X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FsimplCore%2FSimplUtils.lhs;h=60892770d67231ac8482295ef8fd1f7a9eb55cd4;hb=762ec41d13943e954549cfdada35baba3d4df704;hp=1dd3ea1d4fd6d71837ec812f5fa4c330b9bed36b;hpb=30e84fceebee3cf28768b6533f0c1651a351640a;p=ghc-hetmet.git diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs index 1dd3ea1..6089277 100644 --- a/ghc/compiler/simplCore/SimplUtils.lhs +++ b/ghc/compiler/simplCore/SimplUtils.lhs @@ -5,45 +5,46 @@ \begin{code} module SimplUtils ( - simplBinder, simplBinders, simplRecBndrs, simplLetBndr, - simplLamBndrs, simplTopBndrs, - newId, mkLam, mkCase, + simplBinder, simplBinders, simplRecBndrs, + simplLetBndr, simplLamBndrs, + newId, mkLam, prepareAlts, mkCase, -- The continuation type SimplCont(..), DupFlag(..), LetRhsFlag(..), contIsDupable, contResultType, countValArgs, countArgs, pushContArgs, - mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg, + mkBoringStop, mkRhsStop, contIsRhs, contIsRhsOrArg, getContArgs, interestingCallContext, interestingArg, isStrictType ) where #include "HsVersions.h" -import CmdLineOpts ( SimplifierSwitch(..), - opt_SimplDoLambdaEtaExpansion, opt_SimplDoEtaReduction, - opt_SimplCaseMerge, opt_UF_UpdateInPlace - ) +import CmdLineOpts ( SimplifierSwitch(..), opt_UF_UpdateInPlace, + DynFlag(..), dopt ) import CoreSyn -import CoreUtils ( cheapEqExpr, exprType, - etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce, +import CoreFVs ( exprFreeVars ) +import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial, + etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2, findDefault, exprOkForSpeculation, exprIsValue ) import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr ) -import Id ( Id, idType, idInfo, isLocalId, - mkSysLocal, hasNoBinding, isDeadBinder, idNewDemandInfo, +import Id ( Id, idType, idInfo, isDataConWorkId, + mkSysLocal, isDeadBinder, idNewDemandInfo, idUnfolding, idNewStrictness ) import NewDemand ( isStrictDmd, isBotRes, splitStrictSig ) import SimplMonad -import Type ( Type, seqType, - splitTyConApp_maybe, tyConAppArgs, mkTyVarTys, - splitRepFunTys, isStrictType +import Type ( Type, seqType, splitFunTys, dropForAlls, isStrictType, + splitTyConApp_maybe, tyConAppArgs, mkTyVarTys ) -import OccName ( UserFS ) -import TyCon ( tyConDataConsIfAvailable, isAlgTyCon, isNewTyCon ) -import DataCon ( dataConRepArity, dataConSig, dataConArgTys ) -import Var ( mkSysTyVar, tyVarKind ) +import TcType ( isDictTy ) +import Name ( mkSysTvName ) +import OccName ( EncodedFS ) +import TyCon ( tyConDataCons_maybe, isAlgTyCon, isNewTyCon ) +import DataCon ( dataConRepArity, dataConTyVars, dataConArgTys, isVanillaDataCon ) +import Var ( tyVarKind, mkTyVar ) +import VarSet import Util ( lengthExceeds, mapAccumL ) import Outputable \end{code} @@ -77,14 +78,16 @@ data SimplCont -- Strict contexts InId [InAlt] SimplEnv -- The case binder, alts, and subst-env SimplCont - | ArgOf DupFlag -- An arbitrary strict context: the argument + | ArgOf LetRhsFlag -- An arbitrary strict context: the argument -- of a strict function, or a primitive-arg fn -- or a PrimOp - LetRhsFlag + -- No DupFlag because we never duplicate it + OutType -- arg_ty: type of the argument itself OutType -- cont_ty: the type of the expression being sought by the context -- f (error "foo") ==> coerce t (error "foo") -- when f is strict -- We need to know the type t, to which to coerce. + (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result -- The result expression in the OutExprStuff has type cont_ty @@ -98,7 +101,7 @@ instance Outputable LetRhsFlag where instance Outputable SimplCont where ppr (Stop _ is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs) ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont - ppr (ArgOf dup _ _ _) = ptext SLIT("ArgOf...") <+> ppr dup + ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...") ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$ (nest 4 (ppr alts)) $$ ppr cont ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont @@ -112,15 +115,13 @@ instance Outputable DupFlag where ------------------- -mkBoringStop :: OutType -> SimplCont +mkBoringStop, mkRhsStop :: OutType -> SimplCont mkBoringStop ty = Stop ty AnArg (canUpdateInPlace ty) - -mkStop :: OutType -> LetRhsFlag -> SimplCont -mkStop ty is_rhs = Stop ty is_rhs (canUpdateInPlace ty) +mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty) contIsRhs :: SimplCont -> Bool contIsRhs (Stop _ AnRhs _) = True -contIsRhs (ArgOf _ AnRhs _ _) = True +contIsRhs (ArgOf AnRhs _ _ _) = True contIsRhs other = False contIsRhsOrArg (Stop _ _ _) = True @@ -131,11 +132,10 @@ contIsRhsOrArg other = False contIsDupable :: SimplCont -> Bool contIsDupable (Stop _ _ _) = True contIsDupable (ApplyTo OkToDup _ _ _) = True -contIsDupable (ArgOf OkToDup _ _ _) = True contIsDupable (Select OkToDup _ _ _ _) = True contIsDupable (CoerceIt _ cont) = contIsDupable cont -contIsDupable (InlinePlease cont) = contIsDupable cont -contIsDupable other = False +contIsDupable (InlinePlease cont) = contIsDupable cont +contIsDupable other = False ------------------- discardableCont :: SimplCont -> Bool @@ -221,6 +221,9 @@ getContArgs chkr fun orig_cont -- * (error "Hello") arg -- * f (error "Hello") where f is strict -- etc + -- Then, especially in the first of these cases, we'd like to discard + -- the continuation, leaving just the bottoming expression. But the + -- type might not be right, so we may have to add a coerce. go acc ss inl cont | null ss && discardableCont cont = (reverse acc, discardCont cont, inl) | otherwise = (reverse acc, cont, inl) @@ -231,14 +234,14 @@ getContArgs chkr fun orig_cont computed_stricts = zipWith (||) fun_stricts arg_stricts ---------------------------- - (val_arg_tys, _) = splitRepFunTys (idType fun) + (val_arg_tys, _) = splitFunTys (dropForAlls (idType fun)) arg_stricts = map isStrictType val_arg_tys ++ repeat False -- These argument types are used as a cheap and cheerful way to find -- unboxed arguments, which must be strict. But it's an InType -- and so there might be a type variable where we expect a function -- type (the substitution hasn't happened yet). And we don't bother -- doing the type applications for a polymorphic function. - -- Hence the split*Rep*FunTys + -- Hence the splitFunTys*IgnoringForAlls* ---------------------------- -- If fun_stricts is finite, it means the function returns bottom @@ -272,6 +275,9 @@ interestingArg :: OutExpr -> Bool interestingArg (Var v) = hasSomeUnfolding (idUnfolding v) -- Was: isValueUnfolding (idUnfolding v') -- But that seems over-pessimistic + || isDataConWorkId v + -- This accounts for an argument like + -- () or [], which is definitely interesting interestingArg (Type _) = False interestingArg (App fn (Type _)) = interestingArg fn interestingArg (Note _ a) = interestingArg a @@ -364,9 +370,9 @@ interestingCallContext :: Bool -- False <=> no args at all interestingCallContext some_args some_val_args cont = interesting cont where - interesting (InlinePlease _) = True - interesting (Select _ _ _ _ _) = some_args - interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y + interesting (InlinePlease _) = True + interesting (Select _ _ _ _ _) = some_args + interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y -- Perhaps True is a bit over-keen, but I've -- seen (coerce f) x, where f has an INLINE prag, -- So we have to give some motivaiton for inlining it @@ -404,10 +410,10 @@ canUpdateInPlace ty | otherwise = case splitTyConApp_maybe ty of Nothing -> False - Just (tycon, _) -> case tyConDataConsIfAvailable tycon of - [dc] -> arity == 1 || arity == 2 - where - arity = dataConRepArity dc + Just (tycon, _) -> case tyConDataCons_maybe tycon of + Just [dc] -> arity == 1 || arity == 2 + where + arity = dataConRepArity dc other -> False \end{code} @@ -447,26 +453,11 @@ simplLetBndr env id seqBndr id' `seq` returnSmpl (setSubst env subst', id') -simplTopBndrs, simplLamBndrs, simplRecBndrs +simplLamBndrs, simplRecBndrs :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder]) -simplTopBndrs = simplBndrs simplTopBinder simplRecBndrs = simplBndrs Subst.simplLetId simplLamBndrs = simplBndrs Subst.simplLamBndr --- For top-level binders, don't use simplLetId for GlobalIds. --- There are some of these, notably consructor wrappers, and we don't --- want to clone them or fiddle with them at all. --- Rather tiresomely, the specialiser may float a use of a constructor --- wrapper to before its definition (which shouldn't really matter) --- because it doesn't see the constructor wrapper as free in the binding --- it is floating (because it's a GlobalId). --- Then the simplifier brings all top level Ids into scope at the --- beginning, and we don't want to lose the IdInfo on the constructor --- wrappers. It would also be Bad to clone it! -simplTopBinder subst bndr - | isLocalId bndr = Subst.simplLetId subst bndr - | otherwise = (subst, bndr) - simplBndrs simpl_bndr env bndrs = let (subst', bndrs') = mapAccumL simpl_bndr (getSubst env) bndrs @@ -485,7 +476,7 @@ seqBndr b | isTyVar b = b `seq` () \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} @@ -510,15 +501,19 @@ Try three things \begin{code} mkLam env bndrs body cont - | opt_SimplDoEtaReduction, - Just etad_lam <- tryEtaReduce bndrs body - = tick (EtaReduction (head bndrs)) `thenSmpl_` - returnSmpl (emptyFloats env, etad_lam) - - | opt_SimplDoLambdaEtaExpansion, - any isRuntimeVar bndrs - = tryEtaExpansion body `thenSmpl` \ body' -> - returnSmpl (emptyFloats env, mkLams bndrs body') + = getDOptsSmpl `thenSmpl` \dflags -> + mkLam' dflags env bndrs body cont + where + mkLam' dflags env bndrs body cont + | dopt Opt_DoEtaReduction dflags, + Just etad_lam <- tryEtaReduce bndrs body + = tick (EtaReduction (head bndrs)) `thenSmpl_` + returnSmpl (emptyFloats env, etad_lam) + + | dopt Opt_DoLambdaEtaExpansion dflags, + any isRuntimeVar bndrs + = tryEtaExpansion body `thenSmpl` \ body' -> + returnSmpl (emptyFloats env, mkLams bndrs body') {- Sept 01: I'm experimenting with getting the full laziness pass to float out past big lambdsa @@ -531,8 +526,8 @@ mkLam env bndrs body cont returnSmpl (floats, mkLams bndrs body') -} - | otherwise - = returnSmpl (emptyFloats env, mkLams bndrs body) + | otherwise + = returnSmpl (emptyFloats env, mkLams bndrs body) \end{code} @@ -554,16 +549,27 @@ tryEtaReduce bndrs body -- efficient here: -- (a) we already have the binders -- (b) we can do the triviality test before computing the free vars - -- [in fact I take the simple path and look for just a variable] = go (reverse bndrs) body where go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round - go [] (Var fun) | ok_fun fun = Just (Var fun) -- Success! + go [] fun | ok_fun fun = Just fun -- Success! go _ _ = Nothing -- Failure! - ok_fun fun = not (fun `elem` bndrs) && not (hasNoBinding fun) - -- Note the awkward "hasNoBinding" test - -- Details with exprIsTrivial + ok_fun fun = exprIsTrivial fun + && not (any (`elemVarSet` (exprFreeVars fun)) bndrs) + && (exprIsValue fun || all ok_lam bndrs) + ok_lam v = isTyVar v || isDictTy (idType v) + -- The exprIsValue is because eta reduction is not + -- valid in general: \x. bot /= bot + -- So we need to be sure that the "fun" is a value. + -- + -- However, we always want to reduce (/\a -> f a) to f + -- This came up in a RULE: foldr (build (/\a -> g a)) + -- did not match foldr (build (/\b -> ...something complex...)) + -- The type checker can insert these eta-expanded versions, + -- with both type and dictionary lambdas; hence the slightly + -- ad-hoc isDictTy + ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg \end{code} @@ -781,6 +787,138 @@ tryRhsTyLam env tyvars body -- Only does something if