Intel's Manual Documents a CPU Instruction That Doesn't Exist
Who Is Vfmaddrnd231pd?
A blogger investigates vfmaddrnd231pd, an FMA instruction in Intel's 2024 Software Development Manual that promises per-instruction rounding control. But GNU assembler rejects it, compilers lack the intrinsics, the opcode references a mysterious DDS prefix, and running it on a Haswell CPU triggers an undefined instruction exception. The instruction vanished from the 2026 manual, leaving only speculation about a last-minute silicon cancellation.
My best hypothesis is that Intel wanted to add the vfmaddrnd* set, then found an issue with it at the last moment and removed it from the CPU, but didn't quite adjust the SDM correctly.