Editing Constant folding (section)

== The optimizations in action ==

Constant folding and propagation are typically used together to achieve many simplifications and reductions, and their interleaved, iterative application continues until those effects cease.

Consider this unoptimized pseudocode returning a number unknown pending analysis:

<syntaxhighlight lang="c">
int a = 30;
int b = 9 - (a / 5);
int c = b * 4;

if (c > 10) {
   c = c - 10;
}
return c * (60 / a);
</syntaxhighlight>

Applying constant propagation once, followed by constant folding, yields:

<syntaxhighlight lang="c">
int a = 30;
int b = 3;
int c = b * 4;

if (c > 10) {
   c = c - 10;
}
return c * 2;
</syntaxhighlight>

Repeating both steps twice produces:

<syntaxhighlight lang="c">
int a = 30;
int b = 3;
int c = 12;

if (true) {
   c = 2;
}
return c * 2;
</syntaxhighlight>

Having replaced all uses of variables <code>a</code> and <code>b</code> with constants, the compiler's [[dead-code elimination]] applies to those variables, leaving:

<syntaxhighlight lang="c">
int c = 12;

if (true) {
   c = 2;
}
return c * 2;
</syntaxhighlight>

(Boolean constructs vary among languages and compilers, but their details—such as the status, origin, and representation of [[Boolean data type|true]]—do not affect these optimization principles.)

Traditional constant propagation produces no further optimization; it does not restructure programs.

However, a similar optimization, [[sparse conditional constant propagation]], goes further by selecting the appropriate conditional branch,<ref>{{Citation |last1=Wegman |first1=Mark N |last2=Zadeck |first2=F. Kenneth |title=Constant Propagation with Conditional Branches |journal=ACM Transactions on Programming Languages and Systems |volume=13 |issue=2 |date=April 1991 |pages=181–210 |doi= 10.1145/103135.103136|citeseerx=10.1.1.130.3532 |s2cid=52813828 }}</ref> and removing the always-true conditional test. Thus, variable <code>c</code> becomes redundant, and only an operation on a constant remains:

<syntaxhighlight lang="c">
return 4;
</syntaxhighlight>

If that pseudocode constitutes a function body, the compiler knows the function evaluates to integer constant <code>4</code>, allowing replacement of calls to the function with <code>4</code>, and further increasing program efficiency.