On 29 April 2015 at 17:21, Levent Erkok
This proposal is very much in the spirit of the earlier proposal on adding new float/double functions; for instance see here: https://mail.haskell.org/pipermail/libraries/2014-April/022667.html
"fma" (a.k.a. fused-multiply-add) is one of those functions; which is the workhorse in many HPC applications. The idea is to multiply two floats and add a third with just one rounding, and thus preserving more precision. There are a multitude of applications for this operation in engineering data-analysis, and modern processors come with custom implementations and a lot of hardware to support it natively.
I created a ticket along these lines already: https://ghc.haskell.org/trac/ghc/ticket/10364
Edward suggested that the matter should further be discussed here.
I think the proposal is rather straightforward, and should be noncontroversial. To wit, we shall add a new method to the RealFloat class:
class (RealFrac a, Floating a) => RealFloat a where ... fma :: a -> a -> a -> a
The intention is that
fma x y z = x * y + z
Can we please have a better name (even if it's just "fusedMultipleAdd")? I have no real opinion on adding this function or not, but just seeing that function (in other code, doing ":info RealFloat" in ghci, etc.) tells me nothing about what it is. Ideally we wouldn't have to rely upon reading Haddock to understand random TLAs in code.
except the multiplication and addition are done infinitely-precisely, and then rounded only once; as opposed to two roundings as one would get with the above implementation. Most modern architectures directly support this operation so we can map it easily; and in case the architecture does not have it available, we can get it via the C-math libraries, where it appears under the names fma (the double version), and fmaf (the float version.)
There should be no default definitions; as an incorrect (two-rounding version) would essentially beat the purpose of having fma in the first place.
While the name "fma" is well-established in the arithmetic/hardware community and in the C-library, we can also go with "fusedMultiplyAdd," if that is deemed more clear.
Discussion period: 2 weeks.
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