\section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop}
\begin{code}
-{-# OPTIONS -w #-}
--- The above warning supression flag is a temporary kludge.
--- While working on this module you are encouraged to remove it and fix
--- any warnings in the module. See
--- http://hackage.haskell.org/trac/ghc/wiki/CodingStyle#Warnings
--- for details
-
module LiberateCase ( liberateCase ) where
#include "HsVersions.h"
import DynFlags
-import HscTypes
-import CoreLint ( showPass, endPass )
import CoreSyn
import CoreUnfold ( couldBeSmallEnoughToInline )
-import Rules ( RuleBase )
-import UniqSupply ( UniqSupply )
-import SimplMonad ( SimplCount, zeroSimplCount )
import Id
import VarEnv
-import Name ( localiseName )
import Util ( notNull )
\end{code}
Better code, because 'a' is free inside the inner letrec, rather
than needing projection from v.
-Other examples we'd like to catch with this kind of transformation
+Note that this deals with *free variables*. SpecConstr deals with
+*arguments* that are of known form. E.g.
last [] = error
last (x:[]) = x
last (x:xs) = last xs
-We'd like to avoid the redundant pattern match, transforming to
-
- last [] = error
- last (x:[]) = x
- last (x:(y:ys)) = last' y ys
- where
- last' y [] = y
- last' _ (y:ys) = last' y ys
-
- (is this necessarily an improvement)
-
-Similarly drop:
-
- drop n [] = []
- drop 0 xs = xs
- drop n (x:xs) = drop (n-1) xs
-
-Would like to pass n along unboxed.
Note [Scrutinee with cast]
~~~~~~~~~~~~~~~~~~~~~~~~~~
%************************************************************************
\begin{code}
-liberateCase :: HscEnv -> UniqSupply -> RuleBase -> ModGuts
- -> IO (SimplCount, ModGuts)
-liberateCase hsc_env _ _ guts
- = do { let dflags = hsc_dflags hsc_env
-
- ; showPass dflags "Liberate case"
- ; let { env = initEnv dflags
- ; binds' = do_prog env (mg_binds guts) }
- ; endPass dflags "Liberate case" Opt_D_verbose_core2core binds'
- {- no specific flag for dumping -}
- ; return (zeroSimplCount dflags, guts { mg_binds = binds' }) }
+liberateCase :: DynFlags -> [CoreBind] -> [CoreBind]
+liberateCase dflags binds = do_prog (initEnv dflags) binds
where
- do_prog env [] = []
+ do_prog _ [] = []
do_prog env (bind:binds) = bind' : do_prog env' binds
where
(env', bind') = libCaseBind env bind
libCaseBind env (Rec pairs)
= (env_body, Rec pairs')
where
- (binders, rhss) = unzip pairs
+ binders = map fst pairs
env_body = addBinders env binders
pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
- env_rhs = if all rhs_small_enough pairs then extended_env else env
-
-- We extend the rec-env by binding each Id to its rhs, first
-- processing the rhs with an *un-extended* environment, so
-- that the same process doesn't occur for ever!
- --
- extended_env = addRecBinds env [ (adjust binder, libCase env_body rhs)
- | (binder, rhs) <- pairs ]
-
- -- Two subtle things:
- -- (a) Reset the export flags on the binders so
- -- that we don't get name clashes on exported things if the
- -- local binding floats out to top level. This is most unlikely
- -- to happen, since the whole point concerns free variables.
- -- But resetting the export flag is right regardless.
- --
- -- (b) Make the name an Internal one. External Names should never be
- -- nested; if it were floated to the top level, we'd get a name
- -- clash at code generation time.
- adjust bndr = setIdNotExported (setIdName bndr (localiseName (idName bndr)))
-
- rhs_small_enough (id,rhs)
+ env_rhs = addRecBinds env [ (localiseId binder, libCase env_body rhs)
+ | (binder, rhs) <- pairs
+ , rhs_small_enough binder rhs ]
+ -- localiseID : see Note [Need to localiseId in libCaseBind]
+
+
+ rhs_small_enough id rhs -- Note [Small enough]
= idArity id > 0 -- Note [Only functions!]
