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% (c) The AQUA Project, Glasgow University, 1994-1998
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\section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop}
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module LiberateCase ( liberateCase ) where
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#include "HsVersions.h"
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import CoreUnfold ( couldBeSmallEnoughToInline )
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import Util ( notNull )
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The liberate-case transformation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This module walks over @Core@, and looks for @case@ on free variables.
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if there is case on a free on the route to the recursive call,
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then the recursive call is replaced with an unfolding.
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=> the inner f is replaced.
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(note the NEED for shadowing)
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Better code, because 'a' is free inside the inner letrec, rather
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than needing projection from v.
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Note that this deals with *free variables*. SpecConstr deals with
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*arguments* that are of known form. E.g.
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Note [Scrutinee with cast]
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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f = \ t -> case (v `cast` co) of
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Exactly the same optimisation (unrolling one call to f) will work here,
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despite the cast. See mk_alt_env in the Case branch of libCase.
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Note [Only functions!]
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~~~~~~~~~~~~~~~~~~~~~~
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Consider the following code
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f = g (case v of V a b -> a : t f)
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where g is expensive. If we aren't careful, liberate case will turn this into
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V a b -> a : t (letrec f = g (case v of V a b -> a : f t)
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Yikes! We evaluate g twice. This leads to a O(2^n) explosion
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if g calls back to the same code recursively.
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Solution: make sure that we only do the liberate-case thing on *functions*
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To think about (Apr 94)
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Main worry: duplicating code excessively. At the moment we duplicate
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the entire binding group once at each recursive call. But there may
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be a group of recursive calls which share a common set of evaluated
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free variables, in which case the duplication is a plain waste.
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Another thing we could consider adding is some unfold-threshold thing,
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so that we'll only duplicate if the size of the group rhss isn't too
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The ``level'' of a binder tells how many
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recursive defns lexically enclose the binding
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A recursive defn "encloses" its RHS, not its
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letrec f = let g = ... in ...
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Here, the level of @f@ is zero, the level of @g@ is one,
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and the level of @h@ is zero (NB not one).
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%************************************************************************
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%************************************************************************
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liberateCase :: DynFlags -> [CoreBind] -> [CoreBind]
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liberateCase dflags binds = do_prog (initEnv dflags) binds
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do_prog env (bind:binds) = bind' : do_prog env' binds
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(env', bind') = libCaseBind env bind
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%************************************************************************
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%************************************************************************
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libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
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libCaseBind env (NonRec binder rhs)
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= (addBinders env [binder], NonRec binder (libCase env rhs))
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libCaseBind env (Rec pairs)
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= (env_body, Rec pairs')
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binders = map fst pairs
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env_body = addBinders env binders
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pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
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-- We extend the rec-env by binding each Id to its rhs, first
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-- processing the rhs with an *un-extended* environment, so
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-- that the same process doesn't occur for ever!
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env_rhs = addRecBinds env [ (localiseId binder, libCase env_body rhs)
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| (binder, rhs) <- pairs
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, rhs_small_enough binder rhs ]
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-- localiseID : see Note [Need to localiseId in libCaseBind]
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rhs_small_enough id rhs -- Note [Small enough]
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= idArity id > 0 -- Note [Only functions!]
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&& maybe True (\size -> couldBeSmallEnoughToInline size rhs)
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Note [Need to localiseId in libCaseBind]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The call to localiseId is needed for two subtle reasons
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(a) Reset the export flags on the binders so
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that we don't get name clashes on exported things if the
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local binding floats out to top level. This is most unlikely
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to happen, since the whole point concerns free variables.
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But resetting the export flag is right regardless.
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(b) Make the name an Internal one. External Names should never be
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nested; if it were floated to the top level, we'd get a name
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clash at code generation time.
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f = \x. BIG...(case fv of { (a,b) -> ...g.. })...
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Then we *can* do liberate-case on g (small RHS) but not for f (too big).
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But we can choose on a item-by-item basis, and that's what the
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rhs_small_enough call in the comprehension for env_rhs does.
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libCase :: LibCaseEnv
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libCase env (Var v) = libCaseId env v
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libCase _ (Lit lit) = Lit lit
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libCase _ (Type ty) = Type ty
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libCase env (App fun arg) = App (libCase env fun) (libCase env arg)
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libCase env (Note note body) = Note note (libCase env body)
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libCase env (Cast e co) = Cast (libCase env e) co
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libCase env (Lam binder body)
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= Lam binder (libCase (addBinders env [binder]) body)
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libCase env (Let bind body)
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= Let bind' (libCase env_body body)
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(env_body, bind') = libCaseBind env bind
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libCase env (Case scrut bndr ty alts)
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= Case (libCase env scrut) bndr ty (map (libCaseAlt env_alts) alts)
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env_alts = addBinders (mk_alt_env scrut) [bndr]
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mk_alt_env (Var scrut_var) = addScrutedVar env scrut_var
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mk_alt_env (Cast scrut _) = mk_alt_env scrut -- Note [Scrutinee with cast]
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libCaseAlt :: LibCaseEnv -> (AltCon, [CoreBndr], CoreExpr)
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-> (AltCon, [CoreBndr], CoreExpr)
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libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
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libCaseId :: LibCaseEnv -> Id -> CoreExpr
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| Just the_bind <- lookupRecId env v -- It's a use of a recursive thing
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, notNull free_scruts -- with free vars scrutinised in RHS
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= Let the_bind (Var v)
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rec_id_level = lookupLevel env v
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free_scruts = freeScruts env rec_id_level
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freeScruts :: LibCaseEnv
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-> LibCaseLevel -- Level of the recursive Id
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-> [Id] -- Ids that are scrutinised between the binding
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-- of the recursive Id and here
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freeScruts env rec_bind_lvl
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= [v | (v, scrut_bind_lvl, scrut_at_lvl) <- lc_scruts env
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, scrut_bind_lvl <= rec_bind_lvl
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, scrut_at_lvl > rec_bind_lvl]
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-- Note [When to specialise]
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-- Note [Avoiding fruitless liberate-case]
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Note [When to specialise]
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~~~~~~~~~~~~~~~~~~~~~~~~~
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f = \x. letrec g = \y. case x of
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True -> ... (f a) ...
