Copy propagation

In <a href="/facts/Compiler_theory/WNfCDFJe">compiler theory</a>, 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 x = y, which simply assigns the value of y to x.
From the following code:

y = x
z = 3 + y

Copy propagation would yield:

z = 3 + x

Copy propagation often makes use of <a href="/facts/Reaching_definitions/8aJ8dmH9">reaching definitions</a>, <a href="/facts/Use-def_chains/sJ1haL3j">use-def chains</a> and <a href="/facts/Def-use_chains/sJ1haL3j">def-use chains</a> when computing which occurrences of the target may be safely replaced. If all <a href="/facts/Upwards_exposed_uses/aUtQxJcM">upwards exposed uses</a> 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 <a href="/facts/Elimination_of_common_sub_expressions/hlCW19QS">elimination of common sub expressions</a>—require that copy propagation be run afterwards in order to achieve an increase in efficiency.

Copy propagation open-in-new

Copy propagation