2 % (c) The AQUA Project, Glasgow University, 1994-1998
4 \section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop}
7 module LiberateCase ( liberateCase ) where
9 #include "HsVersions.h"
11 import CmdLineOpts ( DynFlags, DynFlag(..), dopt, opt_LiberateCaseThreshold )
12 import CoreLint ( showPass, endPass )
14 import CoreUnfold ( couldBeSmallEnoughToInline )
20 This module walks over @Core@, and looks for @case@ on free variables.
22 if there is case on a free on the route to the recursive call,
23 then the recursive call is replaced with an unfolding.
32 => the inner f is replaced.
41 (note the NEED for shadowing)
43 => Run Andr\'e's wonder pass ...
50 Better code, because 'a' is free inside the inner letrec, rather
51 than needing projection from v.
53 Other examples we'd like to catch with this kind of transformation
59 We'd like to avoid the redundant pattern match, transforming to
63 last (x:(y:ys)) = last' y ys
66 last' _ (y:ys) = last' y ys
68 (is this necessarily an improvement)
75 drop n (x:xs) = drop (n-1) xs
77 Would like to pass n along unboxed.
80 To think about (Apr 94)
83 Main worry: duplicating code excessively. At the moment we duplicate
84 the entire binding group once at each recursive call. But there may
85 be a group of recursive calls which share a common set of evaluated
86 free variables, in which case the duplication is a plain waste.
88 Another thing we could consider adding is some unfold-threshold thing,
89 so that we'll only duplicate if the size of the group rhss isn't too
95 The ``level'' of a binder tells how many
96 recursive defns lexically enclose the binding
97 A recursive defn "encloses" its RHS, not its
100 letrec f = let g = ... in ...
105 Here, the level of @f@ is zero, the level of @g@ is one,
106 and the level of @h@ is zero (NB not one).
109 type LibCaseLevel = Int
111 topLevel :: LibCaseLevel
118 Int -- Bomb-out size for deciding if
119 -- potential liberatees are too big.
120 -- (passed in from cmd-line args)
122 LibCaseLevel -- Current level
124 (IdEnv LibCaseLevel) -- Binds all non-top-level in-scope Ids
125 -- (top-level and imported things have
128 (IdEnv CoreBind) -- Binds *only* recursively defined
129 -- Ids, to their own binding group,
130 -- and *only* in their own RHSs
132 [(Id,LibCaseLevel)] -- Each of these Ids was scrutinised by an
133 -- enclosing case expression, with the
134 -- specified number of enclosing
135 -- recursive bindings; furthermore,
136 -- the Id is bound at a lower level
137 -- than the case expression. The
138 -- order is insignificant; it's a bag
141 initEnv :: Int -> LibCaseEnv
142 initEnv bomb_size = LibCaseEnv bomb_size 0 emptyVarEnv emptyVarEnv []
144 bombOutSize (LibCaseEnv bomb_size _ _ _ _) = bomb_size
151 liberateCase :: DynFlags -> [CoreBind] -> IO [CoreBind]
152 liberateCase dflags binds
154 showPass dflags "Liberate case" ;
155 let { binds' = do_prog (initEnv opt_LiberateCaseThreshold) binds } ;
156 endPass dflags "Liberate case" Opt_D_verbose_core2core binds'
157 {- no specific flag for dumping -}
161 do_prog env (bind:binds) = bind' : do_prog env' binds
163 (env', bind') = libCaseBind env bind
170 libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
172 libCaseBind env (NonRec binder rhs)
173 = (addBinders env [binder], NonRec binder (libCase env rhs))
175 libCaseBind env (Rec pairs)
176 = (env_body, Rec pairs')
178 (binders, rhss) = unzip pairs
180 env_body = addBinders env binders
182 pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
184 env_rhs = if all rhs_small_enough rhss then extended_env else env
186 -- We extend the rec-env by binding each Id to its rhs, first
187 -- processing the rhs with an *un-extended* environment, so
188 -- that the same process doesn't occur for ever!
