X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FsimplCore%2FSimplUtils.lhs;h=56d2795e310d9db0f200e5e3432ee0d2abde31bc;hp=c87e1fcd01cd2b4c86f5cb79fe6711c48230f955;hb=b84ba676034763b3082bbd9405794a4fde499d14;hpb=01b453a5c3608f52707ee55374ca50cb592f567d diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs index c87e1fc..56d2795 100644 --- a/compiler/simplCore/SimplUtils.lhs +++ b/compiler/simplCore/SimplUtils.lhs @@ -6,17 +6,17 @@ \begin{code} module SimplUtils ( -- Rebuilding - mkLam, mkCase, prepareAlts, bindCaseBndr, + mkLam, mkCase, prepareAlts, -- Inlining, preInlineUnconditionally, postInlineUnconditionally, - activeInline, activeRule, + activeUnfolding, activeUnfInRule, activeRule, simplEnvForGHCi, simplEnvForRules, updModeForInlineRules, -- The continuation type SimplCont(..), DupFlag(..), ArgInfo(..), contIsDupable, contResultType, contIsTrivial, contArgs, dropArgs, - countValArgs, countArgs, + pushArgs, countValArgs, countArgs, addArgTo, mkBoringStop, mkRhsStop, mkLazyArgStop, contIsRhsOrArg, interestingCallContext, @@ -40,7 +40,7 @@ import CoreUnfold import Name import Id import Var ( isCoVar ) -import NewDemand +import Demand import SimplMonad import Type hiding( substTy ) import Coercion ( coercionKind ) @@ -99,44 +99,53 @@ data SimplCont | ApplyTo -- C arg DupFlag - InExpr SimplEnv -- The argument and its static env + InExpr StaticEnv -- The argument and its static env SimplCont | Select -- case C of alts DupFlag - InId [InAlt] SimplEnv -- The case binder, alts, and subst-env + InId [InAlt] StaticEnv -- The case binder, alts, and subst-env SimplCont -- The two strict forms have no DupFlag, because we never duplicate them | StrictBind -- (\x* \xs. e) C InId [InBndr] -- let x* = [] in e - InExpr SimplEnv -- is a special case + InExpr StaticEnv -- is a special case SimplCont - | StrictArg -- e C - OutExpr -- e; *always* of form (Var v `App1` e1 .. `App` en) - CallCtxt -- Whether *this* argument position is interesting - ArgInfo -- Whether the function at the head of e has rules, etc - SimplCont -- plus strictness flags for *further* args + | StrictArg -- f e1 ..en C + ArgInfo -- Specifies f, e1..en, Whether f has rules, etc + -- plus strictness flags for *further* args + CallCtxt -- Whether *this* argument position is interesting + SimplCont data ArgInfo = ArgInfo { - ai_rules :: Bool, -- Function has rules (recursively) - -- => be keener to inline in all args - ai_strs :: [Bool], -- Strictness of arguments + ai_fun :: Id, -- The function + ai_args :: [OutExpr], -- ...applied to these args (which are in *reverse* order) + ai_rules :: [CoreRule], -- Rules for this function + + ai_encl :: Bool, -- Flag saying whether this function + -- or an enclosing one has rules (recursively) + -- True => be keener to inline in all args + + ai_strs :: [Bool], -- Strictness of remaining arguments -- Usually infinite, but if it is finite it guarantees -- that the function diverges after being given -- that number of args - ai_discs :: [Int] -- Discounts for arguments; non-zero => be keener to inline + ai_discs :: [Int] -- Discounts for remaining arguments; non-zero => be keener to inline -- Always infinite } +addArgTo :: ArgInfo -> OutExpr -> ArgInfo +addArgTo ai arg = ai { ai_args = arg : ai_args ai } + instance Outputable SimplCont where ppr (Stop interesting) = ptext (sLit "Stop") <> brackets (ppr interesting) ppr (ApplyTo dup arg _ cont) = ((ptext (sLit "ApplyTo") <+> ppr dup <+> pprParendExpr arg) {- $$ nest 2 (pprSimplEnv se) -}) $$ ppr cont ppr (StrictBind b _ _ _ cont) = (ptext (sLit "StrictBind") <+> ppr b) $$ ppr cont - ppr (StrictArg f _ _ cont) = (ptext (sLit "StrictArg") <+> ppr f) $$ ppr cont + ppr (StrictArg ai _ cont) = (ptext (sLit "StrictArg") <+> ppr (ai_fun ai)) $$ ppr cont ppr (Select dup bndr alts _ cont) = (ptext (sLit "Select") <+> ppr dup <+> ppr bndr) $$ (nest 4 (ppr alts)) $$ ppr cont ppr (CoerceIt co cont) = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont @@ -191,13 +200,17 @@ contResultType env ty cont go (Stop {}) ty = ty go (CoerceIt co cont) _ = go cont (snd (coercionKind co)) go (StrictBind _ bs body se cont) _ = go cont (subst_ty se (exprType (mkLams bs body))) - go (StrictArg fn _ _ cont) _ = go cont (funResultTy (exprType fn)) + go (StrictArg ai _ cont) _ = go cont (funResultTy (argInfoResultTy ai)) go (Select _ _ alts se cont) _ = go cont (subst_ty se (coreAltsType alts)) go (ApplyTo _ arg se cont) ty = go cont (apply_to_arg ty arg se) apply_to_arg ty (Type ty_arg) se = applyTy ty (subst_ty se ty_arg) apply_to_arg ty _ _ = funResultTy ty +argInfoResultTy :: ArgInfo -> OutType +argInfoResultTy (ArgInfo { ai_fun = fun, ai_args = args }) + = foldr (\arg fn_ty -> applyTypeToArg fn_ty arg) (idType fun) args + ------------------- countValArgs :: SimplCont -> Int countValArgs (ApplyTo _ (Type _) _ cont) = countValArgs cont @@ -215,6 +228,10 @@ contArgs cont = go [] cont go args (ApplyTo _ arg se cont) = go (substExpr se arg : args) cont go args cont = (reverse args, cont) +pushArgs :: SimplEnv -> [CoreExpr] -> SimplCont -> SimplCont +pushArgs _env [] cont = cont +pushArgs env (arg:args) cont = ApplyTo NoDup arg env (pushArgs env args cont) + dropArgs :: Int -> SimplCont -> SimplCont dropArgs 0 cont = cont dropArgs n (ApplyTo _ _ _ cont) = dropArgs (n-1) cont @@ -275,10 +292,10 @@ interestingCallContext cont -- motivation to inline. See Note [Cast then apply] -- in CoreUnfold - interesting (StrictArg _ cci _ _) = cci - interesting (StrictBind {}) = BoringCtxt - interesting (Stop cci) = cci - interesting (CoerceIt _ cont) = interesting cont + interesting (StrictArg _ cci _) = cci + interesting (StrictBind {}) = BoringCtxt + interesting (Stop cci) = cci + interesting (CoerceIt _ cont) = interesting cont -- If this call is the arg of a strict function, the context -- is a bit interesting. If we inline here, we may get useful -- evaluation information to avoid repeated evals: e.g. @@ -304,18 +321,20 @@ mkArgInfo :: Id mkArgInfo fun rules n_val_args call_cont | n_val_args < idArity fun -- Note [Unsaturated functions] - = ArgInfo { ai_rules = False + = ArgInfo { ai_fun = fun, ai_args = [], ai_rules = rules + , ai_encl = False , ai_strs = vanilla_stricts , ai_discs = vanilla_discounts } | otherwise - = ArgInfo { ai_rules = interestingArgContext rules call_cont + = ArgInfo { ai_fun = fun, ai_args = [], ai_rules = rules + , ai_encl = interestingArgContext rules call_cont , ai_strs = add_type_str (idType fun) arg_stricts , ai_discs = arg_discounts } where vanilla_discounts, arg_discounts :: [Int] vanilla_discounts = repeat 0 arg_discounts = case idUnfolding fun of - CoreUnfolding {uf_guidance = UnfoldIfGoodArgs {ug_args = discounts}} + CoreUnfolding {uf_guidance = UnfIfGoodArgs {ug_args = discounts}} -> discounts ++ vanilla_discounts _ -> vanilla_discounts @@ -323,7 +342,7 @@ mkArgInfo fun rules n_val_args call_cont vanilla_stricts = repeat False arg_stricts - = case splitStrictSig (idNewStrictness fun) of + = case splitStrictSig (idStrictness fun) of (demands, result_info) | not (demands `lengthExceeds` n_val_args) -> -- Enough args, use the strictness given. @@ -392,12 +411,12 @@ interestingArgContext rules call_cont where enclosing_fn_has_rules = go call_cont - go (Select {}) = False - go (ApplyTo {}) = False - go (StrictArg _ cci _ _) = interesting cci - go (StrictBind {}) = False -- ?? - go (CoerceIt _ c) = go c - go (Stop cci) = interesting cci + go (Select {}) = False + go (ApplyTo {}) = False + go (StrictArg _ cci _) = interesting cci + go (StrictBind {}) = False -- ?? + go (CoerceIt _ c) = go c + go (Stop cci) = interesting cci interesting (ArgCtxt rules) = rules interesting _ = False @@ -616,11 +635,18 @@ let-float if you inline windowToViewport However, as usual for Gentle mode, do not inline things that are inactive in the intial stages. See Note [Gentle mode]. +Note [Top-level botomming Ids] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Don't inline top-level Ids that are bottoming, even if they are used just +once, because FloatOut has gone to some trouble to extract them out. +Inlining them won't make the program run