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DataParallel/ClosureConversion Up?

## Closure conversion without indexed types

The following scheme approaches the problem of mixing converted and unconverted code from the point of view of GHC's Core representation, avoiding the use of classes as much as possible. In particular, the scheme gracefully handles any declarations that themselves cannot be converted, but occur in a converted module. The two essential ideas are that (1) we move between converted and unconverted values/code using a conversion isomorphism and (2) we treat unconverted declarations differently depending on whether or not they involve arrows; e.g., the definition of `Int`

by way of unboxed values (which we cannot convert) doesn't prevent us from using `Int`

s *as is* in converted code.

### Conversion status

All `TyCon`

s, `DataCon`

s, and `Id`

s have a *conversion status* that determines how occurences of these entities are treated during conversion. For an `Id`

named `v`

, we have two alternatives:

- The binding of
`v`

was compiled without conversion and we have to use`v`

itself in converted code, which requires the use of an in-place conversion function. - Otherwise, we have a converted variant
`v_CC`

, and we use`v_CC`

instead of`v`

in converted code.

For a type constructor `T`

and its data constructors `C`

, we have three alternatives:

- The declaration introducing
`T`

and its constructors was compiled without conversion or we were unable to convert it, as it uses some language feature that prevents conversion. - A converted variant
`T_CC`

exists, but coincides with`T`

(e.g., because`T`

neither directly nor indirectly involves arrows). - A converted variant
`T_CC`

exists and differs from`T`

.

In the last two cases, we also have a *conversion constructor* `isoT`

whose type and meaning is described below.

An example of a feature that prevents conversion are unboxed values. We cannot make a closure from a function that has an unboxed argument, as we can neither instantiate the parametric polymorphic closure type with unboxed types, nor can we put unboxed values into the existentially quantified environment of a closure.

### Converting types

#### The closure type

We represent closures by

data a :-> b = forall e. !(e -> a -> b) :$ e

and define closure application as

($:) :: (a :-> b) -> a -> b (f :$ e) $: x = f e x

So, we have `(->)_CC == (:->)`

.

#### Conversion of type terms

We determine the converted type `t^`

of `t`

as follows:

T^ = T_CC , if T_CC exists = T , otherwise a^ = a_CC (t1 -> t2)^ = t1 -> t2 , if kindOf t1 == # or kindOf t2 == # = t1^ :-> t2^, otherwise (t1 t2)^ = t1^ t2^ (forall a.t)^ = forall a_CC.t^

Here some examples,

(Int -> Int)^ = Int :-> Int (forall a. [a] -> [a])^ = [a] :-> [a] ([Int -> Int] -> Int)^ = [Int :-> Int] :-> Int (Int# -> Int# -> Int#)^ = Int# -> Int# -> Int# ((Int -> Int) -> Int#)^ = (Int -> Int) -> Int# (Int -> Int -> Int#)^ = Int :-> (Int -> Int#)

Why do we use `(t1 -> t2)^ = t1 -> t2`

when either argument type is unboxed, instead of producing `t1^ -> t2^`

? Because we want to avoid creating conversion constructors (see below) for such types. After all, the conversion constructor `isoArr`

for function arrows works only for arrows of kind `*->*->*`

.

### Conversion constructors

To move between `t`

and `t^`

we use conversion functions. And to deal with type constructors, we need *conversion constructors*; i.e., functions that map conversion functions for type arguments to conversion functions for compound types.

#### Conversion pairs

Conversion functions come in pairs, which we wrap with the following data type for convenience:

data a :<->: b = (:<->:) {to :: a -> b, fr ::b -> a}

The functions witness the isomorphism between the two representations, as usual.

#### Types of convercion constructors

The type of a conversion constructor depends on the kind of the converted type constructor:

isoTy (t::k1->k2) = forall a a_CC. isoTy (a::k1) -> isoTy (t a::k2) isoTy (t::*) = t :<->: t^

where type conversion `t^`

is defined below.

As an example, consider

data T (f::*->*) = T1 (f Int) | T2 (f Bool)

The type of the conversion constructor is as follows :

isoTy (T::(*->*)->*) = forall f f_CC. (forall a a_CC. (a :<->: a_CC) -> (f a :<->: f_CC a_CC)) -> T f :<->: T_CC f_CC

The conversion constructor might be implemented as

isoT isof = toT :<->: frT where toT (T1 x) = T1 (to (isof isoInt ) x) toT (T2 y) = T2 (to (isof isoBool) y) frT (T1 x) = T1 (fr (isof isoInt ) x) frT (T2 y) = T2 (fr (isof isoBool) y)

where `isoInt`

and `isoBool`

are the conversion constructors for `Int`

s and `Bool`

s.