there's a let -} \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, + not (null all_cons), -- This is a tricky corner case. If the data type has no constructors, + -- which GHC allows, then the case expression will have at most a default + -- alternative. We don't want to eliminate that alternative, because the + -- invariant is that there's always one alternative. It's more convenient + -- to leave + -- case x of { DEFAULT -> e } + -- as it is, rather than transform it to + -- error "case cant match" + -- which would be quite legitmate. But it's a really obscure corner, and + -- not worth wasting code on. + 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 + = mk_tv_bndrs missing_con inst_tys `thenSmpl` \ (tv_bndrs, inst_tys') -> + getUniquesSmpl `thenSmpl` \ id_uniqs -> + let arg_tys = dataConArgTys missing_con inst_tys' + arg_ids = zipWith (mkSysLocal FSLIT("a")) id_uniqs arg_tys + in + returnSmpl (tv_bndrs ++ arg_ids) + +mk_tv_bndrs missing_con inst_tys + | isVanillaDataCon missing_con + = returnSmpl ([], inst_tys) + | otherwise + = getUniquesSmpl `thenSmpl` \ tv_uniqs -> + let new_tvs = zipWith mk tv_uniqs (dataConTyVars missing_con) + mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv) + in + returnSmpl (new_tvs, mkTyVarTys new_tvs) +\end{code} + %************************************************************************ %* * @@ -791,11 +929,11 @@ tryRhsTyLam env tyvars body -- Only does something if there's a let mkCase puts a case expression back together, trying various transformations first. \begin{code} -mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr +mkCase :: OutExpr -> OutId -> OutType -> [OutAlt] -> SimplM OutExpr -mkCase scrut case_bndr alts +mkCase scrut case_bndr ty alts = mkAlts scrut case_bndr alts `thenSmpl` \ better_alts -> - mkCase1 scrut case_bndr better_alts + mkCase1 scrut case_bndr ty better_alts \end{code} @@ -817,16 +955,7 @@ mkAlts tries these things: 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 ... ... @@ -881,51 +1010,21 @@ mkAlts scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts) -------------------------------------------------- --- 2. Fill in missing constructor --------------------------------------------------- - -mkAlts scrut 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! - - [missing_con] <- filter is_missing (tyConDataConsIfAvailable tycon) - -- 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 - impossible_cons = otherCons (idUnfolding case_bndr) - handled_data_cons = [data_con | DataAlt data_con <- impossible_cons] ++ - [data_con | (DataAlt data_con, _, _) <- alts] - is_missing con = not (con `elem` handled_data_cons) - --------------------------------------------------- --- 3. Merge nested cases +-- 2. Merge nested cases -------------------------------------------------- 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, - scruting_same_var scrut_var - - = let -- Eliminate any inner alts which are shadowed by the outer ones + = getDOptsSmpl `thenSmpl` \dflags -> + mkAlts' dflags scrut outer_bndr outer_alts + where + mkAlts' dflags scrut outer_bndr outer_alts + | dopt Opt_CaseMerge dflags, + (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts, +-- gaw 2004 + Just (Case (Var scrut_var) inner_bndr _ inner_alts) <- maybe_outer_deflt, + scruting_same_var scrut_var + + = let -- Eliminate any inner alts which are shadowed by the outer ones outer_cons = [con | (con,_,_) <- outer_alts_without_deflt] munged_inner_alts = [ (con, args, munge_rhs rhs) @@ -946,24 +1045,24 @@ mkAlts scrut outer_bndr outer_alts -- mkCase applied to them, so they won't have a case in their default -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr -- in munge_rhs may put a case into the DEFAULT branch! - where + where -- We are scrutinising the same variable if it's -- the outer case-binder, or if the outer case scrutinises a variable -- (and it's the same). Testing