module SimplUtils (
simplBinder, simplBinders, simplRecBndrs,
simplLetBndr, simplLamBndrs,
- newId, mkLam, mkCase,
+ newId, mkLam, prepareAlts, mkCase,
-- The continuation type
SimplCont(..), DupFlag(..), LetRhsFlag(..),
splitTyConApp_maybe, tyConAppArgs, mkTyVarTys
)
import TcType ( isDictTy )
-import OccName ( UserFS )
+import OccName ( EncodedFS )
import TyCon ( tyConDataCons_maybe, isAlgTyCon, isNewTyCon )
import DataCon ( dataConRepArity, dataConSig, dataConArgTys )
import Var ( mkSysTyVar, tyVarKind )
\begin{code}
-newId :: UserFS -> Type -> SimplM Id
+newId :: EncodedFS -> Type -> SimplM Id
newId fs ty = getUniqueSmpl `thenSmpl` \ uniq ->
returnSmpl (mkSysLocal fs uniq ty)
\end{code}
-}
\end{code}
+%************************************************************************
+%* *
+\subsection{Case alternative filtering
+%* *
+%************************************************************************
+
+prepareAlts does two things:
+
+1. Eliminate alternatives that cannot match, including the
+ DEFAULT alternative.
+
+2. If the DEFAULT alternative can match only one possible constructor,
+ then make that constructor explicit.
+ e.g.
+ case e of x { DEFAULT -> rhs }
+ ===>
+ case e of x { (a,b) -> rhs }
+ where the type is a single constructor type. This gives better code
+ when rhs also scrutinises x or e.
+
+It's a good idea do do this stuff before simplifying the alternatives, to
+avoid simplifying alternatives we know can't happen, and to come up with
+the list of constructors that are handled, to put into the IdInfo of the
+case binder, for use when simplifying the alternatives.
+
+Eliminating the default alternative in (1) isn't so obvious, but it can
+happen:
+
+data Colour = Red | Green | Blue
+
+f x = case x of
+ Red -> ..
+ Green -> ..
+ DEFAULT -> h x
+
+h y = case y of
+ Blue -> ..
+ DEFAULT -> [ case y of ... ]
+
+If we inline h into f, the default case of the inlined h can't happen.
+If we don't notice this, we may end up filtering out *all* the cases
+of the inner case y, which give us nowhere to go!
+
+
+\begin{code}
+prepareAlts :: OutExpr -- Scrutinee
+ -> InId -- Case binder
+ -> [InAlt]
+ -> SimplM ([InAlt], -- Better alternatives
+ [AltCon]) -- These cases are handled
+
+prepareAlts scrut case_bndr alts
+ = let
+ (alts_wo_default, maybe_deflt) = findDefault alts
+
+ impossible_cons = case scrut of
+ Var v -> otherCons (idUnfolding v)
+ other -> []
+
+ -- Filter out alternatives that can't possibly match
+ better_alts | null impossible_cons = alts_wo_default
+ | otherwise = [alt | alt@(con,_,_) <- alts_wo_default,
+ not (con `elem` impossible_cons)]
+
+ -- "handled_cons" are handled either by the context,
+ -- or by a branch in this case expression
+ -- (Don't add DEFAULT to the handled_cons!!)
+ handled_cons = impossible_cons ++ [con | (con,_,_) <- better_alts]
+ in
+ -- Filter out the default, if it can't happen,
+ -- or replace it with "proper" alternative if there
+ -- is only one constructor left
+ prepareDefault case_bndr handled_cons maybe_deflt `thenSmpl` \ deflt_alt ->
+
+ returnSmpl (deflt_alt ++ better_alts, handled_cons)
+
+prepareDefault case_bndr handled_cons (Just rhs)
+ | Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr),
+ isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
+ not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
+ -- case x of { DEFAULT -> e }
+ -- and we don't want to fill in a default for them!
