Editing Optimizing compiler (section)

=== Data-flow optimizations ===
[[Dataflow|Data-flow]] optimizations, based on [[data-flow analysis]], primarily depend on how certain properties of data are propagated by control edges in the [[control-flow graph]]. Some of these include:

;[[Common subexpression elimination]]: In the expression <code>(a + b) - (a + b)/4</code>, "common subexpression" refers to the duplicated <code>(a + b)</code>. Compilers implementing this technique realize that <code>(a + b)</code> will not change, and so only calculate its value once.<ref name="aho-sethi-ullman" />{{rp|pages=592–594}}

;[[Constant folding|Constant folding and propagation]]: Replacing expressions consisting of constants (e.g., <code>3 + 5</code>) with their final value (<code>8</code>) at compile time, rather than doing the calculation in run-time.<ref name="MuchnickAssociates1997">{{cite book|author1=Muchnick|first=Steven|url=https://archive.org/details/advancedcompiler00much|title=Advanced Compiler Design Implementation|author2=Muchnick and Associates|date=15 August 1997|publisher=Morgan Kaufmann|isbn=978-1-55860-320-2|pages=[https://archive.org/details/advancedcompiler00much/page/329 329]–|quote=constant folding.|url-access=registration}}</ref> Used in most modern languages.

;[[Induction variable recognition and elimination]]: See discussion above about ''induction variable analysis''.

;[[Strict aliasing|Alias classification and pointer analysis]]: In the presence of [[pointer (computer programming)|pointer]]s, it is difficult to make any optimizations at all, since potentially any variable can have been changed when a memory location is assigned to. By specifying which pointers can alias which variables, unrelated pointers can be ignored.

;[[Dead store|Dead-store]] elimination: Removal of assignments to variables that are not subsequently read, either because the lifetime of the variable ends or because of a subsequent assignment that will overwrite the first value.