; 
        i=i+1;
     }
  }

It can be translated into a 3 address code:

  i:=0                   //an assignment
  L1: if i < 12 goto L2  //a conditional jump
  goto L3                // an unconditional jump
  L2: t0:=i*4
  b[t0]:= i              // indexed assignment
  i:=i+1
  goto L1
  L3 :
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