ARM's dominance in the mobile and low-power market was largely a historical accident. In particular: - At the dawn of the smartphone market, existing x86 platf

ARM's dominance in the mobile and low-power market was largely a historical accident. In particular: - At the dawn of the smartphone market, existing x86 platforms were designed for high-performance applications that were too power-hungry for a mobile phone. - Intel wasn't interested in pursing low power mobile phone applications, and sold their XScale line to Marvell. - AMD couldn't afford to invest in low power mobile phone applications, and sold their entire handset division to Qualcomm. - VIA wasn't in a position to invest in much of anything, much less mobile phones. - MIPS was stagnant and didn't really have a clear roadmap or future, and RISC-V didn't exist yet. - ARM was available for third-parties to license, whereas x86 was not. As for efficiency, modern multi-core processors spend the overwhelming majority of their transistor budget on logic units and SRAM -- the cost of decoding instructions is trivial. In addition, many of the more esoteric x86 instructions are emulated in microcode. As such, I don't see how AArch64 would be inherently more efficient than x86-64.