Moreover, the conversion constructor for function arrows is

isoArr :: a :<->: a_CC -- argument conversion -> b :<->: b_CC -- result conversion -> (a -> b) :<->: (a_CC :-> b_CC) isoArr (toa :<->: fra) (tob :<->: frb) = toArr :<->: frArr where toArr f = const (tob . f . fra) :$ () frArr (f :$ e) = frb . f e . toa

### Converting type declarations

#### Conversion rules

If a type declaration for constructor `T`

occurs in a converted module, we need to decide whether to convert the declaration of `T`

. We decide this as follows:

- If the declaration of
`T`

mentions another algebraic type constructor`S`

for which there is**no**`S_CC`

, then we cannot convert`T`

. - If
**all**algebraic type constructors`S`

mentioned in`T`

's definiton have a conversion`S_CC == S`

, we do not convert`T`

, but set`T_CC == T`

and generate a suitable conversion constructor`isoT`

. (NB: The condition implies that`T`

does not mention any function arrows.) - If the declaration of
`T`

uses any features that we cannot (or for the moment, don't want to) convert, simply don't convert it. - Otherwise, we generate a converted type declaration
`T_CC`

together with a conversion constructor`isoT`

. Conversion proceeds by converting all data constructors (see below).

Moreover, we handle other forms of type constructors as follows:

`FunTyCon`

: We have`(->)_CC = (:->)`

.`TupleTyCon`

: We have`(,..,)_CC = (,..,)`

. We may either have a (long) list of conversion constructors`iso(,..,)`

pre-defined or need to generate them inline by generating a suitable case expression where needed.`SynTyCon`

: Closure conversion operates on`coreView`

; hence, we will see no synonyms. (Well, we may see synonym families, but will treat them as not convertible for the moment.)`PrimTyCon`

: We essentially ignore primitive types during conversion, assuming that their converted and unconverted forms coincide. As they cannot contain values of other types, we need no conversion constructor.`CoercionTyCon`

and`SuperKindTyCon`

: They don't categorise values and are ignored during conversion.

#### Conversion constructor

Whenever we have a converted type constructor `T_CC`

, we also need to generate a conversion constructor `isoT`

. If `T`

has one or more arguments, the conversion is non-trivial, even for `T_CC == T`

.

#### Converting data constructors

We convert a data constructor `C :: t1 -> ... -> tn`

by generating a converted constructor `C_CC :: t1^ -> .. -> tn^`

. This includes the generation of a corresponding new worker `Id`

. For example, if the original worker has the type signature

MkT :: (Int -> Int) -> Int

the converted worker is

MkT_CC :: (Int :-> Int) -> Int

As a consequence, whenever we convert a *partial* wrapper application in an expression, we need to introduce a closure on the spot. (Simon pointed out that this is a rare case anyway.)

We do not specially handle wrappers of data constructors. They are converted just like any other toplevel function.

#### Examples

For example, when we convert

data Int = I# Int#

we get `Int_CC = Int`

and we have

isoInt :: Int :<->: Int isoInt = toInt :<->: frInt where toInt (I# i#) = I# i# frInt (I# i#) = I# i#

As another example,

data Maybe a = Nothing | Just a

implies `Maybe_CC = Maybe`

and

isoMaybe :: (a :<->: a_CC) -> (Maybe a :<->: Maybe a_CC) isoMaybe isoa = toMaybe :<->: frMaybe where toMaybe isoa Nothing = Nothing toMaybe isoa (Just x) = Just (to isoa x) frMaybe isoa Nothing = Nothing frMaybe isoa (Just x) = Just (fr isoa x)

### Converting classes and instances

We don't alter class and instance declarations in any way. However, the dictionary type constructors and dfuns are converted in the same way as other data types and value bindings, respectively.

