Live-variable analysis: Difference between revisions

Revision as of 14:43, 9 May 2014

Revision as of 14:46, 9 May 2014

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The in-state of a block is the set of variables that are live at the start of the block. Its out-state is the set of variables that are live at the end of it. The out-state is the union of the in-states of the block's successors. The transfer function of a statement is applied by making the variables that are written dead, then making the variables that are read live.

{|

<table>

|

<tr><td>

// in: {}

b1: a = 3;

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return b * d + c;

// out:{}

|}

</td>

</tr>

</table>

The in-state of b3 only contains ''b'' and ''d'', since ''c'' has been written. The out-state of b1 is the union of the in-states of b2 and b3. The definition of ''c'' in b2 can be removed, since ''c'' is not live immediately after the statement.