path: root/src/Simplify.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators #-}
module Simplify (
  simplifyN, simplifyFix,
  SimplifyConfig(..), simplifyWith, simplifyFixWith,
) where

import Data.Function (fix)
import Data.Monoid (Any(..))
import Data.Type.Equality (testEquality)

import AST
import AST.Count
import CHAD.Types
import Data


-- | This has no fields now, hence this type is useless as-is. When debugging, however, it's useful to be able to add some.
data SimplifyConfig = SimplifyConfig

defaultSimplifyConfig :: SimplifyConfig
defaultSimplifyConfig = SimplifyConfig

simplifyN :: KnownEnv env => Int -> Ex env t -> Ex env t
simplifyN 0 = id
simplifyN n = simplifyN (n - 1) . simplify

simplify :: forall env t. KnownEnv env => Ex env t -> Ex env t
simplify =
  let ?accumInScope = checkAccumInScope @env knownEnv
      ?config = defaultSimplifyConfig
  in snd . simplify'

simplifyWith :: forall env t. KnownEnv env => SimplifyConfig -> Ex env t -> Ex env t
simplifyWith config =
  let ?accumInScope = checkAccumInScope @env knownEnv
      ?config = config
  in snd . simplify'

simplifyFix :: forall env t. KnownEnv env => Ex env t -> Ex env t
simplifyFix = simplifyFixWith defaultSimplifyConfig

simplifyFixWith :: forall env t. KnownEnv env => SimplifyConfig -> Ex env t -> Ex env t
simplifyFixWith config =
  let ?accumInScope = checkAccumInScope @env knownEnv
      ?config = config
  in fix $ \loop e ->
            let (Any act, e') = simplify' e
            in if act then loop e' else e'

simplify' :: (?accumInScope :: Bool, ?config :: SimplifyConfig) => Ex env t -> (Any, Ex env t)
simplify' = \case
  -- inlining
  ELet _ rhs body
    | cheapExpr rhs
    -> acted $ simplify' (substInline rhs body)

    | Occ lexOcc runOcc <- occCount IZ body
    , ((not ?accumInScope || not (hasAdds rhs)) && lexOcc <= One && runOcc <= One)  -- without effects, normal rules apply
          || (lexOcc == One && runOcc == One)  -- with effects, linear inlining is still allowed, but weakening is not
    -> acted $ simplify' (substInline rhs body)

  -- let splitting / let peeling
  ELet _ (EPair _ a b) body ->
    acted $ simplify' $
      ELet ext a $
      ELet ext (weakenExpr WSink b) $
        subst (\_ t -> \case IZ -> EPair ext (EVar ext (typeOf a) (IS IZ)) (EVar ext (typeOf b) IZ)
                             IS i -> EVar ext t (IS (IS i)))
              body
  ELet _ (EJust _ a) body ->
    acted $ simplify' $ ELet ext a $ subst0 (EJust ext (EVar ext (typeOf a) IZ)) body
  ELet _ (EInl _ t2 a) body ->
    acted $ simplify' $ ELet ext a $ subst0 (EInl ext t2 (EVar ext (typeOf a) IZ)) body
  ELet _ (EInr _ t1 a) body ->
    acted $ simplify' $ ELet ext a $ subst0 (EInr ext t1 (EVar ext (typeOf a) IZ)) body

  -- let rotation
  ELet _ (ELet _ rhs a) b ->
    acted $ simplify' $
      ELet ext rhs $
      ELet ext a $
        weakenExpr (WCopy WSink) (snd (simplify' b))

  -- beta rules for products
  EFst _ (EPair _ e e')
    | not (hasAdds e') -> acted $ simplify' e
    | otherwise -> acted $ simplify' $ ELet ext e' (weakenExpr WSink e)
  ESnd _ (EPair _ e' e)
    | not (hasAdds e') -> acted $ simplify' e
    | otherwise -> acted $ simplify' $ ELet ext e' (weakenExpr WSink e)

  -- beta rules for coproducts
  ECase _ (EInl _ _ e) rhs _ -> acted $ simplify' (ELet ext e rhs)
  ECase _ (EInr _ _ e) _ rhs -> acted $ simplify' (ELet ext e rhs)