faster! + \begin{code} preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool preInlineUnconditionally env top_lvl bndr rhs - | not active = False - | opt_SimplNoPreInlining = False + | not active = False + | isTopLevel top_lvl && isBottomingId bndr = False -- Note [Top-level bottoming Ids] + | opt_SimplNoPreInlining = False | otherwise = case idOccInfo bndr of IAmDead -> True -- Happens in ((\x.1) v) OneOcc in_lam True int_cxt -> try_once in_lam int_cxt @@ -632,12 +658,11 @@ preInlineUnconditionally env top_lvl bndr rhs -- See Note [pre/postInlineUnconditionally in gentle mode] SimplPhase n _ -> isActive n act act = idInlineActivation bndr - try_once in_lam int_cxt -- There's one textual occurrence | not in_lam = isNotTopLevel top_lvl || early_phase | otherwise = int_cxt && canInlineInLam rhs --- Be very careful before inlining inside a lambda, becuase (a) we must not +-- Be very careful before inlining inside a lambda, because (a) we must not -- invalidate occurrence information, and (b) we want to avoid pushing a -- single allocation (here) into multiple allocations (inside lambda). -- Inlining a *function* with a single *saturated* call would be ok, mind you. @@ -720,12 +745,13 @@ postInlineUnconditionally -> Unfolding -> Bool postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding - | not active = False - | isLoopBreaker occ_info = False -- If it's a loop-breaker of any kind, don't inline + | not active = False + | isLoopBreaker occ_info = False -- If it's a loop-breaker of any kind, don't inline -- because it might be referred to "earlier" - | isExportedId bndr = False - | isInlineRule unfolding = False -- Note [InlineRule and postInlineUnconditionally] - | exprIsTrivial rhs = True + | isExportedId bndr = False + | isStableUnfolding unfolding = False -- Note [InlineRule and postInlineUnconditionally] + | exprIsTrivial rhs = True + | isTopLevel top_lvl = False -- Note [Top level and postInlineUnconditionally] | otherwise = case occ_info of -- The point of examining occ_info here is that for *non-values* @@ -738,7 +764,8 @@ postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding -- case v of -- True -> case x of ... -- False -> case x of ... - -- I'm not sure how important this is in practice + -- This is very important in practice; e.g. wheel-seive1 doubles + -- in allocation if you miss this out OneOcc in_lam _one_br int_cxt -- OneOcc => no code-duplication issue -> smallEnoughToInline unfolding -- Small enough to dup -- ToDo: consider discount on smallEnoughToInline if int_cxt is true @@ -751,8 +778,8 @@ postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding -- PRINCIPLE: when we've already simplified an expression once, -- make sure that we only inline it if it's reasonably small. - && ((isNotTopLevel top_lvl && not in_lam) || - -- But outside a lambda, we want to be reasonably aggressive + && (not in_lam || + -- Outside a lambda, we want to be reasonably aggressive -- about inlining into multiple branches of case -- e.g. let x = -- in case y of { C1 -> ..x..; C2 -> ..x..; C3 -> ... } @@ -791,27 +818,56 @@ postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding SimplPhase n _ -> isActive n act act = idInlineActivation bndr -activeInline :: SimplEnv -> OutId -> Bool -activeInline env id - | isNonRuleLoopBreaker (idOccInfo id) -- Things with an INLINE pragma may have - -- an unfolding *and* be a loop breaker - = False -- (maybe the knot is not yet untied) - | otherwise +activeUnfolding :: SimplEnv -> IdUnfoldingFun +activeUnfolding env = case getMode env of - SimplGently { sm_inline = inlining_on } - -> inlining_on && isEarlyActive act - -- See Note [Gentle mode] - - -- NB: we used to have a second exception, for data con wrappers. - -- On the grounds that we use gentle mode for rule LHSs, and - -- they match better when data con wrappers are inlined. - -- But that only really applies to the trivial wrappers (like (:)), - -- and they are now constructed as Compulsory unfoldings (in MkId) - -- so they'll happen anyway. - - SimplPhase n _ -> isActive n act + SimplGently { sm_inline = False } -> active_unfolding_minimal + SimplGently { sm_inline = True } -> active_unfolding_gentle + SimplPhase n _ -> active_unfolding n + +activeUnfInRule :: SimplEnv -> IdUnfoldingFun +-- When matching in RULE, we want to "look through" an unfolding +-- if *rules* are on, even if *inlinings* are not. A notable example +-- is DFuns, which really we want to match in rules like (op dfun) +-- in gentle mode. +activeUnfInRule env + = case getMode env of + SimplGently { sm_rules = False } -> active_unfolding_minimal + SimplGently { sm_rules = True } -> active_unfolding_gentle + SimplPhase n _ -> active_unfolding n + +active_unfolding_minimal :: IdUnfoldingFun +-- Compuslory unfoldings only +-- Ignore SimplGently, because we want to inline regardless; +-- the Id has no top-level binding at all +-- +-- NB: we used to have a second exception, for data con wrappers. +-- On the grounds that we use gentle mode for rule LHSs, and +-- they match better when data con wrappers are inlined. +-- But that only really applies to the trivial wrappers (like (:)), +-- and they are now constructed as Compulsory unfoldings (in MkId) +-- so they'll happen anyway. +active_unfolding_minimal id + | isCompulsoryUnfolding unf = unf + | otherwise = NoUnfolding where - act = idInlineActivation id + unf = realIdUnfolding id -- Never a loop breaker + +active_unfolding_gentle :: IdUnfoldingFun +-- Anything that is early-active +-- See Note [Gentle mode] +active_unfolding_gentle id + | isEarlyActive (idInlineActivation id) = idUnfolding id + | otherwise = NoUnfolding + -- idUnfolding checks for loop-breakers + -- Things with an INLINE pragma may have + -- an unfolding *and* be a loop breaker + -- (maybe the knot is not yet untied) + +active_unfolding :: CompilerPhase -> IdUnfoldingFun +active_unfolding n id + | isActive n (idInlineActivation id) = idUnfolding id + | otherwise = NoUnfolding activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool) -- Nothing => No rules at all @@ -826,6 +882,14 @@ activeRule dflags env SimplPhase n _ -> Just (isActive n) \end{code} +Note [Top level and postInlineUnconditionally] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We don't do postInlineUnconditionally for top-level things (except +ones that are trivial). There is no point, because the main goal is +to get rid of local bindings used in multiple case branches. And +doing so risks replacing a single global allocation with local allocations. + + Note [InlineRule and postInlineUnconditionally] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Do not do postInlineUnconditionally if the Id has an InlineRule, otherwise @@ -1277,83 +1341,61 @@ Historical note: if you use let-bindings instead of a substitution, beware of th prepareAlts tries these things: -1. If several alternatives are identical, merge them into - a single DEFAULT alternative. I've occasionally seen this - making a big difference: +1. Eliminate alternatives that cannot match, including the + DEFAULT alternative. - case e of =====> case e of - C _ -> f x D v -> ....v.... - D v -> ....v.... DEFAULT -> f x - DEFAULT -> f x +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. - The point is that we merge common RHSs, at least for the DEFAULT case. - [One could do something more elaborate but I've never seen it needed.] - To avoid an expensive test, we just merge branches equal to the *first* - alternative; this picks up the common cases - a) all branches equal - b) some branches equal to the DEFAULT (which occurs first) +3. Returns a list of the constructors that cannot holds in the + DEFAULT alternative (if there is one) -2. Case merging: - case e of b { ==> case e of b { - p1 -> rhs1 p1 -> rhs1 - ... ... - pm -> rhsm pm -> rhsm - _ -> case b of b' { pn -> let b'=b in rhsn - pn -> rhsn ... - ... po -> let b'=b in rhso - po -> rhso _ -> let b'=b in rhsd - _ -> rhsd - } - - which merges two cases in one case when -- the default alternative of - the outer case scrutises the same variable as the outer case This - transformation is called Case Merging. It avoids that the same - variable is scrutinised multiple