The CPU instruction that appeared in a manual but would not run

An Intel manual described vfmaddrnd231pd, an instruction promising per-operation rounding for floating-point multiplication and addition. purplesyringa found missing compiler intrinsics, assembled the encoding manually and got an undefined-instruction fault on Haswell. The entry disappeared in a newer manual. Links to AVX10 changes suggest a discarded design, but that origin remains the author’s reconstruction rather than Intel’s confirmation.

purplesyringa · Oct 12, 2026

Why it matters

Even precise-looking reference material deserves cross-checking against toolchains and actual supported hardware.

More context

Read the investigation for its method: compare manual versions, test the documented encoding and trace related compiler changes. The useful lesson is evidence gathering, not adopting an unavailable instruction.

Keep in mind

The historical explanation is partly inferred; the observed failure does not establish the behavior of every Intel processor.

Original read