Unreachable Code - Analysis

Analysis

Detecting unreachable code is a form of static analysis and involves performing control flow analysis to find any code that will never be executed regardless of the values of variables and other conditions at run time. In some languages (e.g. Java ) 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 the internal transformations performed by an optimizing compiler (e.g., common subexpression elimination).

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

int iN = 2 + 1; if (iN == 4) { xyz }

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

double dX = sqrt(2); if (dX > 5) { xyz }

The unreachable code elimination technique is in the same class of optimizations as dead code elimination and redundant code elimination.