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
Famous quotes containing the word purpose:
“A material resurrection seems strange and even absurd except for purposes of punishment, and all punishment which is to revenge rather than correct must be morally wrong, and when the World is at an end, what moral or warning purpose can eternal tortures answer?”
—George Gordon Noel Byron (1788–1824)
“For the purpose of knowledge, one must know how to use that inner current that draws us to a thing, and then the one that, after a time, draws us away from it.”
—Friedrich Nietzsche (1844–1900)
“The technologist was the final guise of the white missionary, industrialization the last gospel of a dying race and living standards a substitute for a purpose in living.”
—Max Frisch (1911–1991)