Editing Constant folding (section)

== Constant folding ==

Constant folding is the process of recognizing and evaluating [[Constant (programming)|constant]] expressions at [[compile time]] rather than computing them at runtime. Terms in constant expressions are typically simple literals, such as the [[integer literal]] <code>2</code>, but they may also be variables whose values are known at compile time. Consider the statement:

<syntaxhighlight lang="c">
i = 320 * 200 * 32;
</syntaxhighlight>

Most compilers would not actually generate two multiply instructions and a store for this statement. Instead, they identify constructs such as these and substitute the computed values at compile time (in this case, <code>2,048,000</code>).

Constant folding can make use of arithmetic identities. If <code>x</code> is numeric, the value of <code>0 * x</code> is zero even if the compiler does not know the value of <code>x</code>. (Note that this is not valid for [[IEEE_754|IEEE floats]], since <code>x</code> could be Infinity or [[NaN]].)

Constant folding may apply to more than just numbers. Concatenation of [[string literal]]s and constant strings can be constant folded.  Code such as <code>"abc" + "def"</code> may be replaced with <code>"abcdef"</code>.