Feature request: Add support for FMA

FMA (fused-multiply-add) has been around for quite some-time, and is natively supported by many of the newer processors. It's an operation that is heavily used in HPC applications. I think it's time for Haskell to have native support for it as well.

Ideally, it should be added to the Num class as a method. Assuming that is a possibility, we would have the following signature and default definition:

class Num a where
   ...
   fma :: a -> a -> a -> a
   fma x y z = x * y + z

Except of course the Float and Double instances would ensure that the rounding is done only once.

If adding it to the Num class is not an option, then RealFloat class would be the next best place; and perhaps that's arguably also a better place because those types are the ones one usually has in mind when using FMA.

I think either Num or RealFloat would be fine choices.

Implementation:

• If the underlying architecture supports it (which is very common), directly emit FMA instruction
• Otherwise, FFI out to C and use fma and fmaf from the math library

A direct software implementation might also be possible for platforms where neither choice above is an option, but that bridge can be crossed when we get there.

As a final note; while supporting these functions might seem going-out-of-our-way; it is indeed a big selling point in HPC applications. Furthermore, hardware manufacturers are putting big resources to make these supported natively in the instruction sets. Supporting FMA right out of the box would be a very good step in wider adaptation of Haskell in the HPC community.