🔍Intel's Mysterious vfmaddrnd231pd Instruction Vanishes
Intel's FMA instruction vanished from docs, leaving devs puzzled
TL;DR
Intel's FMA instruction, vfmaddrnd231pd, has disappeared from the latest Software Development Manual, leaving developers with questions and no clear answers. The instruction's unique rounding control could have sped up certain algorithms, but now it's gone without a trace.
Intel's FMA instruction set includes a peculiar instruction called vfmaddrnd231pd, which allows for per-instruction rounding control. However, it vanished from the latest Software Development Manual, leaving developers puzzled. This instruction could have sped up algorithms relying on rounding, but now it's gone, raising questions about its original purpose and Intel's intentions. The instruction's disappearance is a mystery, with no clear explanation or confirmation from Intel.
Key Points
vfmaddrnd231pd allows per-instruction rounding control, unique in the FMA set.
The instruction vanished from the 2026 edition of the Intel Software Development Manual.
Intel's documentation contains unique tables and pseudocode for vfmaddrnd231pd.
The instruction raises #UD (Undefined Instruction) on CPUs supporting FMA.
The best hypothesis is that Intel found an issue and removed the instruction.
Why It Matters
If you're working on algorithms that rely on precise rounding, the disappearance of vfmaddrnd231pd could mean you're missing out on potential performance gains. The instruction's unique rounding control could have sped up certain operations, but now it's gone, leaving developers to wonder if there's a workaround or if they need to find an alternative approach.
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