X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FsimplCore%2FSimplify.lhs;h=f9cbc0af3eece5dbc288d8135544466dd6cf7964;hp=22c7a5a409d91c5a1a9375de96f8a801a1558cf3;hb=a263737bbf44050a7b5ecbe267ddf85d410b73e5;hpb=e79c9ce01d0ce4412bd4bcd99c8c728a6a2ec569 diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs index 22c7a5a..f9cbc0a 100644 --- a/compiler/simplCore/Simplify.lhs +++ b/compiler/simplCore/Simplify.lhs @@ -13,30 +13,32 @@ import SimplMonad import Type hiding ( substTy, extendTvSubst ) import SimplEnv import SimplUtils -import MkId ( rUNTIME_ERROR_ID ) import FamInstEnv ( FamInstEnv ) import Id +import MkId ( mkImpossibleExpr, seqId ) import Var import IdInfo import Coercion import FamInstEnv ( topNormaliseType ) -import DataCon ( dataConRepStrictness, dataConUnivTyVars ) +import DataCon ( DataCon, dataConWorkId, dataConRepStrictness ) import CoreSyn import NewDemand ( isStrictDmd, splitStrictSig ) import PprCore ( pprParendExpr, pprCoreExpr ) -import CoreUnfold ( mkUnfolding, callSiteInline, CallCtxt(..) ) +import CoreUnfold ( mkUnfolding, mkCoreUnfolding, mkInlineRule, + exprIsConApp_maybe, callSiteInline, CallCtxt(..) ) import CoreUtils +import qualified CoreSubst +import CoreArity ( exprArity ) import Rules ( lookupRule, getRules ) -import BasicTypes ( isMarkedStrict ) -import CostCentre ( currentCCS ) +import BasicTypes ( isMarkedStrict, Arity ) +import CostCentre ( currentCCS, pushCCisNop ) import TysPrim ( realWorldStatePrimTy ) import PrelInfo ( realWorldPrimId ) import BasicTypes ( TopLevelFlag(..), isTopLevel, RecFlag(..), isNonRuleLoopBreaker ) +import MonadUtils ( foldlM ) import Maybes ( orElse ) import Data.List ( mapAccumL ) -import MonadUtils ( foldlM ) -import StaticFlags ( opt_PassCaseBndrToJoinPoints ) import Outputable import FastString \end{code} @@ -202,7 +204,7 @@ expansion at a let RHS can concentrate solely on the PAP case. %************************************************************************ \begin{code} -simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind] +simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv simplTopBinds env0 binds0 = do { -- Put all the top-level binders into scope at the start @@ -215,7 +217,7 @@ simplTopBinds env0 binds0 dopt Opt_D_dump_rule_firings dflags ; env2 <- simpl_binds dump_flag env1 binds0 ; freeTick SimplifierDone - ; return (getFloats env2) } + ; return env2 } where -- We need to track the zapped top-level binders, because -- they should have their fragile IdInfo zapped (notably occurrence info) @@ -341,7 +343,7 @@ simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se ; (env', rhs') <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2) then -- No floating, just wrap up! - do { rhs' <- mkLam tvs' (wrapFloats body_env2 body2) + do { rhs' <- mkLam env tvs' (wrapFloats body_env2 body2) ; return (env, rhs') } else if null tvs then -- Simple floating @@ -351,8 +353,8 @@ simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se else -- Do type-abstraction first do { tick LetFloatFromLet ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2 - ; rhs' <- mkLam tvs' body3 - ; let env' = foldl (addPolyBind top_lvl) env poly_binds + ; rhs' <- mkLam env tvs' body3 + ; env' <- foldlM (addPolyBind top_lvl) env poly_binds ; return (env', rhs') } ; completeBind env' top_lvl bndr bndr1 rhs' } @@ -462,7 +464,8 @@ prepareRhs env0 rhs0 where is_val = n_val_args > 0 -- There is at least one arg -- ...and the fun a constructor or PAP - && (isDataConWorkId fun || n_val_args < idArity fun) + && (isConLikeId fun || n_val_args < idArity fun) + -- See Note [CONLIKE pragma] in BasicTypes go _ env other = return (False, env, other) \end{code} @@ -567,29 +570,23 @@ completeBind :: SimplEnv -- * or by adding to the floats in the envt completeBind env top_lvl old_bndr new_bndr new_rhs - | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding - -- Inline and discard the binding - = do { tick (PostInlineUnconditionally old_bndr) - ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $ - return (extendIdSubst env old_bndr (DoneEx new_rhs)) } - -- Use the substitution to make quite, quite sure that the - -- substitution will happen, since we are going to discard the binding + = do { let old_info = idInfo old_bndr + old_unf = unfoldingInfo old_info + occ_info = occInfo old_info - | otherwise - = return (addNonRecWithUnf env new_bndr new_rhs unfolding wkr) - where - unfolding | omit_unfolding = NoUnfolding - | otherwise = mkUnfolding (isTopLevel top_lvl) new_rhs - old_info = idInfo old_bndr - occ_info = occInfo old_info - wkr = substWorker env (workerInfo old_info) - omit_unfolding = isNonRuleLoopBreaker occ_info - -- or not (activeInline env old_bndr) - -- Do *not* trim the unfolding in SimplGently, else - -- the specialiser can't see it! - ------------------ -addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplEnv + ; new_unfolding <- simplUnfolding env top_lvl old_bndr occ_info new_rhs old_unf + + ; if postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs new_unfolding + -- Inline and discard the binding + then do { tick (PostInlineUnconditionally old_bndr) + ; return (extendIdSubst env old_bndr (DoneEx new_rhs)) } + -- Use the substitution to make quite, quite sure that the + -- substitution will happen, since we are going to discard the binding + + else return (addNonRecWithUnf env new_bndr new_rhs new_unfolding) } + +------------------------------ +addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv -- Add a new binding to the environment, complete