As an example, assume `Num Int`

were defined as

class Num a where (+) :: a -> a -> a negate :: a -> a instance Num Int where (+) = primAddInt negate = primNegateInt

with the Core code being

data Num a = Num { (+) :: a -> a -> a, negate :: a -> a } dNumInt = Num Int dNumInt = Num primAddInt primNegateInt

Then, closure conversion gives us

data Num_CC a = Num_CC { (+_CC) :: a :-> a :-> a, negate_CC :: a :-> a } dNumInt_CC :: Num_CC Int -- as Int_CC = Int dNumInt_CC = Num_CC (to isoIntToIntToInt primAddInt) (to isoIntToInt primNegateInt) where isoIntToIntToInt = isoArr isoInt isoIntToInt isoIntToInt = isoArr isoInt isoInt

### Converting value bindings

#### Bindings

For every binding

f :: t = e

we generate

f_CC :: t^ = e^

#### Toplevel

When converting a toplevel binding for `f :: t`

, we generate `f_CC :: t^`

and redefine `f`

as

f :: t = fr iso<t> f_CC

#### Examples

chak: revision front

### Converting core terms

Apart from the standard rules, we need to handle the following special cases:

- We come across a value variable
`v`

where`idCC v == NoCC`

whose type is`t`

: we generate`convert t v`

(see below). - We come across a case expression where the scrutinised type
`T`

has`tyConCC T == NoCC`

: we leave the case expression as is (i.e., unconverted), but make sure that the`idCC`

field of all variables bound by patterns in the alternatives have their`idCC`

field as`NoCC`

. (This implies that the previous case will kick in and convert the (unconverted) values obtained after decomposition.) - Whenever we have an FC
`cast`

from or to a newtype`T`

, where`tyConCC T == NoCC`

, we need to add a`convert tau`

or`trevnoc tau`

, respectively. We can spot these casts by inspecting the kind of every coercion used in a cast. One side of the equality will have the newtype constructor. - We come across a dfun: If its
`idCC`

field is`NoCC`

, we keep the selection as is, but apply`convert t e`

from it, where`t`

is the type of the selected method and`e`

the selection expression. If`idCC`

is`ConvCC d_CC`

, and the dfun's class is converted,`d_CC`

is fully converted. If it's class is not converted, we also keep the selection unconverted, but have a bit less to do in`convert t e`

.**TODO**This needs to be fully worked out.

### Generating conversions

Whenever we had `convert t e`

above, where `t`

is an unconverted type and `e`

a converted expression, we need to generate some conversion code. This works roughly as follows in a type directed manner:

convert T = id , if tyConCC T == NoCC or AsIsCC = to_T , otherwise convert a = id convert (t1 t2) = convert t1 (convert t2) convert (t1 -> t2) = createClosure using (trevnoc t1) and (convert t2) on argument and result resp.

where `trevnoc`

is the same as `convert`

, but using `from_T`

instead of `to_T`

.

The idea is that conversions for parametrised types are parametrised over conversions of their parameter types. Wherever we call a function using parametrised types, we will know these type parameters (and hence can use `convert`

) to compute their conversions. This fits well, because it is at occurences of `Id`

s that have `idCC == NoCC`

where we have to perform conversion.

The only remaining problem is that a type parameter to a function may itself be a type parameter got from a calling function; so similar to classes, we need to pass conversion functions with every type parameter. So, maybe we want to stick `fr`

and `to`

into a class after all and requires that all functions used in converted contexts have the appropriate contexts in their signatures.

### TODO

#### Examples

Have an example with two modules one unconverted, where the converted imports the unconverted.

Also have an example that motivates why we have to vectorise/CC declarations such as `Int`

.

#### Conversion functions

Similar to `HasGenerics`

and instead of storing `Id`

of conversion constructors, we can derive from the name of the `TyCon`

.

#### Data constructors

How to exactly handle the worker and wrapper? Can we replace arrows by closure types in the worker? Or do we always have to add a wrapper?

**Simpler''' Don't try to make a complete cloned data constructor. By the time of CC, its all just Core and so wrappers are just like any other global function.
**

#### Original functions

The previous story was that when vectorising `f`

and generating `f_CC`

, we now define

f :: tau f = trevnoc tau f_CC

Now, with the approximate conversion scheme above, we may not have `trevnoc tau`

. In this case, we still generate `f_CC`

, but also leave the rhs of `f`

alone (i.e., compile the original functions).

When we give up on converting a complete right-hand side, we still want to convert all subexpressions that we can convert.