I would think a large part of the smaller size was removing the transistors that enabled paths for higher clock speeds. It's going into effectively monolithic hybrid CPUs with 4/4c cores so the area savings can't come a differing process (or even higher density/low perf library).
Marketing messaging aside the efficiency by product is more so that wide work loads become power limited with the conventional "big" cores when they are clocked (specced) for more peak performance (that you want for burst workloads). Yes you can clock they done into the efficiency range but then you basically "waste" transistors that were used to enable those high clock speeds in the first place. Ultimately as with Intel the main goal is performance per area/cost (however you want to phrase) as opposed to actual low end power efficiency as with the mobile hybrids.
The longer term outlook is that there seems to be suggestions that SMT is on it's way out. As such these area efficient cores (from both) are going to be needed to offset the thread count loss.
I'd expect Zen 4c cores to have significantly higher dynamic Perf/W *at the same voltage* but to hit the same frequency they need to run at a higher voltage than Zen 4 cores. So, the Perf/W advantage is only there if you run more cores at a significantly lower frequency, which is something you might want to do for a a highly parallel datacenter/cloud workload or for a handheld device like a Steam Deck
In fact there's no need for me to guess, this chinese article has a lot of measured data:
https://zhuanlan-zhihu-com.translat...uto&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_pto=wapp
The V/F curve shows Zen 4c and Zen 4 voltages both around ~0.69v at 1.5GHz, but at 2.3GHz, it's something like 0.79v vs 0.70v, which feels like it would be more than enough to offset the higher power efficiency at the same voltage. As far as I can tell, the first 3 points on the "Energy Efficiency Curve" graph of that article are 1.5GHz, 1.8GHz, and 2.3GHz where power efficiency is significantly better at 1.5GHz, about the same at 1.8GHz, and noticeably worse at 2.3GHz. The power benefit in that graph at 1.5GHz looks tiny, but that's because at 1.5GHz (single core for that graph I think), the vast majority of the power is outside the CPU (>4W feels high - I wonder if that platform had PCI-E power management issues like some users have had in the past with AMD?), so the actual power consumption difference for the CPU core itself in percentage terms seems very significant.
Zen 4c is good but it isn't that amazing in my opinion though...
Assuming those measurements are correct: ~35% lower area for ~35% lower frequency at Vmin, or ~29% lower frequency at max boost clock (so perf/mm2 if you didn't care about perf/watt hasn't really improved much) with *worse* dynamic Perf/W as early as >2GHz?! It's nice if you're very power limited and running close-ish to Vmin anyway (e.g. Steam Deck) but for multithreaded workloads where you want better Perf/mm2 & Perf/$ than Zen 4c Vmin offers you, it sounds the benefit isn't as big as AMD's marketing made me think originally.
At the same power efficiency as a 3.5GHz Zen4c (according to that specint energy efficiency graph), you can clock Zen4 ~25% higher, so the perf(/mm2) benefit at fixed power and fixed area is roughly +15% at that point of the curve (1.0 / ((1-0.35) / (1-0.25))). But at 2.3GHz, you can only clock Zen 4 about ~15% higher at the same power (2.7GHz vs 2.3GHz) so the advantage is roughly +23%. If you care about power consumption rather than perf/$, then you can trade-off area vs power by reducing the voltage on both chips! (obviously if you want to go below 2.3GHz then you need 4c to reduce the voltage and that's where it starts to shine, but AMD's Zen 4c datacenter chips look to be pretty close to 2.3GHz base clock and much higher boost clocks).
There are definitely some markets in both datacenter and Steam Deck-like devices where these trade-offs make a lot of technical sense... and then there's other markets where it doesn't make as much technical sense and it's kind of a wash, but it still makes a lot of marketing sense to compete with Intel's core counts.
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