with its unfolding -- but *do not* do postInlineUnconditionally, because we have already -- processed some of the scope of the binding @@ -602,71 +599,130 @@ addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplEnv -- opportunity to inline 'y' too. addPolyBind top_lvl env (NonRec poly_id rhs) - = addNonRecWithUnf env poly_id rhs unfolding NoWorker - where - unfolding | not (activeInline env poly_id) = NoUnfolding - | otherwise = mkUnfolding (isTopLevel top_lvl) rhs - -- addNonRecWithInfo adds the new binding in the - -- proper way (ie complete with unfolding etc), - -- and extends the in-scope set + = do { unfolding <- simplUnfolding env top_lvl poly_id NoOccInfo rhs noUnfolding + -- Assumes that poly_id did not have an INLINE prag + -- which is perhaps wrong. ToDo: think about this + ; return (addNonRecWithUnf env poly_id rhs unfolding) } -addPolyBind _ env bind@(Rec _) = extendFloats env bind +addPolyBind _ env bind@(Rec _) = return (extendFloats env bind) -- Hack: letrecs are more awkward, so we extend "by steam" -- without adding unfoldings etc. At worst this leads to -- more simplifier iterations ------------------ +------------------------------ addNonRecWithUnf :: SimplEnv - -> OutId -> OutExpr -- New binder and RHS - -> Unfolding -> WorkerInfo -- and unfolding - -> SimplEnv --- Add suitable IdInfo to the Id, add the binding to the floats, and extend the in-scope set -addNonRecWithUnf env new_bndr rhs unfolding wkr - = ASSERT( isId new_bndr ) + -> OutId -> OutExpr -- New binder and RHS + -> Unfolding -- New unfolding + -> SimplEnv +addNonRecWithUnf env new_bndr new_rhs new_unfolding + = let new_arity = exprArity new_rhs + old_arity = idArity new_bndr + info1 = idInfo new_bndr `setArityInfo` new_arity + + -- Unfolding info: Note [Setting the new unfolding] + info2 = info1 `setUnfoldingInfo` new_unfolding + + -- Demand info: Note [Setting the demand info] + info3 | isEvaldUnfolding new_unfolding = zapDemandInfo info2 `orElse` info2 + | otherwise = info2 + + final_id = new_bndr `setIdInfo` info3 + dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr + in + ASSERT( isId new_bndr ) WARN( new_arity < old_arity || new_arity < dmd_arity, - (ppr final_id <+> ppr old_arity <+> ppr new_arity <+> ppr dmd_arity) $$ ppr rhs ) - final_id `seq` -- This seq forces the Id, and hence its IdInfo, - -- and hence any inner substitutions - addNonRec env final_id rhs - -- The addNonRec adds it to the in-scope set too + (ptext (sLit "Arity decrease:") <+> ppr final_id <+> ppr old_arity + <+> ppr new_arity <+> ppr dmd_arity) ) + -- Note [Arity decrease] + + final_id `seq` -- This seq forces the Id, and hence its IdInfo, + -- and hence any inner substitutions + -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $ + addNonRec env final_id new_rhs + -- The addNonRec adds it to the in-scope set too + +------------------------------ +simplUnfolding :: SimplEnv-> TopLevelFlag + -> Id -- Debug output only + -> OccInfo -> OutExpr + -> Unfolding -> SimplM Unfolding +-- Note [Setting the new unfolding] +simplUnfolding env _ _ _ _ (DFunUnfolding con ops) + = return (DFunUnfolding con ops') where - dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr - old_arity = idArity new_bndr + ops' = map (CoreSubst.substExpr (mkCoreSubst env)) ops + +simplUnfolding env top_lvl _ _ _ + (CoreUnfolding { uf_tmpl = expr, uf_arity = arity + , uf_guidance = guide@(InlineRule {}) }) + = do { expr' <- simplExpr (setMode SimplGently env) expr + -- See Note [Simplifying gently inside InlineRules] in SimplUtils + ; let mb_wkr' = CoreSubst.substInlineRuleInfo (mkCoreSubst env) (ir_info guide) + ; return (mkCoreUnfolding (isTopLevel top_lvl) expr' arity + (guide { ir_info = mb_wkr' })) } + -- See Note [Top-level flag on inline rules] in CoreUnfold + +simplUnfolding _ top_lvl _ occ_info new_rhs _ + | omit_unfolding = return NoUnfolding + | otherwise = return (mkUnfolding (isTopLevel top_lvl) new_rhs) + where + omit_unfolding = isNonRuleLoopBreaker occ_info +\end{code} - -- Arity info - new_arity = exprArity rhs - new_bndr_info = idInfo new_bndr `setArityInfo` new_arity - - -- Unfolding info - -- Add the unfolding *only* for non-loop-breakers - -- Making loop breakers not have an unfolding at all - -- means that we can avoid tests in exprIsConApp, for example. - -- This is important: if exprIsConApp says 'yes' for a recursive - -- thing, then we can get into an infinite loop - - -- Demand info - -- If the unfolding is a value, the demand info may - -- go pear-shaped, so we nuke it. Example: - -- let x = (a,b) in - -- case x of (p,q) -> h p q x - -- Here x is certainly demanded. But after we've nuked - -- the case, we'll get just - -- let x = (a,b) in h a b x - -- and now x is not demanded (I'm assuming h is lazy) - -- This really happens. Similarly - -- let f = \x -> e in ...f..f... - -- After inlining f at some of its call sites the original binding may - -- (for example) be no longer strictly demanded. - -- The solution here is a bit ad hoc... - info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding - `setWorkerInfo` wkr - - final_info | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf - | otherwise = info_w_unf +Note [Arity decrease] +~~~~~~~~~~~~~~~~~~~~~ +Generally speaking the arity of a binding should not decrease. But it *can* +legitimately happen becuase of RULES. Eg + f = g Int +where g has arity 2, will have arity 2. But if there's a rewrite rule + g Int --> h +where h has arity 1, then f's arity will decrease. Here's a real-life example, +which is in the output of Specialise: + + Rec { + $dm {Arity 2} = \d.