Consecutive linked list nodes can occur when you bump allocate them, and in particularly if you have a copying garbage collector which ensures that bump allocation takes place from blank slate heap areas with no gaps.

Idea: what if we implement something that resembles CDR coding, but doesn't compact the cells together (not a space-saving device). The idea as is that when we have two cells A and B such that A->cdr == B, and such that A + 1 == B, then we replace A->cdr with some special constant which says the same thing; indicates that A->cdr is equivalent to A + 1.

Then, I think, we could have a very simple, stable and portable form of the trick in the article:

  while (node) {
    value += node->value;
    if (node->next == NEXT_IS_CONSECUTIVE)
      next = node + 1;
    else
      next = node->next;
    node = next;
  }

This doesn't look like something that can be optimized away, because we are not comparing node->next to node + 1; there is no tautology there.

Yes. But I don’t think the OP is suggesting this as an alternative to using an array. As I read / skimmed it the linked list is just a simplified example. You can use this trick in more complex situations too, eg if you’re searching a tree structure and you know that some paths through the tree are much more common than others.

Yes, as he noted the trick is of limited value in practice

Value prediction and address prediction are very different things.

Address prediction for loads and stores, by detecting various kinds of strides and access patterns, is done by most CPUs designed during the last 25 years and it is used for prefetching the corresponding data. This is as important for loads and stores as branch prediction for branches.

On the other hand, value prediction for loads is done by very few CPUs and for very restricted use cases, because in general it is too costly in comparison with meager benefits. Unlike for branch direction prediction and branch target prediction, where the set from which the predicted value must be chosen is small, the set from which to choose the value that will be returned by a load is huge, except for very specific applications, e.g. which repeatedly load values from a small table.

The application from the parent article is such a very special case, because the value returned by the load can be computed without loading it, except for exceptional cases, which are detected when the loaded value is different from the pre-computed value.

But that works on a different level, right? At least as I understand it data speculation is about prefetching from memory into cache. This trick is about using the branch predictor as an ultra-fast ”L0” cache you could say. At least that’s how I understand it.

Yeah this use case is pretty contrived. Even a simple unrolled linked list would beat his implementation.

> Won't it introduce risk of invalid memory access

no.