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Intel's manuals describe a nonexistent FMA rounding instruction

purplesyringa.moe Editor score 7/10

The post says Intel's manual documents vfmaddrnd231pd, but GNU assembler does not recognize it and the cited compiler intrinsics are absent. Engineers considering instruction-specific optimizations should verify that documented CPU features exist in actual toolchains.

What

vfmaddrnd231pd is an FMA instruction documented in the 2024 Intel manual but absent from the 2026 edition.

How

The assembler and compiler do not recognize it, and running its listed encoding on a Haswell CPU raises an undefined-instruction exception.

Why it matters

Engineers should verify instruction documentation against toolchain and hardware behavior before relying on a documented CPU feature.

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