alt Hacker News> And if that's really the case where are the power efficient x86 chips?
Where are the power inefficient x86 chips? If you normalize for production process and put the chips under synthetic load, ARM and x86 usually end up in a similar ballpark of efficiency. ARM is typically less efficient for wide SIMD/vector workloads, but more efficient at idle.
AMD and Intel aren't smartphone manufacturers. Their cash cows aren't in manufacturing mobile chipsets, and neither of them have sweetheart deals on ARM IP with Softbank like Apple does. For the markets they address, it's not unlikely that ARM would be both unprofitable and more power-hungry.
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Intel spent years trying to get manufacturers to use their x86 chips in phones, but manufacturers turned them down, because the power efficiency was never good enough.
Jim Keller goes into some detail about what difference the ISA makes in general in this clip https://youtu.be/yTMRGERZrQE?si=u-dEXwxp0MWPQumy
Spoiler, it's not much because most of the actual execution time is spent in a handful of basic OPs.
Branch prediction is where the magic happens today.