Version 22 (modified by chak, 11 years ago) (diff)


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.

Conversion status

We add to all declaration that are affected by closure conversion a value of type

data StatusCC a 
  = NoCC      -- Declaration has not been converted
  | ConvCC a  -- Here is the converted version

For example, Id gets a field of type StatusCC Id. A declaration thisDecl can be in one of three categories:

  • NoCC: We did not convert that declaration, either because it was declared in an unconverted module or because it uses some feature that prevents conversion.
  • ConvCC thisDecl: Original and converted declaration coincide (e.g., type declarations not involving arrows directly or indirectly).
  • ConvCC convDecl: The variant convDecl is the closure-converted form of thisDecl.

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}

Converting type declarations


The alternatives of TyCon.TyCon get a new field tyConCC :: StatusCC (TyCon, Id). This field is NoCC for data constructors for which we have no conversion and ConvCC (T_CC, iso_T) if we have a conversion, where the converted declaration T_CC may coincide with T. The value iso_T is a conversion constructor for values inhabitating types formed from the original and converted constructor. The type of these functions is as follows:

isoTy (T::k1->..->kn->*) = forall _1 .. _n _1_CC .. _n_CC.
  isoTy (_1::k1) -> .. -> isoTy (_n::kn) -> 
  (C _1 .. _n :<->: C_CC _1_CC .. _n_CC)

(The type variables beginning with underscores are bound here; we add one underscore for each level of kinding.)

As an example, consider

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

The type of the conversion constructor is as follows (using more meaningful type variable names):

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
    toT (T1 x) = T1 (to (tff tfInt ) x)
    toT (T2 y) = T2 (to (tff tfBool) y)
    frT (T1 x) = T1 (fr (tff tfInt ) x)
    frT (T2 y) = T2 (fr (tff tfBool) y)

where isoInt and isoBool are the conversion constructors for Ints and Bools.

Moreover, we represent closures - the converted form of function arrows - as follows:

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

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

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 type constructor S and we have tyConCC S == NoCC, we do not convert T and set its tyConCC field to NoCC as well.
  • If the declaration of T uses any features that we cannot (or for the moment, don't want to) convert, we do not convert T and set its tyConCC field to NoCC.
  • If all type constructors S mentioned in T's definiton have tyConCC S == AsIsCC, we do not convert T and set its tyConCC field to AsIsCC as well. (NB: This implies that T does not mention any function arrows.)
  • Otherwise, generate a converted type declaration T_CC together two conversion functions fr_T and to_T, and set

tyConCC to ConvCC (T_CC, fr_T, to_T). Note that basic types, such as Int and friends, should have tyConCC set to AsIsCC.

Converting class declarations

If we come across a class declaration for a class C during conversion, we convert it generating C_CC. Like with type constructors, Class.Class gets a classCC :: StatusCC Class field that is ConvCC C_CC for classes that have a conversion. We also ensure that the classTyCon of C, let's call it T_C, refers to T_C_CC and fr_T_C and to_T_C in its tyConCC field, and that the classTyCon of C_CC is T_C_CC.

Converting instance declarations

If we encounter an instance declaration for C tau during conversion, there are two alternatives: we have a conversion for C or not:

  • if we do not have a conversion, we generate an instance (and hence dfun) for C tau^, where tau^ is the closure converted tau;
  • if we have a conversion, we generate an instance for C_CC tau^.

In any case, we add a field is_CC :: StatusCC Instance to InstEnv.Instance that contains the additionally generated instance. And in both cases, we should be able to derive the required code for the dfun from the definition of C tau. We also make sure that the dfun's idCC field (see below) is set to that of the converted dfun.

Converting type terms

We determine the converted type t^ of t as follows:

T^            = T_CC , if available
                T    , otherwise
a^            = a
(t1 t2)^      = t1^ t2^
(t1 -> t2)^   = Clo t1 t2
(forall a.t)^ = forall a.t^
(C t1 => t2)^ = C_CC t1^ => t2^ , if available
                C t1^ => t2^    , otherwise

Converting value bindings

When converting a toplevel binding for f :: t, we generate f_CC :: t^. The alternatives GlobalId and LocalId of Var.Var get a new field idCC :: StatusCC Id whose values, for a declaration f, we determine as follows:

  • If Id's declaration uses any features that we cannot (or currently, don't want to) convert, set idCC to NoCC.
  • If all type constructors involved in f's type are marked NoCC or AsIsCC, we set f's idCC field to AsIsCC.
  • Otherwise, convert f and set its ifCC field to ConvCC f_CC.

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 Ids 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.

Issues, aka rl's complaints

Non-converted versus unchanged type declarations

This issue has now been addressed.

Many type declarations will not be changed by conversion, as they do not contain any arrows. Hence, it is more economic to avoid generating a _CC version of these declarations. I initially thought that we can ignore this for a moment, because it is only an optimisation. However, consider

data T = MkT Int
data S = MkS (Int -> Int)

As we don't convert Int, we cannot convert T and S, which is a shame as their conversion is simple and (in the vectorisation case may affect performance dramatically). As a matter of fact, if we identify declarations that need not be converted, then we would mark Int and T as such and can convert S easily.

FC Coercions

This issue has now been addressed.

Closure conversion happens on Core, which means that constructors, such as MkT of

-- unconverted
newtype T a = MkT (a -> a)

in a definition

-- converted
foo :: (a -> a) -> T a
foo f = MkT f

have vanished, leaving only a coercion. As T is not converted, we need to notice that we need to generate MkT (fr f). So, we need to spot the conversion representing MkT.

Generally, we need a story about treating coercions during conversion.

Function type constructor

It is clear how to treat types involving subtypes of the form a -> b. It is less clear how to deal with partial applications of (->). Consider

-- unconverted
data T f = T (Int -> Int) (f Int Int)

used as T (->) in converted code. What is convert (T (->))?


It might be sufficient to never convert class declarations as a whole, but only their representation types.

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.