> If tests continue to fail on some platforms, I plan to manually handle NaN and INF in the C code, before calling libc fma().

For Windows, you need to do much more than this: it's not just about handling NaNs and infinities, it's about reimplementing the entire function from scratch to give correctly rounded results. Without correctly-rounded results, there's very little point in having fma.

If it were a couple of niche platforms that gave bad results, then we could push this through. But it's Windows. :-(