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Depth First Ordering in Compiler Design

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  • Last Updated : 06 Mar, 2022
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The first depth search of the graph visits all the nodes in the graph once, starting with the entry point and visiting the nodes as far as the entry point as quickly as possible. The search route for in-depth search creates the first expandable tree (DFST). Pre-order visits the site before visiting any of the local children, who also visit repeatedly from left to right. Also, postorder traversal visits node children, repeatedly in sequence from left to right, before visiting the site itself. There is one distinct order that is important in the analysis of the flow graph: the order of the first depth is the retreat of postorder traversal. That is, at a first depth, we visit the node, and then cut its child to the right, to the left, and so on. However, before building a flow graph tree, we have decisions about which node follower becomes the right child in the tree, which node becomes the left child, and so on.

Depth First Ordering:

The complexity of data flow analysis using the data flow solution strategy has been displayed as a sequence of nodes. In this case, a value is defined that is called the depth of the flow graph to determine the maximum number of repetitions required. This requires consideration of basic blocks in a certain order and not in a random sequence. First, the flow graph nodes are given a number, called the first depth number. The depth of the first numbers (dfu) of graph nodes is a distortion of how we last visited each node in the pre-order block of the graph.

Properties:

Depth First Ordering has the following properties:

  1. Vi € dominators(j), dfn(i) <dfn(j),
  2. Vforward edges (i, j), dfn(i) < dfn(j) and
  3. Vbackward edges (i, j), dfn(j) < dfn(i).
Flow Graph

Flow Graph

The d depth of the system flow graph is the largest number of posterior edges on any acyclic path in it. It may be noted that it is not the same as the depth of the nest. For example, in the above flow graph, the nesting depth is 2, but the depth d is equal to 1.

The most important result of the repeated analysis is as follows: 

For the forward data flow problem, when graphs are visited in a deep sequence manner, d + 1 repeated is sufficient to reach a fixed point. For the data back problem, nodes should be visited at the same depth as the first order. The point can be proved by the following observations:

  1. For the problem of data forwarding, we visit the nodes in increasing order with the depth of the initial numbers. If there are no loops in the system, which is d-0, then the data flow information entered by the computer in the first duplication may be a fixed point for data flow statistics.
  2. If one loop is present in the system, then d-1. Now, for a new loop release value, the node listed in the first duplicate can affect the property of the loop entry node. This can only be achieved in subsequent repetitions. Therefore, two repetitions are required.

Algorithmic Representation of DFS:

Algorithmic Representation of DFS

Algorithmic Representation of DFS

Back Edges and Reducibility:

The back edge is the edge a -> b, where its head b dominates the tail a. In a Flow Graph, the entire rear edge is reversed, but not all rear edges are reversed. The flow graph is said to be irreversible if all of its reclining ends on any deep stretch tree are the rear edges. If the graph cannot be minimized, however, all backends are the sloping edges of any DFST, but each DFST may have additional sloping edges other than the rear ends. These retractable edges may differ from one DFST to another.

Thus, if we remove all the back edges of the flowing graph and the remaining graph is circular, the graph cannot be reduced, and vice versa. Flow graphs that occur in performance are almost always reduced. Special use of systematic flow control statements such as then, while-doing, continuous, and break statements produces systems whose flow graphs are constantly reversed. Even programs are written using goto statements often appear reducible, as the editor logically thinks about the traps and branches.

 Non-Reducible Flow Graph

 Non-Reducible Flow Graph

For the above Flow graph with the initial node as 1, Node 1 dominates nodes 2 and 3, but 2 does not rule in 3, or vice versa. Therefore, this flow graph has No rear edges, as it does not have the head of an edge that governs its tail. There are two trees that can be the first depth, depending on whether we choose to call search (2) or search (3) first, from search (l). In the first case, edge 3 ->2 is a sloping edge but not the rear; in the second case, the 2 -> 3 is a sloping edge – but-not-back. Understandably, the reason this flow graph does not decrease is that Cycle 2-3 can be installed in two different locations, nodes 2 and 3.

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