What is dead code?

Dead or unreachable code is code that will never be executed. It manifests itself as variables that are declared but never used, functions that are never called, or code that is skipped because of a branch. Since the dead code is not executed, it is an error, often a logic error. Dead code is created by cavalier programmers. An easy way to create dead code is by use of a goto statement and labels. Here is an example:

goto process_image;
j = j + 1;  // dead code here.

Since we have an unconditional branch to the label processing on the first line, the assignment statement on the second line can never be executed. A return statement can be used to make dead code. Here is an example:

int div_0(double a, double b)
    if (b == 0)
        return 1;
        return 0;
    return a/b;   // dead code here.

The arithmetic expression a/b is performed after two return statements inside an if-else. Since the if-else combination will result in one of the return statements being executed, the final return statement becomes an orphan, because it is impossible to get to. That is, since code execution is linear, and there is no conditional expression wrapping the return statement, any code after the return statements cannot possibly be executed.

Note that very few compilers will identify orphan code.


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