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Control Dependency
Control dependency is a situation in which a program instruction executes if the previous instruction evaluates in a way that allows its execution. An instruction B has a ''control dependency'' on a preceding instruction A if the outcome of A determines whether B should be executed or not. In the following example, the instruction S_2 has a control dependency on instruction S_1. However, S_3 does not depend on S_1 because S_3 is always executed irrespective of the outcome of S_1. S1. if (a b) S2. a = a + b S3. b = a + b Intuitively, there is control dependence between two statements A and B if * B could be possibly executed after A * The outcome of the execution of A will determine whether B will be executed or not. A typical example is that there are control dependences between the condition part of an if statement and the statements in its true/false bodies. A formal definition of control dependence can be presented as follows: A statement S_2 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
If And Only If
In logic and related fields such as mathematics and philosophy, "if and only if" (often shortened as "iff") is paraphrased by the biconditional, a logical connective between statements. The biconditional is true in two cases, where either both statements are true or both are false. The connective is biconditional (a statement of material equivalence), and can be likened to the standard material conditional ("only if", equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other (i.e. either both statements are true, or both are false), though it is controversial whether the connective thus defined is properly rendered by the English "if and only if"—with its pre-existing meaning. For example, ''P if and only if Q'' means that ''P'' is true whenever ''Q'' is true, and the only case in which ''P'' is true is if ''Q'' is also true, whereas in the case of ''P if Q ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dominator (graph Theory)
In computer science, a node of a control-flow graph dominates a node if every path from the ''entry node'' to must go through . Notationally, this is written as (or sometimes ). By definition, every node dominates itself. There are a number of related concepts: * A node ''strictly dominates'' a node if dominates and does not equal . * The ''immediate dominator'' or idom of a node is the unique node that strictly dominates but does not strictly dominate any other node that strictly dominates . Every node reachable from the entry node has an immediate dominator (except the entry node). * The ''dominance frontier'' of a node is the set of all nodes such that dominates an immediate predecessor of , but does not strictly dominate . It is the set of nodes where 's dominance stops. * A ''dominator tree'' is a tree where each node's children are those nodes it immediately dominates. The start node is the root of the tree. History Dominance was first introduced by Re ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Control-flow Graph
In computer science, a control-flow graph (CFG) is a representation, using graph notation, of all paths that might be traversed through a program during its execution. The control-flow graph was conceived by Frances E. Allen, who noted that Reese T. Prosser used boolean connectivity matrices for flow analysis before. The CFG is essential to many compiler optimizations and static-analysis tools. Definition In a control-flow graph each node in the graph represents a basic block, i.e. a straight-line sequence of code with a single entry point and a single exit point, where no branches or jumps occur within the block. Basic blocks start with jump targets and end with jumps or branch instructions. Directed edges are used to represent jumps in the control flow. There are, in most presentations, two specially designated blocks: the ''entry block'', through which control enters into the flow graph, and the ''exit block'', through which all control flow leaves. Because of its c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dependence Analysis
In compiler theory, dependence analysis produces execution-order constraints between statements/instructions. Broadly speaking, a statement ''S2'' depends on ''S1'' if ''S1'' must be executed before ''S2''. Broadly, there are two classes of dependencies--control dependencies and data dependencies. Dependence analysis determines whether it is safe to reorder or parallelize statements. Control dependencies Control dependency is a situation in which a program instruction executes if the previous instruction evaluates in a way that allows its execution. A statement ''S2'' is ''control dependent'' on ''S1'' (written S1\ \delta^c\ S2) if and only if ''S2s execution is conditionally guarded by ''S1''. ''S2'' is ''control dependent'' on ''S1'' if and only if S1 \in PDF(S2) where PDF(S) is the post dominance frontier of statement S. The following is an example of such a control dependence: S1 if x > 2 goto L1 S2 y := 3 S3 L1: z := y + 1 Here, ''S2'' only runs if ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Dependency
A data dependency in computer science is a situation in which a program statement (instruction) refers to the data of a preceding statement. In compiler theory, the technique used to discover data dependencies among statements (or instructions) is called dependence analysis. Description Assuming statement S_1 and S_2, S_2 depends on S_1 if: :\left (S_1) \cap O(S_2)\right\cup \left (S_1) \cap I(S_2)\right\cup \left (S_1) \cap O(S_2)\right\neq \varnothing where: * I(S_i) is the set of memory locations read by * O(S_j) is the set of memory locations written by and * there is a feasible run-time execution path from S_1 to These conditions are called Bernstein's Conditions, named after Arthur J. Bernstein. Three cases exist: * Anti-dependence: I(S_1) \cap O(S_2) \neq \varnothing, S_1 \rightarrow S_2 and S_1 reads something before S_2 overwrites it * Flow (data) dependence: O(S_1) \cap I(S_2) \neq \varnothing, S_1 \rightarrow S_2 and S_1 writes before something read by S_2 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Loop Dependence Analysis
In computer science, loop dependence analysis is a process which can be used to find dependencies within iterations of a loop with the goal of determining different relationships between statements. These dependent relationships are tied to the order in which different statements access memory locations. Using the analysis of these relationships, execution of the loop can be organized to allow multiple processors to work on different portions of the loop in parallel. This is known as parallel processing. In general, loops can consume a lot of processing time when executed as serial code. Through parallel processing, it is possible to reduce the total execution time of a program through sharing the processing load among multiple processors. The process of organizing statements to allow multiple processors to work on different portions of a loop is often referred to as parallelization. In order to see how we can exploit parallelization, we have to first analyze the dependencies wi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
