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CS 201 Compiler Construction. Lecture 6 Code Optimizations: Constant Propagation & Folding. Constant Propagation & Folding. 1. Use of var replaced by a constant value - propagation . 2. Exp. is evaluated or folded if all of its operands are constants.
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CS 201Compiler Construction Lecture 6 Code Optimizations: Constant Propagation & Folding
Constant Propagation & Folding 1. Use of var replaced by a constant value - propagation. 2. Exp. is evaluated or folded if all of its operands are constants.
Constant Propagation & Folding • Evaluation of branch • outcomes may be • enabled. • 2. Can take advantage • of non-executable • paths.
Data Flow Analysis Constant Prop. Lattice for a Single variable.
Contd.. Meet Operator for a Single variable.
Contd.. Lattice for all variables (L,∧‘) VAR – set of variables VAL – set of values (all constants, Τ, ) L = {β: β is a total function β: VARVAL} i.e., set of all states of variables. forall v ε VAR β1 ∧’ β2(v) = β1 (v) ∧ β2 (v)
Contd.. Transfer Function S: A = B op C fS(β) – transfer function of S. fS(β) =
Worklist Algorithm • Each node is a single statement • IN and OUT contain the lattice values at node entry and exit • Algorithm will take advantage of resolved branches. How ? • Mark edges as executable or non-executable and propagate information only along executable edges • Branch (true) (false) (true or false) – mark edges executable accordingly
