Editing Unreachable code (section)

==Analysis==
Detection of unreachable code is a form of [[control flow analysis]] to find code that can never be reached in any possible program state. In some languages (e.g. [[Java (programming language)|Java]]<ref>{{cite web|title=Java Language Specification|url=http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.21}}</ref>) some forms of unreachable code are explicitly disallowed. The optimization that removes unreachable code is known as [[dead code elimination]].

Code may become unreachable as a consequence of transformations performed by an [[optimizing compiler]] (e.g., [[common subexpression elimination]]).

In practice the sophistication of the analysis has a significant impact on the amount of unreachable code that is detected. For example, [[constant folding]] and simple flow analysis shows that the inside of the if-statement in the following code is unreachable:

<syntaxhighlight lang="C">
int N = 2 + 1;

if (N == 4)
{
   /* unreachable */
}
</syntaxhighlight>

However, a great deal more sophistication is needed to work out that the corresponding block is unreachable in the following code:

<syntaxhighlight lang="C">
double X = sqrt(2);

if (X > 5)
{
    /* unreachable */
}
</syntaxhighlight>

Unreachable code elimination technique is in the same class of optimizations as [[dead code elimination]] and [[redundant code]] elimination.

===Unreachability vs. profiling===
In some cases, a practical approach may be a combination of simple unreachability criteria and use of a [[Profiler (computer science)|profiler]] to handle the more complex cases. Profiling in general can not ''prove'' anything about the unreachability of a piece of code, but may be a good [[heuristic]] for finding potentially unreachable code. Once a suspect piece of code is found, other methods, such as a more powerful code analysis tool, or even analysis by hand, could be used to decide whether the code is truly unreachable.