[…] The immediate byte defines several bit fields that control rounding, DAZ, FTZ, and exception suppression (See Table 5-3).
Most x86 floating-point instructions use the global rounding mode and settings, changing which is pretty slow. But vfmaddrnd231pd allows the rounding mode to be controlled on a per-instruction basis, and since FMA can emulate addition, subtraction, and multiplication, this mechanism effectively allows any arithmetic to have controllable rounding without AVX-512 – making some algorithms that rely on rounding incredibly faster.
VEX is the VEX prefix, but what is DDS? This opcode is the only mention of DDS in the SDM, and this SO answer says that DDS is merely used for documentation and was removed from later SDM versions, but the fact that it’s still present here is suspicious. And when you actually run the instruction on a CPU that supports the FMA instruction set, like my Haswell, it raises #UD (“UnDefined instruction”)!
