Safe publication for a singleton using local vars

Question

While going through this article I came across the below code which explained safe publication fields.

public class SafeLocalDCLFactory implements Factory {
  private volatile Singleton instance;

  @Override
  public Singleton getInstance() {
    Singleton res = instance;
    if (res == null) {
      synchronized (this) {
        res = instance;
        if (res == null) {
          res = new Singleton();
          instance = res;
        }
      }
    }
    return res;
  }
}

My doubt is why do we need local var res defined in the getInstance() ? Isn't the volatile and DCL enough to guarantee safe publication ? What exactly is the purpose of defining a local var res here and then comparing it for null ?

EDIT: The article explains the below to justify the need to use local var. But how are we protecting ourselves from returning null?

The introduction of local variable here is a correctness fix, but only partial: there still no happens-before between publishing the Singleton instance, and reading of any of its fields. We are only protecting ourselves from returning "null" instead of Singleton instance

Answer 1

It minimizes the number of reads from the volatile variable instance . Volatile variable access is more expensive than non-volatile memory access, because usually it involves a memory barrier. By assigning it to a temporary variable, this code reads from the volatile variable only once if it is already initialized, instead of twice (once to check, once to return).