  -- beta rules for maybe
  EMaybe _ e1 _ ENothing{} -> acted $ simplify' e1
  EMaybe _ _ e1 (EJust _ e2) -> acted $ simplify' $ ELet ext e2 e1

  -- let floating
  EFst _ (ELet _ rhs body) -> acted $ simplify' (ELet ext rhs (EFst ext body))
  ESnd _ (ELet _ rhs body) -> acted $ simplify' (ELet ext rhs (ESnd ext body))
  ECase _ (ELet _ rhs body) e1 e2 -> acted $ simplify' (ELet ext rhs (ECase ext body (weakenExpr (WCopy WSink) e1) (weakenExpr (WCopy WSink) e2)))
  EIdx0 _ (ELet _ rhs body) -> acted $ simplify' (ELet ext rhs (EIdx0 ext body))
  EIdx1 _ (ELet _ rhs body) e -> acted $ simplify' (ELet ext rhs (EIdx1 ext body (weakenExpr WSink e)))
  EAccum _ t p e1 (ELet _ rhs body) acc ->
    acted $ simplify' $
      ELet ext rhs $
        EAccum ext t p (weakenExpr WSink e1) body (weakenExpr WSink acc)

  -- let () = e in ()  ~>  e
  ELet _ e1 (ENil _) | STNil <- typeOf e1 ->
    acted $ simplify' e1

  -- projection down-commuting
  EFst _ (ECase _ e1 e2 e3) ->
    acted $ simplify' $
      ECase ext e1 (EFst ext e2) (EFst ext e3)
  ESnd _ (ECase _ e1 e2 e3) ->
    acted $ simplify' $
      ECase ext e1 (ESnd ext e2) (ESnd ext e3)

  -- TODO: more array indexing
  EIdx _ (EReplicate1Inner _ _ e2) e3 -> acted $ simplify' $ EIdx ext e2 (EFst ext e3)
  EIdx _ (EUnit _ e1) _ -> acted $ simplify' $ e1

  -- TODO: more constant folding
  EOp _ OIf (EConst _ STBool True) -> (Any True, EInl ext STNil (ENil ext))
  EOp _ OIf (EConst _ STBool False) -> (Any True, EInr ext STNil (ENil ext))

  -- inline cheap array constructors
  ELet _ (EReplicate1Inner _ e1 e2) e3 ->
    acted $ simplify' $
      ELet ext (EPair ext e1 e2) $
        let v = EVar ext (STPair tIx (typeOf e2)) IZ
        in subst0 (EReplicate1Inner ext (EFst ext v) (ESnd ext v)) e3
  -- -- TODO: This is a bad idea and anyway only helps in practice if (!) is
  -- -- cheap, which it can't be because (!) is not cheap if you do AD after.
  -- -- Should do proper SoA representation.
  -- ELet _ (EBuild _ n e1 e2) e3 | cheapExpr e2 ->
  --   acted $ simplify' $
  --     ELet ext e1 $
  --       subst0 (EBuild ext n (EVar ext (tTup (sreplicate n tIx)) IZ) (weakenExpr (WCopy WSink) e2)) e3

  -- eta rule for unit
  e | STNil <- typeOf e, not ?accumInScope || not (hasAdds e) ->
    case e of
      ENil _ -> (Any False, e)
      _      -> (Any True, ENil ext)

  EBuild _ SZ _ e ->
    acted $ simplify' $ EUnit ext (substInline (ENil ext) e)

  -- monoid rules
  EAccum _ t p e1 e2 acc -> do
    e1' <- simplify' e1
    e2' <- simplify' e2
    acc' <- simplify' acc
    simplifyOneHotTerm (OneHotTerm t p e1' e2')
      (Any True, ENil ext)
      (\e -> (Any False, EAccum ext t SAPHere (ENil ext) e acc'))
      (\(OneHotTerm t' p' e1'' e2'') -> return (EAccum ext t' p' e1'' e2'' acc'))
  EPlus _ _ (EZero _ _) e -> acted $ simplify' e
  EPlus _ _ e (EZero _ _) -> acted $ simplify' e
  EOneHot _ t p e1 e2 -> do
    e1' <- simplify' e1
    e2' <- simplify' e2
    simplifyOneHotTerm (OneHotTerm t p e1' e2')
      (Any True, EZero ext t)
      (\e -> (Any True, e))
      (\(OneHotTerm t' p' e1'' e2'') -> return (EOneHot ext t' p' e1'' e2''))

