GoKawiilIntel SDM documents nonexistent instruction VFMADDRND231PD
A researcher investigating an Intel-documented x86 instruction called vfmaddrnd231pd found it does not actually exist on real hardware. The instruction, which purportedly allowed per-instruction rounding control for fused multiply-add operations, is absent from GNU assembler and lacks corresponding intrinsics in both ICC and Clang, and attempting to execute its hand-assembled opcode on a Haswell CPU triggers an undefined instruction fault.
GoKawiil's interpretation of the reporting above, not reported fact.
The finding suggests Intel's Software Development Manual, a primary reference for compiler and assembler developers, contained at least one documented instruction that was never implemented in silicon or toolchains, which could mislead low-level programmers who rely on it as authoritative. The presence of an obsolete internal documentation marker (DDS) alongside the phantom instruction hints this may stem from leftover or speculative documentation rather than a currently supported feature, according to the researcher's investigation.
- vfmaddrnd231pd appears in Intel's SDM but has no working implementation on tested hardware or in major toolchains.
- The instruction's opcode uses a 'DDS' VEX prefix designation that Intel reportedly removed from later manual versions.
- The case illustrates how official CPU documentation can include unimplemented or erroneous instructions that developers might mistakenly trust.
Source: purplesyringa.moe, 2026-10-12
Published there as: “Who Is Vfmaddrnd231pd?”
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