#include "HsVersions.h"
-import DynFlags ( DynFlags, DynFlag(..) )
-import StaticFlags ( opt_LiberateCaseThreshold )
+import DynFlags
+import HscTypes
import CoreLint ( showPass, endPass )
import CoreSyn
import CoreUnfold ( couldBeSmallEnoughToInline )
-import Id ( Id, setIdName, idName, setIdNotExported )
+import Rules ( RuleBase )
+import UniqSupply ( UniqSupply )
+import SimplMonad ( SimplCount, zeroSimplCount )
+import Id
+import FamInstEnv
+import Type
+import Coercion
+import TyCon
import VarEnv
import Name ( localiseName )
-import Outputable
import Util ( notNull )
+import Data.IORef ( readIORef )
\end{code}
+The liberate-case transformation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This module walks over @Core@, and looks for @case@ on free variables.
The criterion is:
if there is case on a free on the route to the recursive call,
Example
-\begin{verbatim}
-f = \ t -> case v of
- V a b -> a : f t
-\end{verbatim}
+ f = \ t -> case v of
+ V a b -> a : f t
=> the inner f is replaced.
-\begin{verbatim}
-f = \ t -> case v of
- V a b -> a : (letrec
+ f = \ t -> case v of
+ V a b -> a : (letrec
f = \ t -> case v of
V a b -> a : f t
- in f) t
-\end{verbatim}
+ in f) t
(note the NEED for shadowing)
=> Simplify
-\begin{verbatim}
-f = \ t -> case v of
- V a b -> a : (letrec
+ f = \ t -> case v of
+ V a b -> a : (letrec
f = \ t -> a : f t
- in f t)
-\begin{verbatim}
+ in f t)
Better code, because 'a' is free inside the inner letrec, rather
than needing projection from v.
(is this necessarily an improvement)
-
Similarly drop:
drop n [] = []
f = \ t -> case (v `cast` co) of
V a b -> a : f t
-Exactly the same optimistaion (unrolling one call to f) will work here,
+Exactly the same optimisation (unrolling one call to f) will work here,
despite the cast. See mk_alt_env in the Case branch of libCase.
Data types
~~~~~~~~~~
-
The ``level'' of a binder tells how many
recursive defns lexically enclose the binding
A recursive defn "encloses" its RHS, not its
Here, the level of @f@ is zero, the level of @g@ is one,
and the level of @h@ is zero (NB not one).
-\begin{code}
-type LibCaseLevel = Int
-
-topLevel :: LibCaseLevel
-topLevel = 0
-\end{code}
-
-\begin{code}
-data LibCaseEnv
- = LibCaseEnv {
- lc_size :: Int, -- Bomb-out size for deciding if
- -- potential liberatees are too big.
- -- (passed in from cmd-line args)
-
- lc_lvl :: LibCaseLevel, -- Current level
+Note [Indexed data types]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ data family T :: * -> *
+ data T Int = TI Int
+
+ f :: T Int -> Bool
+ f x = case x of { DEFAULT -> <body> }
+
+We would like to change this to
+ f x = case x `cast` co of { TI p -> <body> }
+
+so that <body> can make use of the fact that x is already evaluated to
+a TI; and a case on a known data type may be more efficient than a
+polymorphic one (not sure this is true any longer). Anyway the former
+showed up in Roman's experiments. Example:
+ foo :: FooT Int -> Int -> Int
+ foo t n = t `seq` bar n
+ where
+ bar 0 = 0
+ bar n = bar (n - case t of TI i -> i)
+Here we'd like to avoid repeated evaluating t inside the loop, by
+taking advantage of the `seq`.
+
+We implement this as part of the liberate-case transformation by
+spotting
+ case <scrut> of (x::T) tys { DEFAULT -> <body> }
+where x :: T tys, and T is a indexed family tycon. Find the
+representation type (T77 tys'), and coercion co, and transform to
+ case <scrut> `cast` co of (y::T77 tys')
+ DEFAULT -> let x = y `cast` sym co in <body>
+
+The "find the representation type" part is done by looking up in the
+family-instance environment.
+
+NB: in fact we re-use x (changing its type) to avoid making a fresh y;
+this entails shadowing, but that's ok.
