Copy propagation

In compiler theory, copy propagation is the process of replacing the occurrences of targets of direct assignments with their values. A direct assignment is an instruction of the form codice_1, which simply assigns the value of codice_2 to codice_3.
From the following code:
 y = x
 z = 3 + y
Copy propagation would yield:
 z = 3 + x
Copy propagation often makes use of reaching definitions, use-def chains and def-use chains when computing which occurrences of the target may be safely replaced. If all upwards exposed uses of the target may be safely modified, the assignment operation may be eliminated.
Copy propagation is a useful "clean up" optimization frequently used after other compiler passes have already been run. Some optimizations—such as classical implementations of elimination of common sub expressions—"require" that copy propagation be run afterwards in order to achieve an increase in efficiency.
Further reading.
Muchnick, Steven S. "Advanced Compiler Design and Implementation". Morgan Kaufmann. 1997.