\x. op d + {-# RULES forall d. $dm Int d = $s$dm #-} - final_id = new_bndr `setIdInfo` final_info -\end{code} + dInt = MkD .... opInt ... + opInt {Arity 1} = $dm dInt + $s$dm {Arity 0} = \x. op dInt } + +Here opInt has arity 1; but when we apply the rule its arity drops to 0. +That's why Specialise goes to a little trouble to pin the right arity +on specialised functions too. + +Note [Setting the new unfolding] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +* If there's an INLINE pragma, we simplify the RHS gently. Maybe we + should do nothing at all, but simplifying gently might get rid of + more crap. + +* If not, we make an unfolding from the new RHS. But *only* for + non-loop-breakers. Making loop breakers not have an unfolding at all + means that we can avoid tests in exprIsConApp, for example. This is + important: if exprIsConApp says 'yes' for a recursive thing, then we + can get into an infinite loop + +If there's an InlineRule on a loop breaker, we hang on to the inlining. +It's pretty dodgy, but the user did say 'INLINE'. May need to revisit +this choice. + +Note [Setting the demand info] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +If the unfolding is a value, the demand info may +go pear-shaped, so we nuke it. Example: + let x = (a,b) in + case x of (p,q) -> h p q x +Here x is certainly demanded. But after we've nuked +the case, we'll get just + let x = (a,b) in h a b x +and now x is not demanded (I'm assuming h is lazy) +This really happens. Similarly + let f = \x -> e in ...f..f... +After inlining f at some of its call sites the original binding may +(for example) be no longer strictly demanded. +The solution here is a bit ad hoc... %************************************************************************ @@ -764,7 +820,7 @@ simplExprF' env expr@(Lam _ _) cont simplExprF' env (Type ty) cont = ASSERT( contIsRhsOrArg cont ) - do { ty' <- simplType env ty + do { ty' <- simplCoercion env ty ; rebuild env (Type ty') cont } simplExprF' env (Case scrut bndr _ alts) cont @@ -799,6 +855,14 @@ simplType env ty seqType new_ty `seq` return new_ty where new_ty = substTy env ty + +--------------------------------- +simplCoercion :: SimplEnv -> InType -> SimplM OutType +-- The InType isn't *necessarily* a coercion, but it might be +-- (in a type application, say) and optCoercion is a no-op on types +simplCoercion env co + = do { co' <- simplType env co + ; return (optCoercion co') } \end{code} @@ -836,7 +900,7 @@ rebuild env expr cont0 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont -> SimplM (SimplEnv, OutExpr) simplCast env body co0 cont0 - = do { co1 <- simplType env co0 + = do { co1 <- simplCoercion env co0 ; simplExprF env body (addCoerce co1 cont0) } where addCoerce co cont = add_coerce co (coercionKind co) cont @@ -848,8 +912,8 @@ simplCast env body co0 cont0 | (_l1, t1) <- coercionKind co2 -- e |> (g1 :: S1~L) |> (g2 :: L~T1) -- ==> - -- e, if T1=T2 - -- e |> (g1 . g2 :: T1~T2) otherwise + -- e, if S1=T1 + -- e |> (g1 . g2 :: S1~T1) otherwise -- -- For example, in the initial form of a worker -- we may find (coerce T (coerce S (\x.e))) y @@ -920,12 +984,12 @@ simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont) simplLam env bndrs body cont = do { (env', bndrs') <- simplLamBndrs env bndrs ; body' <- simplExpr env' body - ; new_lam <- mkLam bndrs' body' + ; new_lam <- mkLam env' bndrs' body' ; rebuild env' new_lam cont } ------------------ simplNonRecE :: SimplEnv - -> InId -- The binder + -> InBndr -- The binder -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda) -> ([InBndr], InExpr) -- Body of the let/lambda -- \xs.e @@ -981,24 +1045,15 @@ simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont -> SimplM (SimplEnv, OutExpr) simplNote env (SCC cc) e cont + | pushCCisNop cc (getEnclosingCC env) -- scc "f" (...(scc "f" e)...) + = simplExprF env e cont -- ==> scc "f" (...e...) + | otherwise = do { e' <- simplExpr (setEnclosingCC env currentCCS) e ; rebuild env (mkSCC cc e') cont } --- See notes with SimplMonad.inlineMode -simplNote env InlineMe e cont - | Just (inside, outside) <- splitInlineCont cont -- Boring boring continuation; see notes above - = do { -- Don't inline inside an INLINE expression - e' <- simplExprC (setMode inlineMode env) e inside - ; rebuild env (mkInlineMe e') outside } - - | otherwise -- Dissolve the InlineMe note if there's - -- an interesting context of any kind to combine with - -- (even a type application -- anything except Stop) - = simplExprF env e cont - -simplNote env (CoreNote s) e cont = do - e' <- simplExpr env e - rebuild env (Note (CoreNote s) e') cont +simplNote env (CoreNote s) e cont + = do { e' <- simplExpr env e + ; rebuild env (Note (CoreNote s) e') cont } \end{code} @@ -1030,8 +1085,7 @@ simplVar env var cont completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr) completeCall env var cont - = do { dflags <- getDOptsSmpl - ; let (args,call_cont) = contArgs cont + = do { let (args,call_cont) = contArgs cont -- The args are OutExprs, obtained by *lazily* substituting -- in the args found in cont. These args are only examined -- to limited depth (unless a rule fires). But we must do @@ -1047,45 +1101,20 @@ completeCall env var cont -- We used to use the black-listing mechanism to ensure that inlining of -- the wrapper didn't occur for things that have specialisations till a -- later phase, so but now we just try RULES first - -- - -- Note [Rules for recursive