  -- type-specific equations for plus
  EPlus _ STNil _ _ -> (Any True, ENil ext)

  EPlus _ (STPair t1 t2) (EJust _ (EPair _ a1 b1)) (EJust _ (EPair _ a2 b2)) ->
    acted $ simplify' $ EJust ext (EPair ext (EPlus ext t1 a1 a2) (EPlus ext t2 b1 b2))
  EPlus _ STPair{} ENothing{} e -> acted $ simplify' e
  EPlus _ STPair{} e ENothing{} -> acted $ simplify' e

  EPlus _ (STEither t1 _) (EJust _ (EInl _ dt2 a1)) (EJust _ (EInl _ _ a2)) ->
    acted $ simplify' $ EJust ext (EInl ext dt2 (EPlus ext t1 a1 a2))
  EPlus _ (STEither _ t2) (EJust _ (EInr _ dt1 b1)) (EJust _ (EInr _ _ b2)) ->
    acted $ simplify' $ EJust ext (EInr ext dt1 (EPlus ext t2 b1 b2))
  EPlus _ STEither{} ENothing{} e -> acted $ simplify' e
  EPlus _ STEither{} e ENothing{} -> acted $ simplify' e

  EPlus _ (STMaybe t) (EJust _ e1) (EJust _ e2) ->
    acted $ simplify' $ EJust ext (EPlus ext t e1 e2)
  EPlus _ STMaybe{} ENothing{} e -> acted $ simplify' e
  EPlus _ STMaybe{} e ENothing{} -> acted $ simplify' e

  -- fallback recursion
  EVar _ t i -> pure $ EVar ext t i
  ELet _ a b -> ELet ext <$> simplify' a <*> simplify' b
  EPair _ a b -> EPair ext <$> simplify' a <*> simplify' b
  EFst _ e -> EFst ext <$> simplify' e
  ESnd _ e -> ESnd ext <$> simplify' e
  ENil _ -> pure $ ENil ext
  EInl _ t e -> EInl ext t <$> simplify' e
  EInr _ t e -> EInr ext t <$> simplify' e
  ECase _ e a b -> ECase ext <$> simplify' e <*> simplify' a <*> simplify' b
  ENothing _ t -> pure $ ENothing ext t
  EJust _ e -> EJust ext <$> simplify' e
  EMaybe _ a b e -> EMaybe ext <$> simplify' a <*> simplify' b <*> simplify' e
  EConstArr _ n t v -> pure $ EConstArr ext n t v
  EBuild _ n a b -> EBuild ext n <$> simplify' a <*> simplify' b
  EFold1Inner _ cm a b c -> EFold1Inner ext cm <$> simplify' a <*> simplify' b <*> simplify' c
  ESum1Inner _ e -> ESum1Inner ext <$> simplify' e
  EUnit _ e -> EUnit ext <$> simplify' e
  EReplicate1Inner _ a b -> EReplicate1Inner ext <$> simplify' a <*> simplify' b
  EMaximum1Inner _ e -> EMaximum1Inner ext <$> simplify' e
  EMinimum1Inner _ e -> EMinimum1Inner ext <$> simplify' e
  EConst _ t v -> pure $ EConst ext t v
  EIdx0 _ e -> EIdx0 ext <$> simplify' e
  EIdx1 _ a b -> EIdx1 ext <$> simplify' a <*> simplify' b
  EIdx _ a b -> EIdx ext <$> simplify' a <*> simplify' b
  EShape _ e -> EShape ext <$> simplify' e
  EOp _ op e -> EOp ext op <$> simplify' e
  ECustom _ s t p a b c e1 e2 ->
    ECustom ext s t p
      <$> (let ?accumInScope = False in simplify' a)
      <*> (let ?accumInScope = False in simplify' b)
      <*> (let ?accumInScope = False in simplify' c)
      <*> simplify' e1 <*> simplify' e2
  EWith _ t e1 e2 -> EWith ext t <$> simplify' e1 <*> (let ?accumInScope = True in simplify' e2)
  EZero _ t -> pure $ EZero ext t
  EPlus _ t a b -> EPlus ext t <$> simplify' a <*> simplify' b
  EError _ t s -> pure $ EError ext t s