+ Just all_cons <- tyConDataCons_maybe tycon,
+ let handled_data_cons = [data_con | DataAlt data_con <- handled_cons],
+ let missing_cons = [con | con <- all_cons,
+ not (con `elem` handled_data_cons)]
+ = case missing_cons of
+ [] -> returnSmpl [] -- Eliminate the default alternative
+ -- if it can't match
+
+ [con] -> -- It matches exactly one constructor, so fill it in
+ tick (FillInCaseDefault case_bndr) `thenSmpl_`
+ mk_args con inst_tys `thenSmpl` \ args ->
+ returnSmpl [(DataAlt con, args, rhs)]
+
+ two_or_more -> returnSmpl [(DEFAULT, [], rhs)]
+
+ | otherwise
+ = returnSmpl [(DEFAULT, [], rhs)]
+
+prepareDefault case_bndr handled_cons Nothing
+ = returnSmpl []
+
+mk_args missing_con inst_tys
+ = getUniquesSmpl `thenSmpl` \ tv_uniqs ->
+ getUniquesSmpl `thenSmpl` \ id_uniqs ->
+ let
+ (_,_,ex_tyvars,_,_,_) = dataConSig missing_con
+ ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
+ mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
+ arg_tys = dataConArgTys missing_con (inst_tys ++ mkTyVarTys ex_tyvars')
+ arg_ids = zipWith (mkSysLocal FSLIT("a")) id_uniqs arg_tys
+ in
+ returnSmpl (ex_tyvars' ++ arg_ids)
+\end{code}
+
%************************************************************************
%* *
mkCase puts a case expression back together, trying various transformations first.
\begin{code}
-mkCase :: OutExpr -> [AltCon] -> OutId -> [OutAlt] -> SimplM OutExpr
+mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
-mkCase scrut handled_cons case_bndr alts
- = mkAlts scrut handled_cons case_bndr alts `thenSmpl` \ better_alts ->
+mkCase scrut case_bndr alts
+ = mkAlts scrut case_bndr alts `thenSmpl` \ better_alts ->
mkCase1 scrut case_bndr better_alts
\end{code}
a) all branches equal
b) some branches equal to the DEFAULT (which occurs first)
-2. If the DEFAULT alternative can match only one possible constructor,
- then make that constructor explicit.
- e.g.
- case e of x { DEFAULT -> rhs }
- ===>
- case e of x { (a,b) -> rhs }
- where the type is a single constructor type. This gives better code
- when rhs also scrutinises x or e.
-
-3. Case merging:
+2. Case merging:
case e of b { ==> case e of b {
p1 -> rhs1 p1 -> rhs1
... ...
--------------------------------------------------
-- 1. Merge identical branches
--------------------------------------------------
-mkAlts scrut handled_cons case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
+mkAlts scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
| all isDeadBinder bndrs1, -- Remember the default
length filtered_alts < length con_alts -- alternative comes first
= tick (AltMerge case_bndr) `thenSmpl_`
--------------------------------------------------
--- 2. Fill in missing constructor
---------------------------------------------------
-
-mkAlts scrut handled_cons case_bndr alts
- | (alts_no_deflt, Just rhs) <- findDefault alts,
- -- There is a DEFAULT case
-
- Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr),
- isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
- not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
- -- case x of { DEFAULT -> e }
- -- and we don't want to fill in a default for them!
-
- Just all_cons <- tyConDataCons_maybe tycon,
- [missing_con] <- [con | con <- all_cons, not (con `elem` handled_data_cons)]
- -- There is just one missing constructor!
-
- = tick (FillInCaseDefault case_bndr) `thenSmpl_`
- getUniquesSmpl `thenSmpl` \ tv_uniqs ->
- getUniquesSmpl `thenSmpl` \ id_uniqs ->
- let
- (_,_,ex_tyvars,_,_,_) = dataConSig missing_con
- ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
- mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
- arg_ids = zipWith (mkSysLocal SLIT("a")) id_uniqs arg_tys
- arg_tys = dataConArgTys missing_con (inst_tys ++ mkTyVarTys ex_tyvars')
- better_alts = (DataAlt missing_con, ex_tyvars' ++ arg_ids, rhs) : alts_no_deflt
- in
- returnSmpl better_alts
- where
- handled_data_cons = [data_con | DataAlt data_con <- handled_cons]
-
---------------------------------------------------
--- 3. Merge nested cases
+-- 2. Merge nested cases
--------------------------------------------------
-mkAlts scrut handled_cons outer_bndr outer_alts
+mkAlts scrut outer_bndr outer_alts
| opt_SimplCaseMerge,
(outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
Just (Case (Var scrut_var) inner_bndr inner_alts) <- maybe_outer_deflt,
-- Catch-all
--------------------------------------------------
-mkAlts scrut handled_cons case_bndr other_alts = returnSmpl other_alts
+mkAlts scrut case_bndr other_alts = returnSmpl other_alts
\end{code}
-- 2. Identity case
--------------------------------------------------
+#ifdef DEBUG
+mkCase1 scrut case_bndr []
+ = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
+ returnSmpl scrut
+#endif
+
mkCase1 scrut case_bndr alts -- Identity case
| all identity_alt alts
= tick (CaseIdentity case_bndr) `thenSmpl_`
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