functions] - -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -- You might think that we shouldn't apply rules for a loop breaker: - -- doing so might give rise to an infinite loop, because a RULE is - -- rather like an extra equation for the function: - -- RULE: f (g x) y = x+y - -- Eqn: f a y = a-y - -- - -- But it's too drastic to disable rules for loop breakers. - -- Even the foldr/build rule would be disabled, because foldr - -- is recursive, and hence a loop breaker: - -- foldr k z (build g) = g k z - -- So it's up to the programmer: rules can cause divergence + -- + -- See also Note [Rules for recursive functions] ; rule_base <- getSimplRules - ; let in_scope = getInScope env - rules = getRules rule_base var - maybe_rule = case activeRule dflags env of - Nothing -> Nothing -- No rules apply - Just act_fn -> lookupRule act_fn in_scope - var args rules - ; case maybe_rule of { - Just (rule, rule_rhs) -> do - tick (RuleFired (ru_name rule)) - (if dopt Opt_D_dump_rule_firings dflags then - pprTrace "Rule fired" (vcat [ - text "Rule:" <+> ftext (ru_name rule), - text "Before:" <+> ppr var <+> sep (map pprParendExpr args), - text "After: " <+> pprCoreExpr rule_rhs, - text "Cont: " <+> ppr call_cont]) - else - id) $ - simplExprF env rule_rhs (dropArgs (ruleArity rule) cont) + ; let rules = getRules rule_base var + ; mb_rule <- tryRules env var rules args call_cont + ; case mb_rule of { + Just (n_args, rule_rhs) -> simplExprF env rule_rhs (dropArgs n_args cont) ; -- The ruleArity says how many args the rule consumed + ; Nothing -> do -- No rules - ; Nothing -> do -- No rules ------------- Next try inlining ---------------- - { let arg_infos = [interestingArg arg | arg <- args, isValArg arg] + { dflags <- getDOptsSmpl + ; let arg_infos = [interestingArg arg | arg <- args, isValArg arg] n_val_args = length arg_infos interesting_cont = interestingCallContext call_cont active_inline = activeInline env var @@ -1095,7 +1124,7 @@ completeCall env var cont Just unfolding -- There is an inlining! -> do { tick (UnfoldingDone var) ; (if dopt Opt_D_dump_inlinings dflags then - pprTrace ("Inlining done" ++ showSDoc (ppr var)) (vcat [ + pprTrace ("Inlining done: " ++ showSDoc (ppr var)) (vcat [ text "Before:" <+> ppr var <+> sep (map pprParendExpr args), text "Inlined fn: " <+> nest 2 (ppr unfolding), text "Cont: " <+> ppr call_cont]) @@ -1109,7 +1138,8 @@ completeCall env var cont -- Next, look for rules or specialisations that match -- rebuildCall env (Var var) - (mkArgInfo var n_val_args call_cont) cont + (mkArgInfo var rules n_val_args call_cont) + cont }}}} rebuildCall :: SimplEnv @@ -1138,7 +1168,7 @@ rebuildCall env fun (ArgInfo { ai_strs = [] }) cont | otherwise = mkCoerce co expr rebuildCall env fun info (ApplyTo _ (Type arg_ty) se cont) - = do { ty' <- simplType (se `setInScope` env) arg_ty + = do { ty' <- simplCoercion (se `setInScope` env) arg_ty ; rebuildCall env (fun `App` Type ty') info cont } rebuildCall env fun @@ -1191,6 +1221,58 @@ to get the effect that finding (error "foo") in a strict arg position will discard the entire application and replace it with (error "foo"). Getting all this at once is TOO HARD! + +%************************************************************************ +%* * + Rewrite rules +%* * +%************************************************************************ + +\begin{code} +tryRules :: SimplEnv + -> Id -> [CoreRule] -> [OutExpr] -> SimplCont + -> SimplM (Maybe (Arity, CoreExpr)) -- The arity is the number of + -- args consumed by the rule +tryRules env fn rules args call_cont + | null rules + = return Nothing + | otherwise + = do { dflags <- getDOptsSmpl + ; case activeRule dflags env of { + Nothing -> return Nothing ; -- No rules apply + Just act_fn -> + + case lookupRule act_fn (getInScope env) fn args rules of { + Nothing -> return Nothing ; -- No rule matches + Just (rule, rule_rhs) -> + + do { tick (RuleFired (ru_name rule)) + ; (if dopt Opt_D_dump_rule_firings dflags then + pprTrace "Rule fired" (vcat [ + text "Rule:" <+> ftext (ru_name rule), + text "Before:" <+> ppr fn <+> sep (map pprParendExpr args), + text "After: " <+> pprCoreExpr rule_rhs, + text "Cont: " <+> ppr call_cont]) + else + id) $ + return (Just (ruleArity rule, rule_rhs)) }}}} +\end{code} + +Note [Rules for recursive functions] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +You might think that we shouldn't apply rules for a loop breaker: +doing so might give rise to an infinite loop, because a RULE is +rather like an extra equation for the function: + RULE: f (g x) y = x+y + Eqn: f a y = a-y + +But it's too drastic to disable rules for loop breakers. +Even the foldr/build rule would be disabled, because foldr +is recursive, and hence a loop breaker: + foldr k z (build g) = g k z +So it's up to the programmer: rules can cause divergence + + %************************************************************************ %* * Rebuilding a cse expression @@ -1287,26 +1369,40 @@ I don't really know how to improve this situation. --------------------------------------------------------- -- Eliminate the case if possible -rebuildCase :: SimplEnv - -> OutExpr -- Scrutinee - -> InId -- Case binder - -> [InAlt] -- Alternatives (inceasing order) - -> SimplCont - -> SimplM (SimplEnv, OutExpr) +rebuildCase, reallyRebuildCase + :: SimplEnv + -> OutExpr -- Scrutinee + -> InId -- Case binder + -> [InAlt] -- Alternatives (inceasing order) + -> SimplCont + -> SimplM (SimplEnv, OutExpr) -------------------------------------------------- -- 1. Eliminate the