simplCore/CSE.lhs

% % (c) The AQUA Project, Glasgow University, 1993-1998 % \section{Common subexpression} \begin{code}

    module CSE (cseProgram) where

    #include "HsVersions.h"

    import CoreSubst
    import Var              ( Var )
    import Id               ( Id, idType, idInlineActivation, zapIdOccInfo )
    import CoreUtils        ( mkAltExpr
                            , exprIsTrivial)
    import Type             ( tyConAppArgs )
    import CoreSyn
    import Outputable
    import BasicTypes       ( isAlwaysActive )
    import TrieMap

    import Data.List

\end{code} Simple common sub-expression ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we see x1 = C a b x2 = C x1 b we build up a reverse mapping: C a b -> x1 C x1 b -> x2 and apply that to the rest of the program. When we then see y1 = C a b y2 = C y1 b we replace the C a b with x1. But then we *dont* want to add x1 -> y1 to the mapping. Rather, we want the reverse, y1 -> x1 so that a subsequent binding y2 = C y1 b will get transformed to C x1 b, and then to x2. So we carry an extra var->var substitution which we apply *before* looking up in the reverse mapping. Note [Shadowing] ~~~~~~~~~~~~~~~~ We have to be careful about shadowing. For example, consider f = \x -> let y = x+x in h = \x -> x+x in ... Here we must *not* do CSE on the inner x+x! The simplifier used to guarantee no shadowing, but it doesn't any more (it proved too hard), so we clone as we go. We can simply add clones to the substitution already described. Note [Case binders 1] ~~~~~~~~~~~~~~~~~~~~~~ Consider f = \x -> case x of wild { (a:as) -> case a of wild1 { (p,q) -> ...(wild1:as)... Here, (wild1:as) is morally the same as (a:as) and hence equal to wild. But that's not quite obvious. In general we want to keep it as (wild1:as), but for CSE purpose that's a bad idea. So we add the binding (wild1 -> a) to the extra var->var mapping. Notice this is exactly backwards to what the simplifier does, which is to try to replaces uses of 'a' with uses of 'wild1' Note [Case binders 2] ~~~~~~~~~~~~~~~~~~~~~~ Consider case (h x) of y -> ...(h x)... We'd like to replace (h x) in the alternative, by y. But because of the preceding [Note: case binders 1], we only want to add the mapping scrutinee -> case binder to the reverse CSE mapping if the scrutinee is a non-trivial expression. (If the scrutinee is a simple variable we want to add the mapping case binder -> scrutinee to the substitution Note [CSE for INLINE and NOINLINE] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We are careful to do no CSE inside functions that the user has marked as INLINE or NOINLINE. In terms of Core, that means a) we do not do CSE inside an InlineRule b) we do not do CSE on the RHS of a binding b=e unless b's InlinePragma is AlwaysActive Here's why (examples from Roman Leshchinskiy). Consider yes :: Int {-# NOINLINE yes #-} yes = undefined no :: Int {-# NOINLINE no #-} no = undefined foo :: Int -> Int -> Int {-# NOINLINE foo #-} foo m n = n {-# RULES "foo/no" foo no = id #-} bar :: Int -> Int bar = foo yes We do not expect the rule to fire. But if we do CSE, then we get yes=no, and the rule does fire. Worse, whether we get yes=no or no=yes depends on the order of the definitions. In general, CSE should probably never touch things with INLINE pragmas as this could lead to surprising results. Consider {-# INLINE foo #-} foo = {-# NOINLINE bar #-} bar = -- Same rhs as foo If CSE produces foo = bar then foo will never be inlined (when it should be); but if it produces bar = foo bar will be inlined (when it should not be). Even if we remove INLINE foo, we'd still like foo to be inlined if rhs is small. This won't happen with foo = bar. Not CSE-ing inside INLINE also solves an annoying bug in CSE. Consider a worker/wrapper, in which the worker has turned into a single variable: $wf = h f = \x -> ...$wf... Now CSE may transform to f = \x -> ...h... But the WorkerInfo for f still says $wf, which is now dead! This won't happen now that we don't look inside INLINEs (which wrappers are). Note [CSE for case expressions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider case f x of y { pat -> ...let y = f x in ... } Then we can CSE the inner (f x) to y. In fact 'case' is like a strict let-binding, and we can use cseRhs for dealing with the scrutinee. %************************************************************************ %* * \section{Common subexpression} %* * %************************************************************************ \begin{code}

    cseProgram :: CoreProgram -> CoreProgram
    cseProgram binds = cseBinds emptyCSEnv binds

    cseBinds :: CSEnv -> [CoreBind] -> [CoreBind]
    cseBinds _   []     = []
    cseBinds env (b:bs) = (b':bs')
                        where
                          (env1, b') = cseBind  env  b
                          bs'        = cseBinds env1 bs

    cseBind :: CSEnv -> CoreBind -> (CSEnv, CoreBind)
    cseBind env (NonRec b e)
      = (env2, NonRec b' e')
      where
        (env1, b') = addBinder env b
        (env2, e') = cseRhs env1 (b',e)

    cseBind env (Rec pairs)
      = (env2, Rec (bs' `zip` es'))
      where
        (bs,es) = unzip pairs
        (env1, bs') = addRecBinders env bs
        (env2, es') = mapAccumL cseRhs env1 (bs' `zip` es)

    cseRhs :: CSEnv -> (OutBndr, InExpr) -> (CSEnv, OutExpr)
    cseRhs env (id',rhs)
      = case lookupCSEnv env rhs' of
            Nothing -> (extendCSEnv env rhs' id', rhs')
            Just id -> (extendCSSubst env id' id, Var id)
              -- In the Just case, we have
              --        x = rhs
              --        ...
              --        x' = rhs
              -- We are replacing the second binding with x'=x
              -- and so must record that in the substitution so
              -- that subsequent uses of x' are replaced with x,
              -- See Trac #5996
      where
        rhs' | isAlwaysActive (idInlineActivation id') = cseExpr env rhs
             | otherwise                               = rhs
                    -- See Note [CSE for INLINE and NOINLINE]

