Blog investigates Intel’s documented but apparently nonexistent VFMADDRND231PD instruction for per-instruction floating-point rounding

A blogger examines VFMADDRND231PD, an Intel-documented FMA instruction that supposedly enables per-instruction rounding control for packed double-precision math. Testing shows assemblers, compilers, and Haswell CPUs do not recognize it, suggesting the instruction is a documentation artifact rather than real hardware functionality.

The post highlights how even authoritative CPU manuals can contain remnants of abandoned or internal features, complicating low-level optimization work. If such per-instruction rounding existed outside AVX-512, it could have accelerated numerical algorithms, but its absence reinforces AVX-512’s role as the practical path for fine-grained floating-point control.