How do you identify a loop in a flow graph?
One application of dominator information is in determining the loops of a flow graph suitable for improvement. There are two essential properties of loops: Ø A loop must have a single entrypoint, called the header. This entry point-dominates all nodes in the loop, or it would not be the sole entry to the loop.
What are the elements of control flow graph?
o The control flow graph is a graphical representation of a program’s control structure. It uses the elements named process blocks, decisions, and junctions.
What is an edge in a control flow graph?
Control flow graph is process oriented. Control flow graph shows all the paths that can be traversed during a program execution. Control flow graph is a directed graph. Edges in CFG portray control flow paths and the nodes in CFG portray basic blocks.
What are natural loops?
The natural loop of a back edge is the smallest set of nodes that includes the head and tail of the back edge, and has no predecessors outside the set, except for the predecessors of the header.
What does the inner loop do?
In computer programs, an important form of control flow is the loop which causes a block of code to be executed more than once. A common idiom is to have a loop nested inside another loop, with the contained loop being commonly referred to as the inner loop.
HOW IS FOR loop executed?
In for loop, a loop variable is used to control the loop. First, initialize this loop variable to some value, then check whether this variable is less than or greater than the counter value. If the statement is true, then the loop body is executed and the loop variable gets updated.
Which of the following loop is executed at least once?
With a do while loop the condition is not evaluated until the end of the loop. Because of that a do while loop will always execute at least once.
What is the looping probability of a path expression?
For a simple loop, if the loop will be taken a mean of N times, the looping probability is N/(N + 1) and the probability of not looping is 1/(N + 1). A link that is not part of a decision node has a probability of 1. The arithmetic rules are those of ordinary arithmetic.
What are back edges and Reducibility in code optimization?
Reducibility. A reducible CFG is one with edges that can be partitioned into two disjoint sets: forward edges, and back edges, such that: Forward edges form a directed acyclic graph with all nodes reachable from the entry node. For all back edges (A, B), node B dominates node A.
What is back edge?
a Back Edge is an edge that connects a vertex to a vertex that is discovered before it’s parent.
What is loop control variables?
A common use of loops is the one in which the loop makes use of a variable (called control variable) that at each iteration is changed by a constant value, and whose value determines the end of the loop.
DO for loops always run once?
A for-loop always makes sure the condition is true before running the program. Whereas, a do-loop runs the program at least once and then checks the condition. Show activity on this post. An entry controlled loop will never execute if the condition is false , however, exit controlled loop will execute at least once.
What is the arithmetic rule for loop in looping probability?
Weights, Notations and Arithmetic: For a simple loop, if the loop will be taken a mean of N times, the looping probability is N/(N + 1) and the probability of not looping is 1/(N + 1). A link that is not part of a decision node has a probability of 1. The arithmetic rules are those of ordinary arithmetic.
How do you find the independent path on a control flow graph?
In the control flow graph, an independent path is one that introduces at least one new edge that has not been traversed before the path is created. The cyclomatic complexity of a flow graph is the number of independent pathways present.
What is reducible flow graph?
REDUCIBLE CONTROL FLOW GRAPHS. A control flow graph G is said to be reducible if the removal of its back edges leads to an acyclic graph where each node can be reached from the initial node of G. In this case, all the edges of this acyclic graph are called forward edges.
What is a critical edge in a graph?
A critical edge in a graph is an edge whose deletion lowers the chromatic number. Dirac conjectured in 1970 that for some k 2 4 there is a vertex k-critical graph with no critical edge.
What is loop detection in compiler design?
Loop optimization is the phase after the Intermediate Code Generation. The main intention of this phase is to reduce the number of lines in a program.
How do I find my back edges?
Back edge: It is an edge (u, v) such that v is ancestor of node u but not part of DFS tree. Edge from 6 to 2 is a back edge. Presence of back edge indicates a cycle in directed graph. Cross Edge: It is a edge which connects two node such that they do not have any ancestor and a descendant relationship between them.
How do you find your back edge?
To detect a back edge, keep track of vertices currently in the recursion stack of function for DFS traversal. If a vertex is reached that is already in the recursion stack, then there is a cycle in the tree. The edge that connects the current vertex to the vertex in the recursion stack is a back edge.
How do you find all loops in a flow graph?
Ø There must be at least one way to iterate the loop (i.e.)at least one path back to the headerOne way to find all the loops in a flow graph is to search for edges in the flow graph whose heads dominate their tails. If a→b is an edge, b is the head and a is the tail.
What is a control flow graph?
Hence, the control flow graph is comprised of all the building blocks involved in a flow diagram such as the start node, end node and flows between the nodes. Control Flow Graph is represented differently for all statements and loops. Following images describe it:
What is the outermost loop in a flow graph?
Finally, the edge 9 -> 1 has as its natural loop the entire flow graph, and therefore is the outermost loop. In this example, the four loops are nested within one another. It is typical, however, to have two loops, neither of which is a subset of the other.
Is a nonreducible flow graph a natural loop?
For instance, the nonreducible flow graph in Fig. 9.45 has a cycle consisting of nodes 2 and 3. Neither of the edges in the cycle is a back edge, so this cycle does not fit the definition of a natural loop.