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False -> ... (g b) ...
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We get the following levels
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Then 'x' is being scrutinised at a deeper level than its binding, so
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it's added to lc_sruts: [(x,1)]
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We do *not* want to specialise the call to 'f', becuase 'x' is not free
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in 'f'. So here the bind-level of 'x' (=1) is not <= the bind-level of 'f' (=0).
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We *do* want to specialise the call to 'g', because 'x' is free in g.
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Here the bind-level of 'x' (=1) is <= the bind-level of 'g' (=1).
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Note [Avoiding fruitless liberate-case]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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f = \x. case top_lvl_thing of
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I# _ -> let g = \y. ... g ...
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Here, top_lvl_thing is scrutinised at a level (1) deeper than its
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binding site (0). Nevertheless, we do NOT want to specialise the call
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to 'g' because all the structure in its free variables is already
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visible at the definition site for g. Hence, when considering specialising
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an occurrence of 'g', we want to check that there's a scruted-var v st
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a) v's binding site is *outside* g
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b) v's scrutinisation site is *inside* g
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%************************************************************************
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%************************************************************************
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addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
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addBinders env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env }) binders
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= env { lc_lvl_env = lvl_env' }
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lvl_env' = extendVarEnvList lvl_env (binders `zip` repeat lvl)
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addRecBinds :: LibCaseEnv -> [(Id,CoreExpr)] -> LibCaseEnv
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addRecBinds env@(LibCaseEnv {lc_lvl = lvl, lc_lvl_env = lvl_env,
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lc_rec_env = rec_env}) pairs
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= env { lc_lvl = lvl', lc_lvl_env = lvl_env', lc_rec_env = rec_env' }
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lvl_env' = extendVarEnvList lvl_env [(binder,lvl) | (binder,_) <- pairs]
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rec_env' = extendVarEnvList rec_env [(binder, Rec pairs) | (binder,_) <- pairs]
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addScrutedVar :: LibCaseEnv
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-> Id -- This Id is being scrutinised by a case expression
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addScrutedVar env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env,
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lc_scruts = scruts }) scrut_var
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= env { lc_scruts = scruts' }
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-- Add to scruts iff the scrut_var is being scrutinised at
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-- a deeper level than its defn
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scruts' = (scrut_var, bind_lvl, lvl) : scruts
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bind_lvl = case lookupVarEnv lvl_env scrut_var of
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lookupRecId :: LibCaseEnv -> Id -> Maybe CoreBind
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lookupRecId env id = lookupVarEnv (lc_rec_env env) id
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lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
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= case lookupVarEnv (lc_lvl_env env) id of
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%************************************************************************
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%************************************************************************
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type LibCaseLevel = Int
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topLevel :: LibCaseLevel
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lc_size :: Maybe Int, -- Bomb-out size for deciding if
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-- potential liberatees are too big.
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-- (passed in from cmd-line args)
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lc_lvl :: LibCaseLevel, -- Current level
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-- The level is incremented when (and only when) going
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-- inside the RHS of a (sufficiently small) recursive
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lc_lvl_env :: IdEnv LibCaseLevel,
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-- Binds all non-top-level in-scope Ids (top-level and
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-- imported things have a level of zero)
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lc_rec_env :: IdEnv CoreBind,
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-- Binds *only* recursively defined ids, to their own
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-- binding group, and *only* in their own RHSs
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lc_scruts :: [(Id, LibCaseLevel, LibCaseLevel)]
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-- Each of these Ids was scrutinised by an enclosing
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-- case expression, at a level deeper than its binding
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-- The first LibCaseLevel is the *binding level* of
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-- the scrutinised Id,
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-- The second is the level *at which it was scrutinised*.
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-- (see Note [Avoiding fruitless liberate-case])
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-- The former is a bit redundant, since you could always
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-- look it up in lc_lvl_env, but it's just cached here
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-- The order is insignificant; it's a bag really
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-- There's one element per scrutinisation;
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-- in principle the same Id may appear multiple times,
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-- although that'd be unusual:
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-- case x of { (a,b) -> ....(case x of ...) .. }
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initEnv :: DynFlags -> LibCaseEnv
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= LibCaseEnv { lc_size = liberateCaseThreshold dflags,
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lc_lvl_env = emptyVarEnv,
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lc_rec_env = emptyVarEnv,
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bombOutSize :: LibCaseEnv -> Maybe Int
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bombOutSize = lc_size