You're arguing semantics IMHO. In my mind speculatively decoding every single byte offset & then resolving at the end of the cycle which to take is a form of heuristic execution because the heuristic is "decode all possible executions". And 4 vs 8 is a pretty sizeable difference. Moreover, the pre-decoder requiring knowing the op-codes at the end means all instruction decodes are serialized on decoding 16 instructions whereas Apple can just decode each op independently & only decodes 8.
Oh, I see what you are saying. I don't consider it to be a heuristic because it's simply bruteforcing it. IMO a heuristic needs improve over brute force.
> And 4 vs 8 is a pretty sizeable difference.
True, but x86 was doing four instruction 20 years ago. As I mentioned the current state of the art (in a shipping product) is 9, and 9 is larger than 8. Importantly, this Skymont of leapfrogging decoders approach is scalable.
> whereas Apple can just decode each op independently & only decodes 8.
Apple isn't as free from serialisation as you suggest. Like X86, many instructions decode to multiple uops. According to research [1] instructions which decode to two uops are common and a few decode to as many as 12 uops.
It also does instruction fusion, two neighbouring instructions can sometimes decode into a single uop. This all means that there is plenty of serialisation within Apple's decoder. And branching also creates serialisation.
It's just not as simple as independently decoding eight instructions into eight uops every cycle. Simpler than what x86 implementations need to do, but not as brain-dead simple as you suggest.
Actually, Skymont's approach has an advantage over Apple here, because it only needs to serialise within each 3-wide decoder.