both allows us not to replace the -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder). - scruting_same_var = case scrut of + scruting_same_var = case scrut of Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut other -> \ v -> v == outer_bndr - add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts - add_default Nothing alts = alts + add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts + add_default Nothing alts = alts -------------------------------------------------- -- Catch-all -------------------------------------------------- -mkAlts scrut case_bndr other_alts = returnSmpl other_alts + mkAlts' dflags scrut case_bndr other_alts = returnSmpl other_alts \end{code} @@ -1059,13 +1158,62 @@ So the case-elimination algorithm is: If so, then we can replace the case with one of the rhss. +Further notes about case elimination +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider: test :: Integer -> IO () + test = print + +Turns out that this compiles to: + Print.test + = \ eta :: Integer + eta1 :: State# RealWorld -> + case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT -> + case hPutStr stdout + (PrelNum.jtos eta ($w[] @ Char)) + eta1 + of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }} + +Notice the strange '<' which has no effect at all. This is a funny one. +It started like this: + +f x y = if x < 0 then jtos x + else if y==0 then "" else jtos x + +At a particular call site we have (f v 1). So we inline to get + + if v < 0 then jtos x + else if 1==0 then "" else jtos x + +Now simplify the 1==0 conditional: + + if v<0 then jtos v else jtos v + +Now common-up the two branches of the case: + + case (v<0) of DEFAULT -> jtos v + +Why don't we drop the case? Because it's strict in v. It's technically +wrong to drop even unnecessary evaluations, and in practice they +may be a result of 'seq' so we *definitely* don't want to drop those. +I don't really know how to improve this situation. + \begin{code} -------------------------------------------------- +-- 0. Check for empty alternatives +-------------------------------------------------- + +#ifdef DEBUG +mkCase1 scrut case_bndr ty [] + = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $ + returnSmpl scrut +#endif + +-------------------------------------------------- -- 1. Eliminate the case altogether if poss -------------------------------------------------- -mkCase1 scrut case_bndr [(con,bndrs,rhs)] +mkCase1 scrut case_bndr ty [(con,bndrs,rhs)] -- See if we can get rid of the case altogether -- See the extensive notes on case-elimination above -- mkCase made sure that if all the alternatives are equal, @@ -1092,6 +1240,7 @@ mkCase1 scrut case_bndr [(con,bndrs,rhs)] -- Here we must *not* discard the case, because dataToTag# just fetches the tag from -- the info pointer. So we'll be pedantic all the time, and see if that gives any -- other problems +-- Also we don't want to discard 'seq's = tick (CaseElim case_bndr) `thenSmpl_` returnSmpl (bindCaseBndr case_bndr scrut rhs) @@ -1106,7 +1255,7 @@ mkCase1 scrut case_bndr [(con,bndrs,rhs)] -- 2. Identity case -------------------------------------------------- -mkCase1 scrut case_bndr alts -- Identity case +mkCase1 scrut case_bndr ty alts -- Identity case | all identity_alt alts = tick (CaseIdentity case_bndr) `thenSmpl_` returnSmpl (re_note scrut) @@ -1129,14 +1278,15 @@ mkCase1 scrut case_bndr alts -- Identity case -- re_note wraps a coerce if it might be necessary re_note scrut = case head alts of - (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut + (_,_,rhs1@(Note _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut other -> scrut -------------------------------------------------- -- Catch-all -------------------------------------------------- -mkCase1 scrut bndr alts = returnSmpl (Case scrut bndr alts) +-- gaw 2004 +mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts) \end{code}