Editing Use-define chain (section)

==Purpose==
Making the use-define or define-use chains is a step in [[liveness analysis]], so that logical representations of all the variables can be identified and tracked through the code.

Consider the following snippet of code:
<syntaxhighlight lang="c">
 int x = 0;    /* A */
 x = x + y;    /* B */
 /* 1, some uses of x */
 x = 35;       /* C */
 /* 2, some more uses of x */
</syntaxhighlight>

Notice that <code>x</code> is assigned a value at three points (marked A, B, and C). However, at the point marked "1", the use-def chain for <code>x</code> should indicate that its current value must have come from line B (and its value at line B must have come from line A). Contrariwise, at the point marked "2", the use-def chain for <code>x</code> indicates that its current value must have come from line C. Since the value of the <code>x</code> in block 2 does not depend on any definitions in block 1 or earlier, <code>x</code> might as well be a different variable there; practically speaking, it ''is'' a different variable &mdash; call it <code>x2</code>.

<syntaxhighlight lang="c">
 int x = 0;    /* A */
 x = x + y;    /* B */
 /* 1, some uses of x */
 int x2 = 35;  /* C */
 /* 2, some uses of x2 */
</syntaxhighlight>

The process of splitting <code>x</code> into two separate variables is called [[live range splitting]]. See also [[static single assignment form]].