X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FsimplCore%2FSimplUtils.lhs;h=1ff6f8fbceebcd46c434d48aeb4f2d4258bd3176;hp=acd0830ee5ea9cf809b9cc2367d4215c2bb2d44a;hb=e9f23b4cc3df781f2fc84b48716a7779ecc8ab06;hpb=5bf1b7f5742ba6405239692f329e8be35491b350 diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs index acd0830..1ff6f8f 100644 --- a/compiler/simplCore/SimplUtils.lhs +++ b/compiler/simplCore/SimplUtils.lhs @@ -6,7 +6,7 @@ \begin{code} module SimplUtils ( -- Rebuilding - mkLam, mkCase, + mkLam, mkCase, prepareAlts, bindCaseBndr, -- Inlining, preInlineUnconditionally, postInlineUnconditionally, @@ -40,10 +40,12 @@ import SimplMonad import Type import TyCon import DataCon +import TcGadt ( dataConCanMatch ) import VarSet import BasicTypes import Util import Outputable +import List( nub ) \end{code} @@ -1116,26 +1118,11 @@ tryRhsTyLam env tyvars body -- Only does something if there's a let %************************************************************************ %* * -\subsection{Case absorption and identity-case elimination} + prepareAlts %* * %************************************************************************ - -mkCase puts a case expression back together, trying various transformations first. - -\begin{code} -mkCase :: OutExpr -> OutId -> OutType - -> [OutAlt] -- Increasing order - -> SimplM OutExpr - -mkCase scrut case_bndr ty alts - = getDOptsSmpl `thenSmpl` \dflags -> - mkAlts dflags scrut case_bndr alts `thenSmpl` \ better_alts -> - mkCase1 scrut case_bndr ty better_alts -\end{code} - - -mkAlts tries these things: +prepareAlts tries these things: 1. If several alternatives are identical, merge them into a single DEFAULT alternative. I've occasionally seen this @@ -1190,43 +1177,93 @@ This gave rise to a horrible sequence of cases and similarly in cascade for all the join points! - +Note [Dead binders] +~~~~~~~~~~~~~~~~~~~~ +We do this *here*, looking at un-simplified alternatives, because we +have to check that r doesn't mention the variables bound by the +pattern in each alternative, so the binder-info is rather useful. \begin{code} +prepareAlts :: OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt]) +prepareAlts scrut case_bndr' alts + = do { dflags <- getDOptsSmpl + ; alts <- combineIdenticalAlts case_bndr' alts + + ; let (alts_wo_default, maybe_deflt) = findDefault alts + alt_cons = [con | (con,_,_) <- alts_wo_default] + imposs_deflt_cons = nub (imposs_cons ++ alt_cons) + -- "imposs_deflt_cons" are handled either by the context, + -- OR by a branch in this case expression. + -- Don't include DEFAULT!! + + ; default_alts <- prepareDefault dflags scrut case_bndr' mb_tc_app + imposs_deflt_cons maybe_deflt + + ; let trimmed_alts = filter possible_alt alts_wo_default + merged_alts = mergeAlts default_alts trimmed_alts + -- We need the mergeAlts in case the new default_alt + -- has turned into a constructor alternative. + -- The merge keeps the inner DEFAULT at the front, if there is one + -- and eliminates any inner_alts that are shadowed by the outer_alts + + + ; return (imposs_deflt_cons, merged_alts) } + where + mb_tc_app = splitTyConApp_maybe (idType case_bndr') + Just (_, inst_tys) = mb_tc_app + + imposs_cons = case scrut of + Var v -> otherCons (idUnfolding v) + other -> [] + + possible_alt :: CoreAlt -> Bool + possible_alt (con, _, _) | con `elem` imposs_cons = False + possible_alt (DataAlt con, _, _) = dataConCanMatch inst_tys con + possible_alt alt = True + + -------------------------------------------------- -- 1. Merge identical branches -------------------------------------------------- -mkAlts dflags scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts) +combineIdenticalAlts :: OutId -> [InAlt] -> SimplM [InAlt] + +combineIdenticalAlts 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_` - returnSmpl better_alts + -- Also Note [Dead binders] + = do { tick (AltMerge case_bndr) + ; return ((DEFAULT, [], rhs1) : filtered_alts) } where filtered_alts = filter keep con_alts keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1) - better_alts = (DEFAULT, [], rhs1) : filtered_alts - --------------------------------------------------- --- 2. Merge nested cases --------------------------------------------------- - -mkAlts dflags scrut outer_bndr outer_alts - | dopt Opt_CaseMerge dflags, - (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 - munged_inner_alts = [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts] - munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs - - new_alts = mergeAlts outer_alts_without_deflt munged_inner_alts - -- The merge keeps the inner DEFAULT at the front, if there is one - -- and eliminates any inner_alts that are shadowed by the outer_alts - in - tick (CaseMerge outer_bndr) `thenSmpl_` - returnSmpl new_alts - -- Warning: don't call mkAlts recursively! +combineIdenticalAlts case_bndr alts = return alts + +------------------------------------------------------------------------- +-- Prepare the default alternative +------------------------------------------------------------------------- +prepareDefault :: DynFlags + -> OutExpr -- Scrutinee + -> OutId -- Case binder; need just for its type. Note that as an + -- OutId, it has maximum information; this is important. + -- Test simpl013 is an example + -> Maybe (TyCon, [Type]) -- Type of scrutinee, decomposed + -> [AltCon] -- These cons can't happen when matching the default + -> Maybe InExpr -- Rhs + -> SimplM [InAlt] -- Still unsimplified + -- We use a list because it's what mergeAlts expects, + -- And becuase case-merging can cause many to show up + +------- Merge nested cases ---------- +prepareDefault dflags scrut outer_bndr bndr_ty imposs_cons (Just deflt_rhs) + | dopt Opt_CaseMerge dflags + , Case (Var scrut_var) inner_bndr _ inner_alts <- deflt_rhs + , scruting_same_var scrut_var + = do { tick (CaseMerge outer_bndr) + + ; let munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs + ; return [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts] } + -- Warning: don't call prepareAlts recursively! -- Firstly, there's no point, because inner alts have already had -- 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 @@ -1240,18 +1277,54 @@ mkAlts dflags scrut outer_bndr outer_alts Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut other -> \ v -> v == outer_bndr ------------------------------------------------- --- Catch-all ------------------------------------------------- - -mkAlts dflags scrut case_bndr other_alts = returnSmpl other_alts +--------- Fill in known constructor ----------- +prepareDefault dflags scrut case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rhs) + | -- This branch handles the case where we are + -- scrutinisng an algebraic data type + 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 imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type + is_possible con = not (con `elem` imposs_data_cons) + && dataConCanMatch inst_tys con + = case filter is_possible all_cons of + [] -> return [] -- Eliminate the default alternative + -- altogether if it can't match + + [con] -> -- It matches exactly one constructor, so fill it in + do { tick (FillInCaseDefault case_bndr) + ; us <- getUniquesSmpl + ; let (ex_tvs, co_tvs, arg_ids) = + dataConRepInstPat us con inst_tys + ; return [(DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, deflt_rhs)] } + + two_or_more -> return [(DEFAULT, [], deflt_rhs)] + +--------- Catch-all cases ----------- +prepareDefault dflags scrut case_bndr bndr_ty imposs_cons (Just deflt_rhs) + = return [(DEFAULT, [], deflt_rhs)] + +prepareDefault dflags scrut case_bndr bndr_ty imposs_cons Nothing + = return [] -- No default branch \end{code} ================================================================================= -mkCase1 tries these things +mkCase tries these things 1. Eliminate the case altogether if possible @@ -1264,192 +1337,41 @@ mkCase1 tries these things and similar friends. -Start with a simple situation: - - case x# of ===> e[x#/y#] - y# -> e - -(when x#, y# are of primitive type, of course). We can't (in general) -do this for algebraic cases, because we might turn bottom into -non-bottom! - -Actually, we generalise this idea to look for a case where we're -scrutinising a variable, and we know that only the default case can -match. For example: -\begin{verbatim} - case x of - 0# -> ... - other -> ...(case x of - 0# -> ... - other -> ...) ... -\end{verbatim} -Here the inner case can be eliminated. This really only shows up in -eliminating error-checking code. - -We also make sure that we deal with this very common case: - - case e of - x -> ...x... - -Here we are using the case as a strict let; if x is used only once -then we want to inline it. We have to be careful that this doesn't -make the program terminate when it would have diverged before, so we -check that - - x is used strictly, or - - e is already evaluated (it may so if e is a variable) - -Lastly, we generalise the transformation to handle this: - - case e of ===> r - True -> r - False -> r - -We only do this for very cheaply compared r's (constructors, literals -and variables). If pedantic