acted :: (Any, a) -> (Any, a)
acted (_, x) = (Any True, x)

cheapExpr :: Expr x env t -> Bool
cheapExpr = \case
  EVar{} -> True
  ENil{} -> True
  EConst{} -> True
  EFst _ e -> cheapExpr e
  ESnd _ e -> cheapExpr e
  EUnit _ e -> cheapExpr e
  _ -> False

-- | This can be made more precise by tracking (and not counting) adds on
-- locally eliminated accumulators.
hasAdds :: Expr x env t -> Bool
hasAdds = \case
  EVar _ _ _ -> False
  ELet _ rhs body -> hasAdds rhs || hasAdds body
  EPair _ a b -> hasAdds a || hasAdds b
  EFst _ e -> hasAdds e
  ESnd _ e -> hasAdds e
  ENil _ -> False
  EInl _ _ e -> hasAdds e
  EInr _ _ e -> hasAdds e
  ECase _ e a b -> hasAdds e || hasAdds a || hasAdds b
  ENothing _ _ -> False
  EJust _ e -> hasAdds e
  EMaybe _ a b e -> hasAdds a || hasAdds b || hasAdds e
  EConstArr _ _ _ _ -> False
  EBuild _ _ a b -> hasAdds a || hasAdds b
  EFold1Inner _ _ a b c -> hasAdds a || hasAdds b || hasAdds c
  ESum1Inner _ e -> hasAdds e
  EUnit _ e -> hasAdds e
  EReplicate1Inner _ a b -> hasAdds a || hasAdds b
  EMaximum1Inner _ e -> hasAdds e
  EMinimum1Inner _ e -> hasAdds e
  ECustom _ _ _ _ a b c d e -> hasAdds a || hasAdds b || hasAdds c || hasAdds d || hasAdds e
  EConst _ _ _ -> False
  EIdx0 _ e -> hasAdds e
  EIdx1 _ a b -> hasAdds a || hasAdds b
  EIdx _ a b -> hasAdds a || hasAdds b
  EShape _ e -> hasAdds e
  EOp _ _ e -> hasAdds e
  EWith _ _ a b -> hasAdds a || hasAdds b
  EAccum _ _ _ _ _ _ -> True
  EZero _ _ -> False
  EPlus _ _ a b -> hasAdds a || hasAdds b
  EOneHot _ _ _ a b -> hasAdds a || hasAdds b
  EError _ _ _ -> False

checkAccumInScope :: SList STy env -> Bool
checkAccumInScope = \case SNil -> False
                          SCons t env -> check t || checkAccumInScope env
  where
    check :: STy t -> Bool
    check STNil = False
    check (STPair s t) = check s || check t
    check (STEither s t) = check s || check t
    check (STMaybe t) = check t
    check (STArr _ t) = check t
    check (STScal _) = False
    check STAccum{} = True

data OneHotTerm env p a b where
  OneHotTerm :: STy a -> SAcPrj p a b -> Ex env (AcIdx p a) -> Ex env (D2 b) -> OneHotTerm env p a b
deriving instance Show (OneHotTerm env p a b)

simplifyOneHotTerm :: OneHotTerm env p a b
                   -> (Any, r)  -- ^ Zero case (onehot is actually zero)
                   -> (Ex env (D2 a) -> (Any, r))  -- ^ Trivial case (no zeros in onehot)
                   -> (forall p' b'. OneHotTerm env p' a b' -> (Any, r))
                   -> (Any, r)
simplifyOneHotTerm (OneHotTerm _ _ _ (EZero _ _)) kzero _ _ = kzero

