# 05 - Putting Data Flow Analysis to Work Last Time Iterative...

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Putting Data Flow Analysis to Work Last Time Iterative Worklist Algorithm via Reaching Definitions Why it terminates. What it computes. Why it works. How fast it goes. Today Live Variable Analysis (backward problem) Constant Propagation: A Progression in Analysis CS 380C Lecture 5 1 Data Flow Analysis Live Variable Analysis Can a variable v at a point p be used before it is redefined along some path starting at p ? USE( p ) - the set of variables that may be used before they are defined by this statement or basic block . DEF( p ) - the set of variables that may be defined by this statement or basic block. 0: read I 1: read N 2: call check (N) 3: I = 1 4: while (I < N) do 5: A(i) = A(i) + I 6: I = I + 1 7: endwhile 8: print A(N) CS 380C Lecture 5 2 Data Flow Analysis

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Live Variable Analysis A backward data flow problem: For each point p in the program and each variable x , determine whether x can be used before being redefined along some path starting at p . For a basic block, x is live if it is used before being redefined within that block, or if it is live going out of the block. IN( v ) is the set variables live coming into a block, and OUT( v ) is the set of variables live going out of a block. USE(v): x USE(v) i ff x may be used before it is defined in v DEF(v): x DEF(v) i ff x must be defined before it is used in v (DEF(v) == KILL(v)) OUT ( v ) = S s SUCC ( v ) IN ( s ) IN ( v ) = USE ( v ) S ( OUT ( v ) - DEF ( v )) The monotone data flow framework uses powerset of X (all variables) lattice. The transfer function T v ( x ) = USE ( v ) [ ( x - DEF ( v )) The meet is set union. The operation space is monotone and distributive, therefore the solution will result in the MOP solution. CS 380C Lecture 5 3 Data Flow Analysis Work List Iterative Algorithm Rehashed initialization worklist the set of all nodes while ( worklist = /0 ) pick and remove a node n from worklist recompute Data Flow Equations if the answer changed then add a ff ected nodes to worklist Initialization: OUT( v ) IN( v ) Data flow equations: OUT( v ) IN( v ) CS 380C Lecture 5 4 Data Flow Analysis
Live Variable Algorithm Algorithm: for all v OUT( v ) = /0 IN(

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