bottoms is on, we only do it when the -scrutinee is a PrimOp which can't fail. - -We do it *here*, looking at un-simplified alternatives, because we -have to check that r doesn't mention the variables bound by the -pattern in each alternative, so the binder-info is rather useful. - -So the case-elimination algorithm is: - - 1. Eliminate alternatives which can't match - - 2. Check whether all the remaining alternatives - (a) do not mention in their rhs any of the variables bound in their pattern - and (b) have equal rhss - - 3. Check we can safely ditch the case: - * PedanticBottoms is off, - or * the scrutinee is an already-evaluated variable - or * the scrutinee is a primop which is ok for speculation - -- ie we want to preserve divide-by-zero errors, and - -- calls to error itself! - - or * [Prim cases] the scrutinee is a primitive variable - - or * [Alg cases] the scrutinee is a variable and - either * the rhs is the same variable - (eg case x of C a b -> x ===> x) - or * there is only one alternative, the default alternative, - and the binder is used strictly in its scope. - [NB this is helped by the "use default binder where - possible" transformation; see below.] - - -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} +mkCase :: OutExpr -> OutId -> OutType + -> [OutAlt] -- Increasing order + -> SimplM OutExpr + -------------------------------------------------- --- 0. Check for empty alternatives +-- 1. Check for empty alternatives -------------------------------------------------- -- This isn't strictly an error. It's possible that the simplifer might "see" -- that an inner case has no accessible alternatives before it "sees" that the -- entire branch of an outer case is inaccessible. So we simply -- put an error case here insteadd -mkCase1 scrut case_bndr ty [] - = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $ +mkCase scrut case_bndr ty [] + = pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $ return (mkApps (Var eRROR_ID) [Type ty, Lit (mkStringLit "Impossible alternative")]) --------------------------------------------------- --- 1. Eliminate the case altogether if poss --------------------------------------------------- - -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, - -- then there is now only one (DEFAULT) rhs - | all isDeadBinder bndrs, - - -- Check that the scrutinee can be let-bound instead of case-bound - exprOkForSpeculation scrut - -- OK not to evaluate it - -- This includes things like (==# a# b#)::Bool - -- so that we simplify - -- case ==# a# b# of { True -> x; False -> x } - -- to just - -- x - -- This particular example shows up in default methods for - -- comparision operations (e.g. in (>=) for Int.Int32) - || exprIsHNF scrut -- It's already evaluated - || var_demanded_later scrut -- It'll be demanded later - --- || not opt_SimplPedanticBottoms) -- Or we don't care! --- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on, --- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate --- its argument: case x of { y -> dataToTag# y } --- 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) - - where - -- The case binder is going to be evaluated later, - -- and the scrutinee is a simple variable - var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr) - var_demanded_later other = False - -------------------------------------------------- -- 2. Identity case -------------------------------------------------- -mkCase1 scrut case_bndr ty alts -- Identity case +mkCase scrut case_bndr ty alts -- Identity case | all identity_alt alts = tick (CaseIdentity case_bndr) `thenSmpl_` returnSmpl (re_cast scrut) where - identity_alt (con, args, rhs) = de_cast rhs `cheapEqExpr` mk_id_rhs con args + identity_alt (con, args, rhs) = check_eq con args (de_cast rhs) - mk_id_rhs (DataAlt con) args = mkConApp con (arg_tys ++ varsToCoreExprs args) - mk_id_rhs (LitAlt lit) _ = Lit lit - mk_id_rhs DEFAULT _ = Var case_bndr + check_eq DEFAULT _ (Var v) = v == case_bndr + check_eq (LitAlt lit') _ (Lit lit) = lit == lit' + check_eq (DataAlt con) args rhs = rhs `cheapEqExpr` mkConApp con (arg_tys ++ varsToCoreExprs args) + || rhs `cheapEqExpr` Var case_bndr + check_eq con args rhs = False arg_tys = map Type (tyConAppArgs (idType case_bndr)) @@ -1474,7 +1396,7 @@ mkCase1 scrut case_bndr ty alts -- Identity case -------------------------------------------------- -- Catch-all -------------------------------------------------- -mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts) +mkCase scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts) \end{code}