M1 really is a counterexample to all theory that Jim is saying etc. The real proof would be if same results were also reproduced on M1 instead of Zen
I have doubts about that. I-cache word lines are much larger than instructions anyway, and it was the reduction in memory fetch operations that made THUMB more energy-efficient on ARM (and even there, there's lots of discussion on whether that claim holds up). And if you're going for fixed-width instructions then many instructions will use more space than they use now, reducing the overall effectiveness of the I-cache.
So even if you can prove that a fixed-size decoder uses less power, you will still need to prove that that gain in decoder power efficiency is greater than the increased power usage due to reduced instruction density and accompanying loss in cache efficiency.
That's not too bad for the first instruction in a line but the second instruction is dependant on how the first instruction decides, and the third dependent on the second. Etc. So it's not only a big multiplexer tree, but a content dependent multiplexer tree. If you're trying to unpack multiple instructions each clock (or course you are. You've got six schedulers to feed) then that's a big pile of logic.
Even RISC-V has this problem, but there they've limited it to two sizes of instruction (2 and 4 bytes), and the size is in the first 2 bits of each instruction (so no fancy decode needed)
Apples M series looks very impressive because typically, at launch, they are node ahead of the competition, so early access deals with TSMC is the secret weapon this buys them about 6 months. They also are primarily laptop chips, AMD has competitive technology but always launches the low power chips after the desktop & server parts.
Aren't Apple typically 2 years ahead? M1 came out 2020, other CPUs from the same node level (5 nm TSMC) came out 2022. If you mean apple launches their 6 months ahead of the rest of the industry gets on the previous node, sure, but not the current node.
What you are thinking about is maybe that AMD 7nm is comparable to Apple 5nm, but really what you should compare is todays AMD cpus with the Apple cpu from 2022, since they are on the same architecture.
But yeah, all the impressive bits about Apple performance disapears once you take architecture into account.
There only seems to be comparisons between laptop CPUs which are quiet limited.
Apple is just that good.