2 ms·
> Value prediction and address prediction are very different things. Both are classes of data prediction, and Apple CPUs do both. > Address prediction for loa
by stinkbeetle 1y ago
> Value prediction and address prediction are very different things.
Both are classes of data prediction, and Apple CPUs do both.
> 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.
That is not what is known as load/store address prediction. That is cache prefetching, which of course has to predict addresses in some manner too.
> 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.
I'm talking about load address prediction specifically. Apple has both, but load value prediction would not trigger here because I don't think it does pattern/stride detection like load address, but rather is value based and you'd have to see the same values coming from the load. Their load address predictor does do strides though.
I don't know if it needs cache misses or other long latency sources to kick in and start training or not, so I'm not entirely sure if it would capture this pattern. But it can capture similar for sure. I have an M4 somewhere, I should dig it out and try.
> 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.