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:
int x = 0; /* A */
x = x + y; /* B */
/* 1, some uses of x */
x = 35; /* C */
/* 2, some more uses of x */
Notice that x is assigned a value at three points (marked A, B, and C). However, at the point marked "1", the use-def chain for x 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 x indicates that its current value must have come from line C. Since the value of the x in block 2 does not depend on any definitions in block 1 or earlier, x might as well be a different variable there; practically speaking, it is a different variable — call it x2.
int x = 0; /* A */
x = x + y; /* B */
/* 1, some uses of x */
int x2 = 35; /* C */
/* 2, some uses of x2 */
The process of splitting x into two separate variables is called live range splitting. See also static single assignment form.
Read more about this topic: Use-define Chain
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