simplifyOneHotTerm (OneHotTerm t1 prj1 idx1 (EOneHot _ t2 prj2 idx2 val)) kzero ktriv k
  | Just Refl <- testEquality (acPrjTy prj1 t1) t2
  = do (Any True, ())  -- record, whatever happens later, that we've modified something
       concatOneHots t1 prj1 idx1 prj2 idx2 $ \prj12 idx12 ->
         simplifyOneHotTerm (OneHotTerm t1 prj12 idx12 val) kzero ktriv k

simplifyOneHotTerm (OneHotTerm t SAPHere idx e) kzero ktriv k = case (t, e) of
  (STNil, _) -> kzero

  (STPair{}, ENothing _ _) -> kzero
  (STPair{}, EJust _ (EPair _ e1 EZero{})) ->
    simplifyOneHotTerm (OneHotTerm t (SAPFst SAPHere) idx e1) kzero ktriv k
  (STPair{}, EJust _ (EPair _ EZero{} e2)) ->
    simplifyOneHotTerm (OneHotTerm t (SAPSnd SAPHere) idx e2) kzero ktriv k

  (STEither{}, ENothing _ _) -> kzero
  (STEither{}, EJust _ (EInl _ _ e1)) ->
    simplifyOneHotTerm (OneHotTerm t (SAPLeft SAPHere) idx e1) kzero ktriv k
  (STEither{}, EJust _ (EInr _ _ e2)) ->
    simplifyOneHotTerm (OneHotTerm t (SAPRight SAPHere) idx e2) kzero ktriv k

  (STMaybe{}, ENothing _ _) -> kzero
  (STMaybe{}, EJust _ e1) ->
    simplifyOneHotTerm (OneHotTerm t (SAPJust SAPHere) idx e1) kzero ktriv k

  (STArr{}, ENothing _ _) -> kzero

  (STScal STI32, _) -> kzero
  (STScal STI64, _) -> kzero
  (STScal STF32, EConst _ _ 0.0) -> kzero
  (STScal STF64, EConst _ _ 0.0) -> kzero
  (STScal STBool, _) -> kzero

  _ -> ktriv e

simplifyOneHotTerm term _ _ k = k term

concatOneHots :: STy a
              -> SAcPrj p1 a b -> Ex env (AcIdx p1 a)
              -> SAcPrj p2 b c -> Ex env (AcIdx p2 b)
              -> (forall p12. SAcPrj p12 a c -> Ex env (AcIdx p12 a) -> r) -> r
concatOneHots t1 prj1 idx1 prj2 idx2 k = case (t1, prj1) of
  (_, SAPHere) -> k prj2 idx2

  (STPair a _, SAPFst prj1') ->
    concatOneHots a prj1' idx1 prj2 idx2 $ \prj12 idx12 -> k (SAPFst prj12) idx12
  (STPair _ b, SAPSnd prj1') ->
    concatOneHots b prj1' idx1 prj2 idx2 $ \prj12 idx12 -> k (SAPSnd prj12) idx12

  (STEither a _, SAPLeft prj1') ->
    concatOneHots a prj1' idx1 prj2 idx2 $ \prj12 idx12 -> k (SAPLeft prj12) idx12
  (STEither _ b, SAPRight prj1') ->
    concatOneHots b prj1' idx1 prj2 idx2 $ \prj12 idx12 -> k (SAPRight prj12) idx12

  (STMaybe a, SAPJust prj1') ->
    concatOneHots a prj1' idx1 prj2 idx2 $ \prj12 idx12 -> k (SAPJust prj12) idx12

  (STArr n a, SAPArrIdx prj1' _) ->
    concatOneHots a prj1' (ESnd ext (EVar ext (typeOf idx1) IZ)) prj2 (weakenExpr WSink idx2) $ \prj12 idx12 ->
      k (SAPArrIdx prj12 n) (ELet ext idx1 $ EPair ext (EFst ext (EVar ext (typeOf idx1) IZ)) idx12)