times. - - -The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs): +Here "cannot match" includes knowledge from GADTs - x | p `is` 1 -> e1 - | p `is` 2 -> e2 - ...etc... +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. -where @is@ was something like - - p `is` n = p /= (-1) && p == n +Eliminating the default alternative in (1) isn't so obvious, but it can +happen: -This gave rise to a horrible sequence of cases +data Colour = Red | Green | Blue - case p of - (-1) -> $j p - 1 -> e1 - DEFAULT -> $j p +f x = case x of + Red -> .. + Green -> .. + DEFAULT -> h x -and similarly in cascade for all the join points! +h y = case y of + Blue -> .. + DEFAULT -> [ case y of ... ] -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. +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 :: SimplEnv -> OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt]) -prepareAlts env scrut case_bndr' alts - = do { dflags <- getDOptsSmpl - ; alts <- combineIdenticalAlts case_bndr' alts - - ; let (alts_wo_default, maybe_deflt) = findDefault alts +prepareAlts :: OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt]) +prepareAlts scrut case_bndr' alts + = do { 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 non-DEFAULT branch in this case expression. - ; default_alts <- prepareDefault dflags env case_bndr' mb_tc_app + ; default_alts <- prepareDefault case_bndr' mb_tc_app imposs_deflt_cons maybe_deflt ; let trimmed_alts = filterOut impossible_alt alts_wo_default - merged_alts = mergeAlts trimmed_alts default_alts + merged_alts = mergeAlts trimmed_alts default_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 @@ -1374,29 +1416,7 @@ prepareAlts env scrut case_bndr' alts impossible_alt _ = False --------------------------------------------------- --- 1. Merge identical branches --------------------------------------------------- -combineIdenticalAlts :: OutId -> [InAlt] -> SimplM [InAlt] - -combineIdenticalAlts case_bndr ((_con1,bndrs1,rhs1) : con_alts) - | all isDeadBinder bndrs1, -- Remember the default - length filtered_alts < length con_alts -- alternative comes first - -- 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) - -combineIdenticalAlts _ alts = return alts - -------------------------------------------------------------------------- --- Prepare the default alternative -------------------------------------------------------------------------- -prepareDefault :: DynFlags - -> SimplEnv - -> OutId -- Case binder; need just for its type. Note that as an +prepareDefault :: 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 @@ -1404,42 +1424,9 @@ prepareDefault :: DynFlags -> 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 env outer_bndr _bndr_ty imposs_cons (Just deflt_rhs) - | dopt Opt_CaseMerge dflags - , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs - , DoneId inner_scrut_var' <- substId env inner_scrut_var - -- Remember, inner_scrut_var is an InId, but outer_bndr is an OutId - , inner_scrut_var' == outer_bndr - -- NB: the substId means that if the outer scrutinee was a - -- variable, and inner scrutinee is the same variable, - -- then inner_scrut_var' will be outer_bndr - -- via the magic of simplCaseBinder - = 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, - not (con `elem` imposs_cons) ] - -- NB: filter out any imposs_cons. Example: - -- case x of - -- A -> e1 - -- DEFAULT -> case x of - -- A -> e2 - -- B -> e3 - -- When we merge, we must ensure that e1 takes - -- precedence over e2 as the value for A! - } - -- 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 - -- in munge_rhs may put a case into the DEFAULT branch! - --------- Fill in known constructor ----------- -prepareDefault _ _ case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rhs) +prepareDefault 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. @@ -1458,7 +1445,7 @@ prepareDefault _ _ case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rh -- 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 - impossible con = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con + impossible con = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con = case filterOut impossible all_cons of [] -> return [] -- Eliminate the default alternative -- altogether if it can't match @@ -1473,27 +1460,48 @@ prepareDefault _ _ case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rh _ -> return [(DEFAULT, [], deflt_rhs)] | debugIsOn, isAlgTyCon tycon, not (isOpenTyCon tycon), null (tyConDataCons tycon) - -- This can legitimately happen for type families, so don't report that + -- Check for no data constructors + -- This can legitimately happen for type families, so don't report that = pprTrace "prepareDefault" (ppr case_bndr <+> ppr tycon) $ return [(DEFAULT, [], deflt_rhs)] --------- Catch-all cases ----------- -prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons (Just deflt_rhs) +prepareDefault _case_bndr _bndr_ty _imposs_cons (Just deflt_rhs) = return [(DEFAULT, [], deflt_rhs)] -prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons Nothing +prepareDefault _case_bndr _bndr_ty _imposs_cons Nothing = return [] -- No default branch \end{code} -================================================================================= +%************************************************************************ +%* * + mkCase +%* * +%************************************************************************ mkCase tries these things -1. Eliminate the case altogether if possible +1. Merge Nested Cases + + case e of b { ==> case e of b { + p1 -> rhs1 p1 -> rhs1 + ... ... + pm -> rhsm pm -> rhsm + _ -> case b of b' { pn -> let b'=b in rhsn + pn -> rhsn ... + ... po -> let b'=b in rhso + po -> rhso _ -> let b'=b in rhsd + _ -> rhsd + } + + which merges two cases in one case when -- the default alternative of + the outer case scrutises the same variable as the outer case. This + transformation is called Case Merging. It avoids that the same + variable is scrutinised multiple times. -2. Case-identity: +2. Eliminate Identity Case case e of ===> e True -> True; @@ -1501,19 +1509,99 @@ mkCase tries these things and similar friends. +3. Merge identical alternatives. + If several alternatives are identical, merge them into + a single DEFAULT alternative. I've occasionally seen this + making a big difference: + + case e of =====> case e of + C _ -> f x D v -> ....v.... + D v -> ....v.... DEFAULT -> f x + DEFAULT -> f x + + The point is that we merge common RHSs, at least for the DEFAULT case. + [One could do something more elaborate but I've never seen it needed.] + To avoid an expensive test, we just merge branches equal to the *first* + alternative; this picks up the common cases + a) all branches equal + b) some branches equal to the DEFAULT (which occurs first) + +The case where Merge Identical Alternatives transformation showed up +was like this (base/Foreign/C/Err/Error.lhs): + + x | p `is` 1 -> e1 + | p `is` 2 -> e2 + ...etc... + +where @is@ was something like + + p `is` n = p /= (-1) && p == n + +This gave rise to a horrible sequence of cases + + case p of + (-1) -> $j p + 1 -> e1 + DEFAULT -> $j p + +and similarly in cascade for all the join points! + \begin{code} -mkCase :: OutExpr -> OutId -> [OutAlt] -- Increasing order - -> SimplM OutExpr +mkCase, mkCase1, mkCase2 + :: DynFlags + -> OutExpr -> OutId + -> [OutAlt] -- Alternatives in standard (increasing) order + -> SimplM OutExpr -------------------------------------------------- --- 2. Identity case +-- 1. Merge Nested Cases -------------------------------------------------- -mkCase scrut case_bndr alts -- Identity case +mkCase dflags scrut outer_bndr ((DEFAULT, _, deflt_rhs) : outer_alts) + | dopt Opt_CaseMerge dflags + , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs + , inner_scrut_var == outer_bndr + = do { tick (CaseMerge outer_bndr) + + ; let wrap_alt (con, args, rhs) = ASSERT( outer_bndr `notElem` args ) + (con, args, wrap_rhs rhs) + -- Simplifier's no-shadowing invariant should ensure + -- that outer_bndr is not shadowed by the inner patterns + wrap_rhs rhs = Let (NonRec inner_bndr (Var outer_bndr)) rhs + -- The let is OK even for unboxed