+
+%************************************************************************
+%* *
+ Top-level code
+%* *
+%************************************************************************
- lc_lvl_env :: IdEnv LibCaseLevel,
- -- 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
-
--- lc_fams :: FamInstEnvs
- -- Instance env for indexed data types
- }
-
-initEnv :: Int -> LibCaseEnv
-initEnv bomb_size
- = LibCaseEnv { lc_size = bomb_size, lc_lvl = 0,
- lc_lvl_env = emptyVarEnv, lc_rec_env = emptyVarEnv,
- lc_scruts = [] }
-
-bombOutSize = lc_size
-\end{code}
-
-
-Programs
-~~~~~~~~
\begin{code}
-liberateCase :: DynFlags -> [CoreBind] -> IO [CoreBind]
-liberateCase dflags binds
- = do {
- showPass dflags "Liberate case" ;
- let { binds' = do_prog (initEnv opt_LiberateCaseThreshold) binds } ;
- endPass dflags "Liberate case" Opt_D_verbose_core2core binds'
- {- no specific flag for dumping -}
- }
+liberateCase :: HscEnv -> UniqSupply -> RuleBase -> ModGuts
+ -> IO (SimplCount, ModGuts)
+liberateCase hsc_env _ _ guts
+ = do { let dflags = hsc_dflags hsc_env
+ ; eps <- readIORef (hsc_EPS hsc_env)
+ ; let fam_envs = (eps_fam_inst_env eps, mg_fam_inst_env guts)
+
+ ; showPass dflags "Liberate case"
+ ; let { env = initEnv dflags fam_envs
+ ; 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' }) }
where
do_prog env [] = []
do_prog env (bind:binds) = bind' : do_prog env' binds
(env', bind') = libCaseBind env bind
\end{code}
+
+%************************************************************************
+%* *
+ Main payload
+%* *
+%************************************************************************
+
Bindings
~~~~~~~~
-
\begin{code}
libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
(env_body, bind') = libCaseBind env bind
libCase env (Case scrut bndr ty alts)
- = Case (libCase env scrut) bndr ty (map (libCaseAlt env_alts) alts)
+ = mkCase env (libCase env scrut) bndr ty (map (libCaseAlt env_alts) alts)
where
env_alts = addBinders (mk_alt_env scrut) [bndr]
mk_alt_env (Var scrut_var) = addScrutedVar env scrut_var
libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
\end{code}
+\begin{code}
+mkCase :: LibCaseEnv -> CoreExpr -> Id -> Type -> [CoreAlt] -> CoreExpr
+-- See Note [Indexed data types]
+mkCase env scrut bndr ty [(DEFAULT,_,rhs)]
+ | Just (tycon, tys) <- splitTyConApp_maybe (idType bndr)
+ , [(subst, fam_inst)] <- lookupFamInstEnv (lc_fams env) tycon tys
+ = let
+ rep_tc = famInstTyCon fam_inst
+ rep_tys = map (substTyVar subst) (tyConTyVars rep_tc)
+ bndr' = setIdType bndr (mkTyConApp rep_tc rep_tys)
+ Just co_tc = tyConFamilyCoercion_maybe rep_tc
+ co = mkTyConApp co_tc rep_tys
+ bind = NonRec bndr (Cast (Var bndr') (mkSymCoercion co))
+ in mkCase env (Cast scrut co) bndr' ty [(DEFAULT,[],Let bind rhs)]
+mkCase env scrut bndr ty alts
+ = Case scrut bndr ty alts
+\end{code}
+
Ids
~~~
\begin{code}
\end{code}
+%************************************************************************
+%* *
+ Utility functions
+%* *
+%************************************************************************
-Utility functions
-~~~~~~~~~~~~~~~~~
\begin{code}
addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
addBinders env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env }) binders
freeScruts env rec_bind_lvl
= [v | (v,scrut_lvl) <- lc_scruts env, scrut_lvl > rec_bind_lvl]
\end{code}
+
+%************************************************************************
+%* *
+ The environment
+%* *
+%************************************************************************
+
+\begin{code}
+type LibCaseLevel = Int
+
+topLevel :: LibCaseLevel
+topLevel = 0
+\end{code}
+
+\begin{code}
+data LibCaseEnv
+ = LibCaseEnv {
+ lc_size :: Int, -- Bomb-out size for deciding if
+ -- potential liberatees are too big.
+ -- (passed in from cmd-line args)
+
+ lc_lvl :: LibCaseLevel, -- Current level
+
+ lc_lvl_env :: IdEnv LibCaseLevel,
+ -- 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
+
+ lc_fams :: FamInstEnvs
+ -- Instance env for indexed data types
+ }
+
+initEnv :: DynFlags -> FamInstEnvs -> LibCaseEnv
+initEnv dflags fams
+ = LibCaseEnv { lc_size = specThreshold dflags,
+ lc_lvl = 0,
+ lc_lvl_env = emptyVarEnv,
+ lc_rec_env = emptyVarEnv,
+ lc_scruts = [],
+ lc_fams = fams }
+
+bombOutSize = lc_size
+\end{code}
+
+
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