case if there's a known constructor -------------------------------------------------- rebuildCase env scrut case_bndr alts cont - | Just (con,args) <- exprIsConApp_maybe scrut - -- Works when the scrutinee is a variable with a known unfolding - -- as well as when it's an explicit constructor application - = knownCon env scrut (DataAlt con) args case_bndr alts cont - | Lit lit <- scrut -- No need for same treatment as constructors -- because literals are inlined more vigorously - = knownCon env scrut (LitAlt lit) [] case_bndr alts cont + = do { tick (KnownBranch case_bndr) + ; case findAlt (LitAlt lit) alts of + Nothing -> missingAlt env case_bndr alts cont + Just (_, bs, rhs) -> simple_rhs bs rhs } + + | Just (con, ty_args, other_args) <- exprIsConApp_maybe scrut + -- Works when the scrutinee is a variable with a known unfolding + -- as well as when it's an explicit constructor application + = do { tick (KnownBranch case_bndr) + ; case findAlt (DataAlt con) alts of + Nothing -> missingAlt env case_bndr alts cont + Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs + Just (_, bs, rhs) -> knownCon env scrut con ty_args other_args + case_bndr bs rhs cont + } + where + simple_rhs bs rhs = ASSERT( null bs ) + do { env' <- simplNonRecX env case_bndr scrut + ; simplExprF env' rhs cont } -------------------------------------------------- @@ -1353,12 +1449,31 @@ rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont -- exprOkForSpeculation was intended for. var_demanded_later _ = False +rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont + | all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq' + = -- For this case, see Note [User-defined RULES for seq] in MkId + do { let rhs' = substExpr env rhs + out_args = [Type (substTy env (idType case_bndr)), + Type (exprType rhs'), scrut, rhs'] + -- Lazily evaluated, so we don't do most of this + + ; rule_base <- getSimplRules + ; let rules = getRules rule_base seqId + ; mb_rule <- tryRules env seqId rules out_args cont + ; case mb_rule of + Just (n_args, res) -> simplExprF (zapSubstEnv env) + (mkApps res (drop n_args out_args)) + cont + Nothing -> reallyRebuildCase env scrut case_bndr alts cont } + +rebuildCase env scrut case_bndr alts cont + = reallyRebuildCase env scrut case_bndr alts cont -------------------------------------------------- -- 3. Catch-all case -------------------------------------------------- -rebuildCase env scrut case_bndr alts cont +reallyRebuildCase env scrut case_bndr alts cont = do { -- Prepare the continuation; -- The new subst_env is in place (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont @@ -1367,17 +1482,7 @@ rebuildCase env scrut case_bndr alts cont ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont -- Check for empty alternatives - ; if null alts' then - -- This isn't strictly an error, although it is unusual. - -- 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 instead. - pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $ - let res_ty' = contResultType env' (substTy env' (coreAltsType alts)) dup_cont - lit = mkStringLit "Impossible alternative" - in return (env', mkApps (Var rUNTIME_ERROR_ID) [Type res_ty', lit]) - + ; if null alts' then missingAlt env case_bndr alts cont else do { case_expr <- mkCase scrut' case_bndr' alts' @@ -1408,6 +1513,19 @@ The point is that we bring into the envt a binding after the outer case, and that makes (a,b) alive. At least we do unless the case binder is guaranteed dead. +In practice, the scrutinee is almost always a variable, so we pretty +much always zap the OccInfo of the binders. It doesn't matter much though. + + +Note [Case of cast] +~~~~~~~~~~~~~~~~~~~ +Consider case (v `cast` co) of x { I# y -> + ... (case (v `cast` co) of {...}) ... +We'd like to eliminate the inner case. We can get this neatly by +arranging that inside the outer case we add the unfolding + v |-> x `cast` (sym co) +to v. Then we should inline v at the inner case, cancel the casts, and away we go + Note [Improving seq] ~~~~~~~~~~~~~~~~~~~ Consider @@ -1422,10 +1540,31 @@ where x::F Int. Then we'd like to rewrite (F Int) to Int, getting I# x# -> let x = x' `cast` sym co in rhs -so that 'rhs' can take advantage of the form of x'. Notice that Note -[Case of cast] may then apply to the result. - -This showed up in Roman's experiments. Example: +so that 'rhs' can take advantage of the form of x'. + +Notice that Note [Case of cast] may then apply to the result. + +Nota Bene: We only do the [Improving seq] transformation if the +case binder 'x' is actually used in the rhs; that is, if the case +is *not* a *pure* seq. + a) There is no point in adding the cast to a pure seq. + b) There is a good reason not to: doing so would interfere + with seq rules (Note [Built-in RULES for seq] in MkId). + In particular, this [Improving seq] thing *adds* a cast + while [Built-in RULES for seq] *removes* one, so they + just flip-flop. + +You might worry about + case v of x { __DEFAULT -> + ... case (v `cast` co) of y { I# -> ... }} +This is a pure seq (since x is unused), so [Improving seq] won't happen. +But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get + case v of x { __DEFAULT -> + ... case (x `cast` co) of y { I# -> ... }} +Now the outer case is not a pure seq, so [Improving seq] will happen, +and then the inner case will disappear. + +The need for [Improving seq] showed up in Roman's experiments. Example: foo :: F Int -> Int -> Int foo t n = t `seq` bar n where @@ -1434,64 +1573,9 @@ This showed up in Roman's experiments. Example: Here we'd like to avoid repeated evaluating t inside the loop, by taking advantage of the `seq`. -At one point I did transformation in LiberateCase, but it's more robust here. -(Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before -LiberateCase gets to see it.) - - -Historical note [no-case-of-case] -~~~~~~~~~~~~~~~~~~~~~~ -We *used* to suppress the binder-swap in case expressoins when --fno-case-of-case is on. Old remarks: - "This happens in the first simplifier pass, - and enhances full laziness. Here's the bad case: - f = \ y -> ...