Editing Constant folding (section)

== Constant propagation ==

Constant propagation is the process of substituting the values of known constants in expressions at compile time. Such constants include those defined above, as well as [[intrinsic function]]s applied to constant values. Consider the following pseudocode:

<syntaxhighlight lang="c">
int x = 14;
int y = 7 - x / 2;
return y * (28 / x + 2);
</syntaxhighlight>

Propagating x yields:

<syntaxhighlight lang="c">
int x = 14;
int y = 7 - 14 / 2;
return y * (28 / 14 + 2);
</syntaxhighlight>

Continuing to propagate yields the following (which would likely be further optimized by [[dead-code elimination]] of both x and y.)

<syntaxhighlight lang="c">
int x = 14;
int y = 0;
return 0;
</syntaxhighlight>

Constant propagation is implemented in compilers using [[reaching definition]] analysis results. If all a variable's reaching definitions are the same assignment - which assigns a same constant to the variable - then the variable will always have the same value, and can be replaced with the constant.

Constant propagation can also cause conditional branches to simplify to one or more unconditional statements, if the conditional expression can be evaluated to true or false at compile time to determine the only possible outcome.