Copy propagation

In compiler theory, copy propagation is the process of replacing the occurrences of targets of direct assignments with their values.^[1] 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 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^[1]—require that copy propagation be run afterwards in order to achieve an increase in efficiency.

References

^ ^a ^b Aho, Alfred V.; Lam, Monica S.; Sethi, Ravi; Ullman, Jeffrey D. (2007). Compilers, Principles, Techniques, & Tools Second edition. Pearson/Addison Wesley. ISBN 978-0-321-48681-3.

v t e Compiler optimizations
Basic block	Peephole optimization Local value numbering
Loop	Automatic parallelization Automatic vectorization Induction variable Loop fusion Loop-invariant code motion Loop inversion Loop interchange Loop nest optimization Loop splitting Loop unrolling Loop unswitching Software pipelining Strength reduction
Data-flow analysis	Available expression Common subexpression elimination Constant folding Dead store elimination Induction variable recognition and elimination Live-variable analysis Upwards exposed uses Use-define chain Reaching definitions
SSA-based	Global value numbering Sparse conditional constant propagation
Code generation	Instruction scheduling Instruction selection Register allocation Rematerialization
Functional	Deforestation Tail-call elimination
Global	Interprocedural optimization
Other	Bounds-checking elimination Compile-time function execution Dead-code elimination Expression templates Inline expansion Jump threading Partial evaluation Profile-guided optimization
Static analysis	Alias analysis Array-access analysis Control-flow analysis Data-flow analysis Dependence analysis Escape analysis Pointer analysis Shape analysis Value range analysis

Copy propagation

See also

References

Further reading