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Use-define chain
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<p>Within <a href="/facts/Computer_science/lN3pEyPz">computer science</a>, a use-definition chain (or UD chain) is a <a href="/facts/Data_structure/yNsNvL1I">data structure</a> that consists of a use <i>U</i>, of a <a href="/facts/Variable_(programming)/VoCKtwml">variable</a>, and all the definitions <i>D</i> of that variable that can reach that use without any other intervening definitions. A UD Chain generally means the <a href="/facts/Assignment_(computer_science)/kLevAoyJ">assignment</a> of some value to a variable. </p><p>A counterpart of a <i>UD Chain</i> is a definition-use chain (or DU chain), which consists of a definition <i>D</i> of a variable and all the uses <i>U</i> reachable from that definition without any other intervening definitions. </p><p>Both UD and DU chains are created by using a form of <a href="/facts/Static_code_analysis/b56nr6QM">static code analysis</a> known as <a href="/facts/Data_flow_analysis/jz2oI6sX">data flow analysis</a>. Knowing the use-def and def-use chains for a program or subprogram is a prerequisite for many <a href="/facts/Compiler_optimization/JXnv4LUd">compiler optimizations</a>, including <a href="/facts/Constant_propagation/1NmVLgm1">constant propagation</a> and <a href="/facts/Common_subexpression_elimination/hlCW19QS">common subexpression elimination</a>. </p>