(case x of I# v -> ...(case x of ...) ... ) - If we eliminate the inner case, we trap it inside the I# v -> arm, - which might prevent some full laziness happening. I've seen this - in action in spectral/cichelli/Prog.hs: - [(m,n) | m <- [1..max], n <- [1..max]] - Hence the check for NoCaseOfCase." -However, now the full-laziness pass itself reverses the binder-swap, so this -check is no longer necessary. - -Historical note [Suppressing the case binder-swap] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -There is another situation when it might make sense to suppress the -case-expression binde-swap. If we have - - case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 } - ...other cases .... } - -We'll perform the binder-swap for the outer case, giving - - case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 } - ...other cases .... } - -But there is no point in doing it for the inner case, because w1 can't -be inlined anyway. Furthermore, doing the case-swapping involves -zapping w2's occurrence info (see paragraphs that follow), and that -forces us to bind w2 when doing case merging. So we get - - case x of w1 { A -> let w2 = w1 in e1 - B -> let w2 = w1 in e2 - ...other cases .... } - -This is plain silly in the common case where w2 is dead. - -Even so, I can't see a good way to implement this idea. I tried -not doing the binder-swap if the scrutinee was already evaluated -but that failed big-time: - - data T = MkT !Int - - case v of w { MkT x -> - case x of x1 { I# y1 -> - case x of x2 { I# y2 -> ... - -Notice that because MkT is strict, x is marked "evaluated". But to -eliminate the last case, we must either make sure that x (as well as -x1) has unfolding MkT y1. THe straightforward thing to do is to do -the binder-swap. So this whole note is a no-op. +At one point I did transformation in LiberateCase, but it's more +robust here. (Otherwise, there's a danger that we'll simply drop the +'seq' altogether, before LiberateCase gets to see it.) \begin{code} @@ -1500,8 +1584,9 @@ improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv -> SimplM (SimplEnv, OutExpr, OutId) -- Note [Improving seq] improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)] - | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1) - = do { case_bndr2 <- newId (fsLit "nt") ty2 + | not (isDeadBinder case_bndr) -- Not a pure seq! See the Note! + , Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1) + = do { case_bndr2 <- newId (fsLit "nt") ty2 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co) env2 = extendIdSubst env case_bndr rhs ; return (env2, scrut `Cast` co, case_bndr2) } @@ -1710,34 +1795,15 @@ and then All this should happen in one sweep. \begin{code} -knownCon :: SimplEnv -> OutExpr -> AltCon - -> [OutExpr] -- Args *including* the universal args - -> InId -> [InAlt] -> SimplCont - -> SimplM (SimplEnv, OutExpr) - -knownCon env scrut con args bndr alts cont - = do { tick (KnownBranch bndr) - ; knownAlt env scrut args bndr (findAlt con alts) cont } - -knownAlt :: SimplEnv -> OutExpr -> [OutExpr] - -> InId -> (AltCon, [CoreBndr], InExpr) -> SimplCont +knownCon :: SimplEnv + -> OutExpr -- The scrutinee + -> DataCon -> [OutType] -> [OutExpr] -- The scrutinee (in pieces) + -> InId -> [InBndr] -> InExpr -- The alternative + -> SimplCont -> SimplM (SimplEnv, OutExpr) -knownAlt env scrut _ bndr (DEFAULT, bs, rhs) cont - = ASSERT( null bs ) - do { env' <- simplNonRecX env bndr scrut - -- This might give rise to a binding with non-atomic args - -- like x = Node (f x) (g x) - -- but simplNonRecX will atomic-ify it - ; simplExprF env' rhs cont } -knownAlt env scrut _ bndr (LitAlt _, bs, rhs) cont - = ASSERT( null bs ) - do { env' <- simplNonRecX env bndr scrut - ; simplExprF env' rhs cont } - -knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont - = do { let n_drop_tys = length (dataConUnivTyVars dc) - ; env' <- bind_args env bs (drop n_drop_tys the_args) +knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont + = do { env' <- bind_args env bs dc_args ; let -- It's useful to bind bndr to scrut, rather than to a fresh -- binding x = Con arg1 .. argn @@ -1746,12 +1812,12 @@ knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont -- BUT, if scrut is a not a variable, we must be careful -- about duplicating the arg redexes; in that case, make -- a new con-app from the args - bndr_rhs = case scrut of - Var _ -> scrut - _ -> con_app - con_app = mkConApp dc (take n_drop_tys the_args ++ con_args) - con_args = [substExpr env' (varToCoreExpr b) | b <- bs] - -- args are aready OutExprs, but bs are InIds + bndr_rhs | exprIsTrivial scrut = scrut + | otherwise = con_app + con_app = Var (dataConWorkId dc) + `mkTyApps` dc_ty_args + `mkApps` [substExpr env' (varToCoreExpr b) | b <- bs] + -- dc_ty_args are aready OutTypes, but bs are InBndrs ; env'' <- simplNonRecX env' bndr bndr_rhs ; simplExprF env'' rhs cont } @@ -1777,8 +1843,21 @@ knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont ; bind_args env'' bs' args } bind_args _ _ _ = - pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$ + pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$ text "scrut:" <+> ppr scrut + +------------------- +missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr) + -- This isn't strictly an error, although it is unusual. + -- 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 instead. +missingAlt env case_bndr alts cont + = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr ) + return (env, mkImpossibleExpr res_ty) + where + res_ty = contResultType env (substTy env (coreAltsType alts)) cont \end{code} @@ -1817,11 +1896,20 @@ mkDupableCont env (CoerceIt ty cont) mkDupableCont env cont@(StrictBind {}) = return (env, mkBoringStop, cont) - -- See Note [Duplicating strict continuations] + -- See Note [Duplicating StrictBind] -mkDupableCont env cont@(StrictArg {}) - = return (env, mkBoringStop, cont) - -- See Note [Duplicating strict continuations] +mkDupableCont env (StrictArg fun cci ai cont) + -- See Note [Duplicating StrictArg] + = do { (env', dup, nodup) <- mkDupableCont env cont + ; (env'', fun') <- mk_dupable_call env' fun + ; return (env'', StrictArg fun' cci ai dup, nodup) } + where + mk_dupable_call env (Var v) = return (env, Var v) + mk_dupable_call env (App fun arg) = do { (env', fun') <- mk_dupable_call env fun + ; (env'', arg') <- makeTrivial env' arg + ; return (env'', fun' `App` arg') } + mk_dupable_call _ other = pprPanic "mk_dupable_call" (ppr other) + -- The invariant of StrictArg is that the first arg is always an App chain mkDupableCont env (ApplyTo _ arg se cont) = -- e.g. [...hole...] (...arg...) @@ -1890,97 +1978,91 @@ mkDupableAlts env case_bndr' the_alts mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr) -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr)) -mkDupableAlt env case_bndr1 (con, bndrs1, rhs1) - | exprIsDupable rhs1 -- Note [Small alternative rhs] - = return (env, (con, bndrs1, rhs1)) +mkDupableAlt env case_bndr (con, bndrs', rhs') + | exprIsDupable rhs' -- Note [Small alternative rhs] + = return (env, (con, bndrs', rhs')) | otherwise - = do { let abstract_over bndr + = do { let rhs_ty' = exprType rhs' + scrut_ty = idType case_bndr + case_bndr_w_unf + = case con of + DEFAULT -> case_bndr + DataAlt dc -> setIdUnfolding case_bndr unf + where + -- See Note [Case binders and join points] + unf = mkInlineRule InlSat rhs 0 + rhs = mkConApp dc (map Type (tyConAppArgs scrut_ty) + ++ varsToCoreExprs bndrs') + + LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt") + <+> ppr case_bndr <+> ppr con ) + case_bndr + -- The case binder is alive but trivial, so why has + -- it not been substituted away? + + used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs' + | otherwise = bndrs' ++ [case_bndr_w_unf] + + abstract_over bndr | isTyVar bndr = True -- Abstract over all type variables just in case | otherwise = not (isDeadBinder bndr) -- The deadness info on the new Ids is preserved by simplBinders - inst_tys1 = tyConAppArgs (idType case_bndr1) - con_app dc = mkConApp dc (map Type inst_tys1 ++ varsToCoreExprs bndrs1) - - (rhs2, final_bndrs) -- See Note [Passing the case binder to join points] - | isDeadBinder case_bndr1 - = (rhs1, filter abstract_over bndrs1) - | opt_PassCaseBndrToJoinPoints, not (null bndrs1) - = (rhs1, (case_bndr1 : filter abstract_over bndrs1)) - | otherwise - = case con of - DataAlt dc -> (Let (NonRec case_bndr1 (con_app dc)) rhs1, bndrs1) - LitAlt lit -> ASSERT( null bndrs1 ) (Let (NonRec case_bndr1 (Lit lit)) rhs1, []) - DEFAULT -> ASSERT( null bndrs1 ) (rhs1, [case_bndr1]) - - ; (final_bndrs1, final_args) -- Note [Join point abstraction] - <- if (any isId final_bndrs) - then return (final_bndrs, varsToCoreExprs final_bndrs) + ; (final_bndrs', final_args) -- Note [Join point abstraction] + <- if (any isId used_bndrs') + then return (used_bndrs', varsToCoreExprs used_bndrs') else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy - ; return (rw_id : final_bndrs, - Var realWorldPrimId : varsToCoreExprs final_bndrs) } + ; return ([rw_id], [Var realWorldPrimId]) } - ; let rhs_ty1 = exprType rhs1 - ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs1 rhs_ty1) + ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty') -- Note [Funky mkPiTypes] ; let -- We make the lambdas into one-shot-lambdas. The -- join point is sure to be applied at most once, and doing so -- prevents the body of the join point being floated out by -- the full laziness pass - really_final_bndrs = map one_shot final_bndrs1 + really_final_bndrs = map one_shot final_bndrs' one_shot v | isId v = setOneShotLambda v | otherwise = v - join_rhs = mkLams really_final_bndrs rhs2 + join_rhs = mkLams really_final_bndrs rhs' join_call = mkApps (Var join_bndr) final_args - ; env1 <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs) - ; return (env1, (con, bndrs1, join_call)) } + ; env' <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs) + ; return (env', (con, bndrs', join_call)) } -- See Note [Duplicated env] \end{code} -Note [Passing the case binder to join points] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Suppose we have - case e of cb { C1 -> r1[cb]; C2 x y z -> r2[cb,x] } -and we want to make join points for the two alternatives, -which mention the case binder 'cb'. Should we pass 'cb' to -the join point, or reconstruct it? Here are the two alternatives -for the C2 alternative: +Note [Case binders and join points] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider this + case (case .. ) of c { + I# c# -> ....c.... + +If we make a join point with c but not c# we get + $j = \c -> ....c.... - Plan A(pass cb): j2 cb x = r2[cb,x] +But if later inlining scrutines the c, thus - Plan B(reconstruct cb): j2 x y z = let cb = C2 x y z in r2[cb,x] + $j = \c -> ... case c of { I# y -> ... } ... -The advantge of Plan B is that we can "see" the definition of cb -in r2, and that may be important when we inline stuff in r2. The -disadvantage is that if this optimisation doesn't happen, we end up -re-allocating C2, when it already exists. This does happen occasionally; -an example is the function nofib/spectral/cichelli/Auxil.