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

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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 18
After topological sorting t2 = b – 1 t1 = a + b a = t1 t3 = b + 5 b = t2 Fig 9.8 DAG for the intermediate code Reordering of code helps in eliminating unnecessary use of temporaries. Hence the code would be as follows. a = a + b b = b – 1 t3 = b + 5 DAG gives the information of how many references exists for node. This helps in good register allocation. If a value has many references then it can be retained in registers for long time. If the value has no reference it can be removed from the register. 19
Page 1 and 2: Code Generation Code generator phas
Page 3 and 4: d. Index addressing mode- this is o
Page 5 and 6: 1. Call. 2. Return 3. end 4. action
Page 7 and 8: * code for b */ action 3 return /*
Page 9 and 10: Code Optimization has to mainly ach
Page 11 and 12: In order to generate more optimized
Page 13 and 14: Each instruction takes some machine
Page 15 and 16: 6 H go to 13 B6 7 8 9 10 11 12 13 1
Page 17: Fig 9.4 DAG for the three address c

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

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