The legend of “x86 CPUs decode instructions into RISC form internally” (2020)

The idea that modern high-performance x86 processors work by decoding x86 instructions into RISC-like instructions is a widespread but misleading myth. An analysis of how different x86 processors handle a simple loop shows that while the P6 microarchitecture of the late 1990s did indeed use RISC-like micro-operations, this is no longer the case for newer high-performance Intel processors such as the Sandy Bridge and beyond. While some improvements such as micro-fusion and branch fusion have been made, these have made the correspondence between micro-operations and RISC instructions increasingly tenuous. However, some other out-of-order x86 microarchitecture families have always decoded instructions directly into single micro-operations without any splitting.

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