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:
“We need not only a purpose in life to give meaning to our existence but also something to give meaning to our suffering. We need as much something to suffer for as something to live for.”
—Eric Hoffer (1902–1983)
“I beg to assure you that I have never written you, or spoken to you, in greater kindness or feeling than now, nor with a fuller purpose to sustain you, so far as in my most anxious judgement, I consistently can. But you must act.”
—Abraham Lincoln (1809–1865)
“The United States is the only great nation whose government is operated without a budget. The fact is to be the more striking when it is considered that budgets and budget procedures are the outgrowth of democratic doctrines and have an important part in developing the modern constitutional rights.... The constitutional purpose of a budget is to make government responsive to public opinion and responsible for its acts.”
—William Howard Taft (1857–1930)