Directed Acyclic Graph Representation of basic blocks
- 1. Seminar on Principles of Compiler Design “Directed Acyclic Graph Representation of basic blocks” Presented By: Mohd Vaseem Akaram ID: 17370206 Presented To: Ms. Nivetha Ma'am Dept of Computer Science
- 2. • In compiler design, a directed acyclic graph (DAG) is on abstract syntax tree(AST) with a unique node for each value. OR • A directed acyclic graph(DAG) is a directed graph that contains no cycles. Definitions
- 3. • DAG is useful data structure for implementing transformation on basic blocks. • A basic block can optimized by the construction of DAG. • A DAG can be constructed for a block and certain transformations such as common sub- expression elimination and dead code elimination applied for performance the local optimization. • To apply the transformation on basic block, a DAG is constructed from three address code. Use of DAG for optimizing basic blocks
- 4. The DAG is Used In: • Determine the common sub expression (Expression computed more than one). • Determine which names are used in block and computed outside the block. • Determine which statements of the block could their computed value. Applications of DAG
- 5. Rule 1: In a DAG • Leaf node represent identifiers, names or constants. • Interior node represent operators. Rule 2: • While constructing DAG, there is a check made to find if there is on existing node with the same children. A new node is created only when such a node doesn't exist This action allow us to detect common sub expression and eliminate the re computation of the same. Rule 3: • The assignment of the from x:= must not be performed until and unless it is a must. Rules of the constructing DAG
- 6. • Problem 1: Construct DAG for the given expression (a+b) * (a+b+c) Solution: Three address code for the given expression. t1=a+b t2=t1+c t3=t1*t2 The DAG is: Problems
- 7. • From the constructed DAG, we observed that the common sub expression (a+b) is translated into a single node in the DAG. The computation is carried out only once and stored in the identifier t1 and reused later. • This illustrates how the DAG construction scheme identifies the common sub expression and helps in elimination its re-computation later. Explanation
- 8. • Problem 2: Construct DAG for the given expression (((a+a) + (a+a))+ ((a+a) + (a+a))) Solution: DAG for this Expression is-
- 9. Problem 3: Construct DAG for the given expression a = b * c d = b e = d * c b = e f = b + c g = f + d Solution: Step 1: Step 2:
- 10. Step 3: Step 4: Step 5: Step 6:
- 11. Thank You!