Dead store

In computer programming, a local variable that is assigned a value but is not read by any subsequent instruction is referred to as a dead store. Dead stores waste processor time and memory, and may be detected through the use of static program analysis, and removed by an optimizing compiler.

If the purpose of a store is intentionally to overwrite data, for example when a password is being removed from memory, dead store optimizations can cause the write not to happen, leading to a security issue.[1] Some system libraries have specific functions designed to avoid such dangerous optimizations, e.g. explicit_bzero on OpenBSD.[2]

Examples

Java example of a dead store:

// DeadStoreExample.java
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class DeadStoreExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<String>(); // This is a Dead Store, as the ArrayList is never read. 
        list = getList();
        System.out.println(list);
    }

    private static List<String> getList() {
        return new ArrayList<String>(Arrays.asList("Hello"));
    }
}

In the above code an ArrayList<String> object was instantiated but never used. Instead, in the next line the variable which references it is set to point to a different object. The ArrayList which was created when list was declared will now need to be de-allocated, for instance by a garbage collector.

JavaScript example of a dead store:

function func(a, b) {
    var x;
    var i = 300;
    while (i--) {
        x = a + b; // dead store
    }
}

The code in the loop repeatedly overwrites the same variable, so it can be reduced to only one call.[3]

See also

References
  1. "Insecure Compiler Optimization | OWASP".
  2. "OpenBSD manual pages". man.openbsd.org. Retrieved 2016-05-14.
  3. "HTML5, and Real World Site Performance: Seventh IE9 Platform Preview Available for Developers".
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