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Control Flow Analysis

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Construct representations for the structure of flow-of-control of programs Control flow graphs represent the structure of flow-of-control within a procedure Call graphs represent the structure of flow- of-control between procedures

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Control Flow Graphs Nodes in the graph are basic blocks A basic block is a sequence of consecutive statements in which control can only enter at the beginning and leave at the end There is an edge from a basic block b 1 to a basic block b 2 if control can flow from the end of b 1 to the beginning of b 2 ; we say b 1 is a predecessor of b 2 and b 2 is a successor of b 1

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An Example (1)prod := 0 (2)i := 1 (3)t1 := 4 * i (4)t2 := a[t1] (5)t3 := 4 * i (6)t4 := b[t3] (7)t5 := t2 * t4 (8)t6 := prod + t5 (9)prod := t6 (10)t7 := i + 1 (11)i := t7 (12)if i <= 20 goto (3)

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Construction of Basic Blocks Determine the set of leaders –the first statement is a leader –the target of a jump is a leader –any statement immediately following a jump is a leader For each leader, its basic block consists of the leader and all statements up to but not including the next leader or the end of the program

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Representation of Basic Blocks Each basic block is represented by a record consisting of –a count of the number of statements –a pointer to the leader –a list of predecessors –a list of successors

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An Example (1)prod := 0 (2)i := 1 (3)t1 := 4 * i (4)t2 := a[t1] (5)t3 := 4 * i (6)t4 := b[t3] (7)t5 := t2 * t4 (8)t6 := prod + t5 (9)prod := t6 (10)t7 := i + 1 (11)i := t7 (12)if i <= 20 goto (3)

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Call Graphs Nodes in the graphs are procedures There is an edge from a procedure p 1 to a procedure p 2 if p 2 is called within p 1 ; we say p 1 is a predecessor of p 2 and p 2 is a successor of p 1

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An Example procedure main( ) one( ) end procedure one( ) two( ) end procedure two( ) one( ) end main one two

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