Who Is Vfmaddrnd231pd?

VFMADDRND231PD is a documented x86 instruction in Intel's Software Development Manual that would theoretically allow per-instruction rounding control for floating-point operations, but it does not actually exist—assemblers don't recognize it, compilers lack its intrinsics, and attempting to execute it on compatible CPUs raises an undefined instruction error. The instruction mysteriously disappeared from the 2026 version of the SDM despite being present in 2024, suggesting Intel may have planned the feature but removed it from hardware at the last moment without fully updating the documentation.