Safe Haskell | None |
---|---|

Language | Haskell2010 |

Vectorisation of expressions.

# Vectorise right-hand sides of toplevel bindings

vectTopExpr :: Var -> CoreExpr -> VM (Maybe (Bool, Inline, CoreExpr)) #

Vectorise a polymorphic expression that forms a *non-recursive* binding.

Return `Nothing`

if the expression is scalar; otherwise, the first component of the result
(which is of type `Bool`

) indicates whether the expression is parallel (i.e., whether it is
tagged as `VIParr`

).

We have got the non-recursive case as a special case as it doesn't require to compute vectorisation information twice.

vectScalarFun :: CoreExpr -> VM VExpr #

Vectorise an expression of functional type, where all arguments and the result are of primitive
types (i.e., `Int`

, `Float`

, `Double`

etc., which have instances of the `Scalar`

type class) and
which does not contain any subcomputations that involve parallel arrays. Such functionals do not
require the full blown vectorisation transformation; instead, they can be lifted by application
of a member of the zipWith family (i.e., `map`

, `zipWith`

, zipWith3', etc.)

Dictionary functions are also scalar functions (as dictionaries themselves are not vectorised,
instead they become dictionaries of vectorised methods). We treat them differently, though see
"Note [Scalar dfuns]" in `Vectorise`

.

Vectorise a dictionary function that has a 'VECTORISE SCALAR instance' pragma.

In other words, all methods in that dictionary are scalar functions — to be vectorised with
`vectScalarFun`

. The dictionary "function" itself may be a constant, though.

NB: You may think that we could implement this function guided by the struture of the Core
expression of the right-hand side of the dictionary function. We cannot proceed like this as
`vectScalarDFun`

must also work for *imported* dfuns, where we don't necessarily have access
to the Core code of the unvectorised dfun.

Here an example — assume,

class Eq a where { (==) :: a -> a -> Bool } instance (Eq a, Eq b) => Eq (a, b) where { (==) = ... } {-# VECTORISE SCALAR instance Eq (a, b) }

The unvectorised dfun for the above instance has the following signature:

$dEqPair :: forall a b. Eq a -> Eq b -> Eq (a, b)

We generate the following (scalar) vectorised dfun (liberally using TH notation):

$v$dEqPair :: forall a b. V:Eq a -> V:Eq b -> V:Eq (a, b) $v$dEqPair = /\a b -> \dEqa :: V:Eq a -> \dEqb :: V:Eq b -> D:V:Eq $(vectScalarFun True recFns [| (==) @(a, b) ($dEqPair @a @b $(unVect dEqa) $(unVect dEqb)) |])

NB: * '(,)' vectorises to '(,)' — hence, the type constructor in the result type remains the same. * We share the '$(unVect di)' sub-expressions between the different selectors, but duplicate the application of the unvectorised dfun, to enable the dictionary selection rules to fire.