    tryForCSE :: CSEnv -> InExpr -> OutExpr
    tryForCSE env expr
      | exprIsTrivial expr'                   = expr'       -- No point
      | Just smaller <- lookupCSEnv env expr' = Var smaller
      | otherwise                             = expr'
      where
        expr' = cseExpr env expr

    cseExpr :: CSEnv -> InExpr -> OutExpr
    cseExpr env (Type t)               = Type (substTy (csEnvSubst env) t)
    cseExpr env (Coercion c)           = Coercion (substCo (csEnvSubst env) c)
    cseExpr _   (Lit lit)              = Lit lit
    cseExpr env (Var v)                = lookupSubst env v
    cseExpr env (App f a)              = App (cseExpr env f) (tryForCSE env a)
    cseExpr env (Tick t e)             = Tick t (cseExpr env e)
    cseExpr env (Cast e co)            = Cast (cseExpr env e) (substCo (csEnvSubst env) co)
    cseExpr env (Lam b e)              = let (env', b') = addBinder env b
                                         in Lam b' (cseExpr env' e)
    cseExpr env (Let bind e)           = let (env', bind') = cseBind env bind
                                         in Let bind' (cseExpr env' e)
    cseExpr env (Case scrut bndr ty alts) = Case scrut' bndr'' ty alts'
                              where
                                    alts' = cseAlts env2 scrut' bndr bndr'' alts
                                    (env1, bndr') = addBinder env bndr
                                    bndr'' = zapIdOccInfo bndr'
                                    -- The swizzling from Note [Case binders 2] may
                                    -- cause a dead case binder to be alive, so we
                                    -- play safe here and bring them all to life
                                    (env2, scrut') = cseRhs env1 (bndr'', scrut)
                                    -- Note [CSE for case expressions]

    cseAlts :: CSEnv -> OutExpr -> InBndr -> InBndr -> [InAlt] -> [OutAlt]

    cseAlts env scrut' bndr bndr' alts
      = map cse_alt alts
      where
        (con_target, alt_env)
            = case scrut' of
                Var v' -> (v',     extendCSSubst env bndr v')    -- See Note [Case binders 1]
                                                                 -- map: bndr -> v'

                _      ->  (bndr', extendCSEnv env scrut' bndr') -- See Note [Case binders 2]
                                                                 -- map: scrut' -> bndr'

        arg_tys = tyConAppArgs (idType bndr)

        cse_alt (DataAlt con, args, rhs)
            | not (null args)
                    -- Don't try CSE if there are no args; it just increases the number
                    -- of live vars.  E.g.
                    --      case x of { True -> ....True.... }
                    -- Don't replace True by x!
                    -- Hence the 'null args', which also deal with literals and DEFAULT
            = (DataAlt con, args', tryForCSE new_env rhs)
            where
              (env', args') = addBinders alt_env args
              new_env       = extendCSEnv env' (mkAltExpr (DataAlt con) args' arg_tys)
                                               con_target

        cse_alt (con, args, rhs)
            = (con, args', tryForCSE env' rhs)
            where
              (env', args') = addBinders alt_env args

\end{code} %************************************************************************ %* * \section{The CSE envt} %* * %************************************************************************ \begin{code}

    type InExpr  = CoreExpr         -- Pre-cloning
    type InBndr  = CoreBndr
    type InAlt   = CoreAlt

    type OutExpr  = CoreExpr        -- Post-cloning
    type OutBndr  = CoreBndr
    type OutAlt   = CoreAlt

    data CSEnv  = CS { cs_map    :: CoreMap (OutExpr, Id)   -- Key, value
                     , cs_subst  :: Subst }

    emptyCSEnv :: CSEnv
    emptyCSEnv = CS { cs_map = emptyCoreMap, cs_subst = emptySubst }

    lookupCSEnv :: CSEnv -> OutExpr -> Maybe Id
    lookupCSEnv (CS { cs_map = csmap }) expr
      = case lookupCoreMap csmap expr of
          Just (_,e) -> Just e
          Nothing    -> Nothing

    extendCSEnv :: CSEnv -> OutExpr -> Id -> CSEnv
    extendCSEnv cse expr id
      = cse { cs_map = extendCoreMap (cs_map cse) expr (expr,id) }

    csEnvSubst :: CSEnv -> Subst
    csEnvSubst = cs_subst

    lookupSubst :: CSEnv -> Id -> OutExpr
    lookupSubst (CS { cs_subst = sub}) x = lookupIdSubst (text "CSE.lookupSubst") sub x

    extendCSSubst :: CSEnv -> Id  -> Id -> CSEnv
    extendCSSubst cse x y = cse { cs_subst = extendIdSubst (cs_subst cse) x (Var y) }

    addBinder :: CSEnv -> Var -> (CSEnv, Var)
    addBinder cse v = (cse { cs_subst = sub' }, v')
                    where
                      (sub', v') = substBndr (cs_subst cse) v

    addBinders :: CSEnv -> [Var] -> (CSEnv, [Var])
    addBinders cse vs = (cse { cs_subst = sub' }, vs')
                    where
                      (sub', vs') = substBndrs (cs_subst cse) vs

    addRecBinders :: CSEnv -> [Id] -> (CSEnv, [Id])
    addRecBinders cse vs = (cse { cs_subst = sub' }, vs')
                    where
                      (sub', vs') = substRecBndrs (cs_subst cse) vs

\end{code}