Three-address code

In computer science, three-address code (often abbreviated to TAC or 3AC) is an 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). The name derives from the fact that three variables are referenced by each statement.

Syntax

Each statement has the general form of:

${\text{result}}:={\text{operand}}_{1}\,\operatorname {operator} \,{\text{operand}}_{2}\,$

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

$p:=x+y\times z\,$

$t_{1}:=y\times z\,$

$p:=x+t_{1}\,$

T

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

Examples

 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
      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
L2:

Another example:

if(a<b)
{
  x=x+1;
}
else if(c<d)
{
  y=y+1
}

1..if(a<b) then goto 4 
2..if(c<d) then goto 7
3..go to next 
4..t1=x+1
5..x=t1
6..go to next
7..t2=y+1
8..y=t2
9..go to next

See also

• Intermediate language
• Reduced instruction set computer
• Jimple