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:
“Natural selection, the blind, unconscious, automatic process which Darwin discovered, and which we now know is the explanation for the existence and apparently purposeful form of all life, has no purpose in mind. It has no mind and no minds eye. It does not plan for the future. It has no vision, no foresight, no sight at all. If it can be said to play the role of the watchmaker in nature, it is the blind watchmaker.”
—Richard Dawkins (b. 1941)
“Possibly the Creator did not make the world chiefly for the purpose of providing studies for gifted novelists; but if he had done so, we can scarcely imagine that He could have offered anything much better in the way of material ...”
—Elizabeth Stuart Phelps (18441911)
“Patience and tenacity of purpose are worth more than twice their weight of cleverness.”
—Thomas Henry Huxley (18251895)