$whinsert. +we won't see that 'c' has already been scrutinised. This actually +happens in the 'tabulate' function in wave4main, and makes a significant +difference to allocation. -Plan B is always better if the constructor is nullary. +An alternative plan is this: -In both cases we don't have liveness info for cb on a branch-by-branch -basis, and it's possible that 'cb' is used in some branches but not -others. Well, the absence analyser will find that out later, so it's -not too bad. + $j = \c# -> let c = I# c# in ...c.... -Sadly, at the time of writing, neither choice seems an unequivocal -win. Here are nofib results, for adding -fpass-case-bndr-to-join-points -(all others are zero effect): +but that is bad if 'c' is *not* later scrutinised. - Program Size Allocs Runtime Elapsed --------------------------------------------------------------------------------- - cichelli +0.0% -4.4% 0.13 0.13 - pic +0.0% -0.7% 0.01 0.04 - transform -0.0% +2.8% -0.4% -9.1% - wave4main +0.0% +10.5% +3.1% +3.4% --------------------------------------------------------------------------------- - Min -0.0% -4.4% -7.0% -31.9% - Max +0.1% +10.5% +3.1% +15.0% - Geometric Mean +0.0% +0.1% -1.7% -6.1% +So instead we do both: we pass 'c' and 'c#' , and record in c's inlining +that it's really I# c#, thus + + $j = \c# -> \c[=I# c#] -> ...c.... +Absence analysis may later discard 'c'. + Note [Duplicated env] ~~~~~~~~~~~~~~~~~~~~~ Some of the alternatives are simplified, but have not been turned into a join point @@ -1990,7 +2072,7 @@ we'd lose that when zapping the subst-env. We could have a per-alt subst-env, but zapping it (as we do in mkDupableCont, the Select case) is safe, and at worst delays the join-point inlining. -Note [Small alterantive rhs] +Note [Small alternative rhs] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It is worth checking for a small RHS because otherwise we get extra let bindings that may cause an extra iteration of the simplifier to @@ -2059,32 +2141,71 @@ It's a bit silly to add the realWorld dummy arg in this case, making True -> $j s (the \v alone is enough to make CPR happy) but I think it's rare -Note [Duplicating strict continuations] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Do *not* duplicate StrictBind and StritArg continuations. We gain -nothing by propagating them into the expressions, and we do lose a -lot. Here's an example: - && (case x of { T -> F; F -> T }) E +Note [Duplicating StrictArg] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The original plan had (where E is a big argument) +e.g. f E [..hole..] + ==> let $j = \a -> f E a + in $j [..hole..] + +But this is terrible! Here's an example: + && E (case x of { T -> F; F -> T }) Now, && is strict so we end up simplifying the case with an ArgOf continuation. If we let-bind it, we get - - let $j = \v -> && v E + let $j = \v -> && E v in simplExpr (case x of { T -> F; F -> T }) (ArgOf (\r -> $j r) And after simplifying more we get - - let $j = \v -> && v E + let $j = \v -> && E v in case x of { T -> $j F; F -> $j T } Which is a Very Bad Thing +What we do now is this + f E [..hole..] + ==> let a = E + in f a [..hole..] +Now if the thing in the hole is a case expression (which is when +we'll call mkDupableCont), we'll push the function call into the +branches, which is what we want. Now RULES for f may fire, and +call-pattern specialisation. Here's an example from Trac #3116 + go (n+1) (case l of + 1 -> bs' + _ -> Chunk p fpc (o+1) (l-1) bs') +If we can push the call for 'go' inside the case, we get +call-pattern specialisation for 'go', which is *crucial* for +this program. + +Here is the (&&) example: + && E (case x of { T -> F; F -> T }) + ==> let a = E in + case x of { T -> && a F; F -> && a T } +Much better! + +Notice that + * Arguments to f *after* the strict one are handled by + the ApplyTo case of mkDupableCont. Eg + f [..hole..] E + + * We can only do the let-binding of E because the function + part of a StrictArg continuation is an explicit syntax + tree. In earlier versions we represented it as a function + (CoreExpr -> CoreEpxr) which we couldn't take apart. + +Do *not* duplicate StrictBind and StritArg continuations. We gain +nothing by propagating them into the expressions, and we do lose a +lot. + +The desire not to duplicate is the entire reason that +mkDupableCont returns a pair of continuations. + +Note [Duplicating StrictBind] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Unlike StrictArg, there doesn't seem anything to gain from +duplicating a StrictBind continuation, so we don't. + The desire not to duplicate is the entire reason that mkDupableCont returns a pair of continuations. -The original plan had: -e.g. (...strict-fn...) [...hole...] - ==> - let $j = \a -> ...strict-fn... - in $j [...hole...] Note [Single-alternative cases] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~