- && couldBeSmallEnoughToInline (bombOutSize env) rhs
+ && maybe True (\size -> couldBeSmallEnoughToInline size rhs)
+ (bombOutSize env)
\end{code}
+Note [Need to localiseId in libCaseBind]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The call to localiseId is needed for two subtle reasons
+(a) Reset the export flags on the binders so
+ that we don't get name clashes on exported things if the
+ local binding floats out to top level. This is most unlikely
+ to happen, since the whole point concerns free variables.
+ But resetting the export flag is right regardless.
+
+(b) Make the name an Internal one. External Names should never be
+ nested; if it were floated to the top level, we'd get a name
+ clash at code generation time.
+
+Note [Small enough]
+~~~~~~~~~~~~~~~~~~~
+Consider
+ \fv. letrec
+ f = \x. BIG...(case fv of { (a,b) -> ...g.. })...
+ g = \y. SMALL...f...
+Then we *can* do liberate-case on g (small RHS) but not for f (too big).
+But we can choose on a item-by-item basis, and that's what the
+rhs_small_enough call in the comprehension for env_rhs does.
Expressions
~~~~~~~~~~~
-> CoreExpr
-> CoreExpr
-libCase env (Var v) = libCaseId env v
-libCase env (Lit lit) = Lit lit
-libCase env (Type ty) = Type ty
+libCase env (Var v) = libCaseId env v
+libCase _ (Lit lit) = Lit lit
+libCase _ (Type ty) = Type ty
+libCase _ (Coercion co) = Coercion co
libCase env (App fun arg) = App (libCase env fun) (libCase env arg)
libCase env (Note note body) = Note note (libCase env body)
libCase env (Cast e co) = Cast (libCase env e) co
env_alts = addBinders (mk_alt_env scrut) [bndr]
mk_alt_env (Var scrut_var) = addScrutedVar env scrut_var
mk_alt_env (Cast scrut _) = mk_alt_env scrut -- Note [Scrutinee with cast]
- mk_alt_env otehr = env
+ mk_alt_env _ = env
+libCaseAlt :: LibCaseEnv -> (AltCon, [CoreBndr], CoreExpr)
+ -> (AltCon, [CoreBndr], CoreExpr)
libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
\end{code}
where
rec_id_level = lookupLevel env v
free_scruts = freeScruts env rec_id_level
+
+freeScruts :: LibCaseEnv
+ -> LibCaseLevel -- Level of the recursive Id
+ -> [Id] -- Ids that are scrutinised between the binding
+ -- of the recursive Id and here
+freeScruts env rec_bind_lvl
+ = [v | (v, scrut_bind_lvl, scrut_at_lvl) <- lc_scruts env
+ , scrut_bind_lvl <= rec_bind_lvl
+ , scrut_at_lvl > rec_bind_lvl]
+ -- Note [When to specialise]
+ -- Note [Avoiding fruitless liberate-case]
\end{code}
+Note [When to specialise]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ f = \x. letrec g = \y. case x of
+ True -> ... (f a) ...
+ False -> ... (g b) ...
+
+We get the following levels
+ f 0
+ x 1
+ g 1
+ y 2
+
+Then 'x' is being scrutinised at a deeper level than its binding, so
+it's added to lc_sruts: [(x,1)]
+
+We do *not* want to specialise the call to 'f', becuase 'x' is not free
+in 'f'. So here the bind-level of 'x' (=1) is not <= the bind-level of 'f' (=0).
+
+We *do* want to specialise the call to 'g', because 'x' is free in g.
+Here the bind-level of 'x' (=1) is <= the bind-level of 'g' (=1).
+
+Note [Avoiding fruitless liberate-case]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider also:
+ f = \x. case top_lvl_thing of
+ I# _ -> let g = \y. ... g ...
