Types of Three-address codes

Three-address code is a sequence of statements of the general form A := B op C, where A, B, C are either programmer defined names, constants or compiler-generated temporary names; op stands for an operation which is applied on A, B.In simple words, a code having at most three addresses in a line is called three address codes.



The three address code for above expression is:


In compiler design the most popular intermediate code representation is Three-address code. It is globally accepted and is most widely used. There are a lot of three-address statements. All the complex three-address statements are generally a combination of simpler three-address statements.

These statements come under following seven categories and can be called as building block for three-address statements-

Statement Meaning
X = Y op Z Binary Operation
X= op Z Unary Operation
X = Y Assignment
if X(rel op)Y goto L Conditional Goto
goto L Unconditional Goto
A[i] = X
Y= A[i]
Array Indexing
P = addr X
Y = *P
*P = Z
Pointer Operations

Now, by using the above statements let us convert some popular high level constructs in Three-address code.

1. While Statement –

while E do S

Three-address Code:

L:if(E==0) goto L1
  goto L

2. For Statement –

for(E1;E2;E3) do S

Three-address Code:

L:if(E2) goto L1
  goto L2

3. Switch Statement –

case 1: S1
case 2: S2
default: S3

Three-address Code:

  goto test
  goto Last
   goto Last
   goto Last
test: if(E==1) goto L1
      if(E==2) goto L2
      goto L3

Thus, we can see by using the seven statements we can replicate statements from higher level language into address codes.

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