binders, + + wrapped_alts | isDeadBinder inner_bndr = inner_alts + | otherwise = map wrap_alt inner_alts + + merged_alts = mergeAlts outer_alts wrapped_alts + -- NB: mergeAlts gives priority to the left + -- case x of + -- A -> e1 + -- DEFAULT -> case x of + -- A -> e2 + -- B -> e3 + -- When we merge, we must ensure that e1 takes + -- precedence over e2 as the value for A! + + ; mkCase1 dflags scrut outer_bndr merged_alts + } + -- Warning: don't call mkCase 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 + -- in munge_rhs may put a case into the DEFAULT branch! + +mkCase dflags scrut bndr alts = mkCase1 dflags scrut bndr alts + +-------------------------------------------------- +-- 2. Eliminate Identity Case +-------------------------------------------------- + +mkCase1 _dflags scrut case_bndr alts -- Identity case | all identity_alt alts - = do tick (CaseIdentity case_bndr) - return (re_cast scrut) + = do { tick (CaseIdentity case_bndr) + ; return (re_cast scrut) } where identity_alt (con, args, rhs) = check_eq con args (de_cast rhs) @@ -1541,22 +1629,93 @@ mkCase scrut case_bndr alts -- Identity case (_,_,Cast _ co) -> Cast scrut co _ -> scrut +-------------------------------------------------- +-- 3. Merge Identical Alternatives +-------------------------------------------------- +mkCase1 dflags scrut case_bndr ((_con1,bndrs1,rhs1) : con_alts) + | all isDeadBinder bndrs1 -- Remember the default + , length filtered_alts < length con_alts -- alternative comes first + -- Also Note [Dead binders] + = do { tick (AltMerge case_bndr) + ; mkCase2 dflags scrut case_bndr alts' } + where + alts' = (DEFAULT, [], rhs1) : filtered_alts + filtered_alts = filter keep con_alts + keep (_con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1) +mkCase1 dflags scrut bndr alts = mkCase2 dflags scrut bndr alts -------------------------------------------------- -- Catch-all -------------------------------------------------- -mkCase scrut bndr alts = return (Case scrut bndr (coreAltsType alts) alts) +mkCase2 _dflags scrut bndr alts + = return (Case scrut bndr (coreAltsType alts) alts) \end{code} - -When adding auxiliary bindings for the case binder, it's worth checking if -its dead, because it often is, and occasionally these mkCase transformations -cascade rather nicely. - -\begin{code} -bindCaseBndr :: Id -> CoreExpr -> CoreExpr -> CoreExpr -bindCaseBndr bndr rhs body - | isDeadBinder bndr = body - | otherwise = bindNonRec bndr rhs body -\end{code} +Note [Dead binders] +~~~~~~~~~~~~~~~~~~~~ +Note that dead-ness is maintained by the simplifier, so that it is +accurate after simplification as well as before. + + +Note [Cascading case merge] +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Case merging should cascade in one sweep, because it +happens bottom-up + + case e of a { + DEFAULT -> case a of b + DEFAULT -> case b of c { + DEFAULT -> e + A -> ea + B -> eb + C -> ec +==> + case e of a { + DEFAULT -> case a of b + DEFAULT -> let c = b in e + A -> let c = b in ea + B -> eb + C -> ec +==> + case e of a { + DEFAULT -> let b = a in let c = b in e + A -> let b = a in let c = b in ea + B -> let b = a in eb + C -> ec + + +However here's a tricky case that we still don't catch, and I don't +see how to catch it in one pass: + + case x of c1 { I# a1 -> + case a1 of c2 -> + 0 -> ... + DEFAULT -> case x of c3 { I# a2 -> + case a2 of ... + +After occurrence analysis (and its binder-swap) we get this + + case x of c1 { I# a1 -> + let x = c1 in -- Binder-swap addition + case a1 of c2 -> + 0 -> ... + DEFAULT -> case x of c3 { I# a2 -> + case a2 of ... + +When we simplify the inner case x, we'll see that +x=c1=I# a1. So we'll bind a2 to a1, and get + + case x of c1 { I# a1 -> + case a1 of c2 -> + 0 -> ... + DEFAULT -> case a1 of ... + +This is corect, but we can't do a case merge in this sweep +because c2 /= a1. Reason: the binding c1=I# a1 went inwards +without getting changed to c1=I# c2. + +I don't think this is worth fixing, even if I knew how. It'll +all come out in the next pass anyway. + + \ No newline at end of file