+ in ...
+
+Here, top_lvl_thing is scrutinised at a level (1) deeper than its
+binding site (0). Nevertheless, we do NOT want to specialise the call
+to 'g' because all the structure in its free variables is already
+visible at the definition site for g. Hence, when considering specialising
+an occurrence of 'g', we want to check that there's a scruted-var v st
+
+ a) v's binding site is *outside* g
+ b) v's scrutinisation site is *inside* g
+
%************************************************************************
%* *
| otherwise = env
where
- scruts' = (scrut_var, lvl) : scruts
+ scruts' = (scrut_var, bind_lvl, lvl) : scruts
bind_lvl = case lookupVarEnv lvl_env scrut_var of
Just lvl -> lvl
Nothing -> topLevel
= case lookupVarEnv (lc_lvl_env env) id of
Just lvl -> lvl
Nothing -> topLevel
-
-freeScruts :: LibCaseEnv
- -> LibCaseLevel -- Level of the recursive Id
- -> [Id] -- Ids that are scrutinised between the binding
- -- of the recursive Id and here
-freeScruts env rec_bind_lvl
- = [v | (v,scrut_lvl) <- lc_scruts env, scrut_lvl > rec_bind_lvl]
\end{code}
%************************************************************************
\begin{code}
data LibCaseEnv
= LibCaseEnv {
- lc_size :: Int, -- Bomb-out size for deciding if
+ lc_size :: Maybe Int, -- Bomb-out size for deciding if
-- potential liberatees are too big.
-- (passed in from cmd-line args)
lc_lvl :: LibCaseLevel, -- Current level
+ -- The level is incremented when (and only when) going
+ -- inside the RHS of a (sufficiently small) recursive
+ -- function.
lc_lvl_env :: IdEnv LibCaseLevel,
- -- Binds all non-top-level in-scope Ids
- -- (top-level and imported things have
- -- a level of zero)
+ -- Binds all non-top-level in-scope Ids (top-level and
+ -- imported things have a level of zero)
lc_rec_env :: IdEnv CoreBind,
- -- Binds *only* recursively defined ids,
- -- to their own binding group,
- -- and *only* in their own RHSs
-
- lc_scruts :: [(Id,LibCaseLevel)]
- -- Each of these Ids was scrutinised by an
- -- enclosing case expression, with the
- -- specified number of enclosing
- -- recursive bindings; furthermore,
- -- the Id is bound at a lower level
- -- than the case expression. The order is
- -- insignificant; it's a bag really
+ -- Binds *only* recursively defined ids, to their own
+ -- binding group, and *only* in their own RHSs
+
+ lc_scruts :: [(Id, LibCaseLevel, LibCaseLevel)]
+ -- Each of these Ids was scrutinised by an enclosing
+ -- case expression, at a level deeper than its binding
+ -- level.
+ --
+ -- The first LibCaseLevel is the *binding level* of
+ -- the scrutinised Id,
+ -- The second is the level *at which it was scrutinised*.
+ -- (see Note [Avoiding fruitless liberate-case])
+ -- The former is a bit redundant, since you could always
+ -- look it up in lc_lvl_env, but it's just cached here
+ --
+ -- The order is insignificant; it's a bag really
+ --
+ -- There's one element per scrutinisation;
+ -- in principle the same Id may appear multiple times,
+ -- although that'd be unusual:
+ -- case x of { (a,b) -> ....(case x of ...) .. }
}
initEnv :: DynFlags -> LibCaseEnv
initEnv dflags
- = LibCaseEnv { lc_size = specThreshold dflags,
+ = LibCaseEnv { lc_size = liberateCaseThreshold dflags,
lc_lvl = 0,
lc_lvl_env = emptyVarEnv,
lc_rec_env = emptyVarEnv,
lc_scruts = [] }
+bombOutSize :: LibCaseEnv -> Maybe Int
bombOutSize = lc_size
\end{code}
-
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