190 extended_env = addRecBinds env [ (binder, libCase env_body rhs)
191 | (binder, rhs) <- pairs ]
193 rhs_small_enough rhs = couldBeSmallEnoughToInline lIBERATE_BOMB_SIZE rhs
194 lIBERATE_BOMB_SIZE = bombOutSize env
202 libCase :: LibCaseEnv
206 libCase env (Var v) = libCaseId env v
207 libCase env (Lit lit) = Lit lit
208 libCase env (Type ty) = Type ty
209 libCase env (App fun arg) = App (libCase env fun) (libCase env arg)
210 libCase env (Note note body) = Note note (libCase env body)
212 libCase env (Lam binder body)
213 = Lam binder (libCase (addBinders env [binder]) body)
215 libCase env (Let bind body)
216 = Let bind' (libCase env_body body)
218 (env_body, bind') = libCaseBind env bind
220 libCase env (Case scrut bndr alts)
221 = Case (libCase env scrut) bndr (map (libCaseAlt env_alts) alts)
223 env_alts = addBinders env_with_scrut [bndr]
224 env_with_scrut = case scrut of
225 Var scrut_var -> addScrutedVar env scrut_var
228 libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
234 libCaseId :: LibCaseEnv -> Id -> CoreExpr
236 | Just the_bind <- lookupRecId env v, -- It's a use of a recursive thing
237 there_are_free_scruts -- with free vars scrutinised in RHS
238 = Let the_bind (Var v)
244 rec_id_level = lookupLevel env v
245 there_are_free_scruts = freeScruts env rec_id_level
253 addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
254 addBinders (LibCaseEnv bomb lvl lvl_env rec_env scruts) binders
255 = LibCaseEnv bomb lvl lvl_env' rec_env scruts
257 lvl_env' = extendVarEnvList lvl_env (binders `zip` repeat lvl)
259 addRecBinds :: LibCaseEnv -> [(Id,CoreExpr)] -> LibCaseEnv
260 addRecBinds (LibCaseEnv bomb lvl lvl_env rec_env scruts) pairs
261 = LibCaseEnv bomb lvl' lvl_env' rec_env' scruts
264 lvl_env' = extendVarEnvList lvl_env [(binder,lvl) | (binder,_) <- pairs]
265 rec_env' = extendVarEnvList rec_env [(binder, Rec pairs) | (binder,_) <- pairs]
267 addScrutedVar :: LibCaseEnv
268 -> Id -- This Id is being scrutinised by a case expression
271 addScrutedVar env@(LibCaseEnv bomb lvl lvl_env rec_env scruts) scrut_var
273 = LibCaseEnv bomb lvl lvl_env rec_env scruts'
274 -- Add to scruts iff the scrut_var is being scrutinised at
275 -- a deeper level than its defn
279 scruts' = (scrut_var, lvl) : scruts
280 bind_lvl = case lookupVarEnv lvl_env scrut_var of
284 lookupRecId :: LibCaseEnv -> Id -> Maybe CoreBind
285 lookupRecId (LibCaseEnv bomb lvl lvl_env rec_env scruts) id
287 = lookupVarEnv rec_env id
289 = case (lookupVarEnv rec_env id) of
294 lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
295 lookupLevel (LibCaseEnv bomb lvl lvl_env rec_env scruts) id
296 = case lookupVarEnv lvl_env id of
300 freeScruts :: LibCaseEnv
301 -> LibCaseLevel -- Level of the recursive Id
302 -> Bool -- True <=> there is an enclosing case of a variable
303 -- bound outside (ie level <=) the recursive Id.
304 freeScruts (LibCaseEnv bomb lvl lvl_env rec_env scruts) rec_bind_lvl
305 = not (null free_scruts)
307 free_scruts = [v | (v,lvl) <- scruts, lvl <= rec_bind_lvl]