8.5.5 DAGs
The previous sections looked at the control flow graph nodes of basic blocks. DAGs, on the other hands, create a useful structure for the intermediate representation within the basic blocks.
A directed graph with no cycles, called a DAG (Directed Acyclic Graph), is used to represent a basic block. Thus, the nodes of a flow graph are themselves graphs!
We can create a DAG instead of an abstract syntax tree by modifying the operations for constructing the nodes. If the node already exists, the operation returns a pointer to the existing node. Example 4 shows this for the two-line assignment statement example.
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In Example 4, there are two references
to a and two references to the quantity
bb * 12 .