1 Code Generation Code generator phase ... - VTU e-Learning

Fig 9.6 DAG for the three address code
Fig 9.6 shows the DAG for the expression a = -b * c + d represented as three address code
Example: Consider the following expression
a = b * d + b * d + c
The three address code will be
t1 = b * d
b2 = b * d
t3 = t1 + t2
t4 = t3 + c
a = t4
Fig 9.7 DAG for the three address code
Fig 9.7 shows the DAG for the expression a = b * d + b * d + c represented as three address code.
From the above DAG it is found that node * has 2 labels t1 & t2. Hence there is no necessary to
generate code twice for the same expression. Final code can be generated from DAG by topological
sorting. Topological sorting is the traversal of tree from leaf to root in which children are visited
before their parents. As there can be multiple topological sorts. There can be many code sequences for
single DAG.
Example:
Consider intermediate code
t1 = a + b
a = t1
t2 = b – 1
b = t2
t3 = b + 5
t1, t2 *
b d
t4 , a ⊕
t3 ⊕ c
⊕ t3
a1 t2 ⊕ Ө t2 , b1 5
a0 b 1
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