X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FsimplCore%2FSimplUtils.lhs;h=501dd60ec9c538413b545b7c60d592264428422b;hb=ebf2c80221ccf11aeb7a0a2be27bfc72529855a5;hp=ac24d65fc4d102da3599355897e614863be32de9;hpb=dabfa71f33eabc5a2d10959728f772aa016f1c84;p=ghc-hetmet.git diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs index ac24d65..501dd60 100644 --- a/ghc/compiler/simplCore/SimplUtils.lhs +++ b/ghc/compiler/simplCore/SimplUtils.lhs @@ -1,414 +1,841 @@ % -% (c) The AQUA Project, Glasgow University, 1993-1996 +% (c) The AQUA Project, Glasgow University, 1993-1998 % \section[SimplUtils]{The simplifier utilities} \begin{code} -#include "HsVersions.h" - module SimplUtils ( + simplBinder, simplBinders, simplRecIds, simplLetId, + tryRhsTyLam, tryEtaExpansion, + mkCase, - floatExposesHNF, - - mkTyLamTryingEta, mkValLamTryingEta, - - etaExpandCount, + -- The continuation type + SimplCont(..), DupFlag(..), contIsDupable, contResultType, + countValArgs, countArgs, mkRhsStop, mkStop, + getContArgs, interestingCallContext, interestingArg, isStrictType, discardInline - mkIdentityAlts, - - simplIdWantsToBeINLINEd, - - type_ok_for_let_to_case ) where -import Ubiq{-uitous-} +#include "HsVersions.h" -import BinderInfo -import CmdLineOpts ( SimplifierSwitch(..) ) +import CmdLineOpts ( switchIsOn, SimplifierSwitch(..), + opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge, opt_DictsStrict, + opt_UF_UpdateInPlace + ) import CoreSyn -import CoreUtils ( manifestlyWHNF ) -import Id ( idType, isBottomingId, idWantsToBeINLINEd, dataConArgTys, - getIdArity, GenId{-instance Eq-} +import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, + etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce, + findDefault + ) +import Subst ( InScopeSet, mkSubst, substExpr ) +import qualified Subst ( simplBndrs, simplBndr, simplLetId ) +import Id ( idType, idName, + idUnfolding, idStrictness, + mkLocalId, idInfo ) -import IdInfo ( arityMaybe ) -import Maybes ( maybeToBool ) -import PrelVals ( augmentId, buildId ) -import PrimOp ( primOpIsCheap ) -import SimplEnv +import IdInfo ( StrictnessInfo(..) ) +import Maybes ( maybeToBool, catMaybes ) +import Name ( setNameUnique ) +import Demand ( isStrict ) import SimplMonad -import Type ( eqTy, isPrimType, maybeAppDataTyConExpandingDicts, getTyVar_maybe ) -import TysWiredIn ( realWorldStateTy ) -import TyVar ( GenTyVar{-instance Eq-} ) -import Util ( isIn, panic ) - +import Type ( Type, mkForAllTys, seqType, repType, + splitTyConApp_maybe, tyConAppArgs, mkTyVarTys, + isDictTy, isDataType, isUnLiftedType, + splitRepFunTys + ) +import TyCon ( tyConDataConsIfAvailable ) +import DataCon ( dataConRepArity ) +import VarEnv ( SubstEnv ) +import Util ( lengthExceeds, mapAccumL ) +import Outputable \end{code} -Floating -~~~~~~~~ -The function @floatExposesHNF@ tells whether let/case floating will -expose a head normal form. It is passed booleans indicating the -desired strategy. +%************************************************************************ +%* * +\subsection{The continuation data type} +%* * +%************************************************************************ \begin{code} -floatExposesHNF - :: Bool -- Float let(rec)s out of rhs - -> Bool -- Float cheap primops out of rhs - -> Bool -- OK to duplicate code - -> GenCoreExpr bdr Id tyvar uvar - -> Bool - -floatExposesHNF float_lets float_primops ok_to_dup rhs - = try rhs - where - try (Case (Prim _ _) (PrimAlts alts deflt) ) - | float_primops && (null alts || ok_to_dup) - = or (try_deflt deflt : map try_alt alts) - - try (Let bind body) | float_lets = try body - - -- `build g' - -- is like a HNF, - -- because it *will* become one. - -- likewise for `augment g h' - -- - try (App (App (Var bld) _) _) | bld == buildId = True - try (App (App (App (Var aug) _) _) _) | aug == augmentId = True - - try other = manifestlyWHNF other - {- but *not* necessarily "manifestlyBottom other"... - - We may want to float a let out of a let to expose WHNFs, - but to do that to expose a "bottom" is a Bad Idea: - let x = let y = ... - in ...error ...y... -- manifestly bottom using y - in ... - =/=> - let y = ... - in let x = ...error ...y... - in ... - - as y is only used in case of an error, we do not want - to allocate it eagerly as that's a waste. - -} - - try_alt (lit,rhs) = try rhs - - try_deflt NoDefault = False - try_deflt (BindDefault _ rhs) = try rhs +data SimplCont -- Strict contexts + = Stop OutType -- Type of the result + Bool -- True => This is the RHS of a thunk whose type suggests + -- that update-in-place would be possible + -- (This makes the inliner a little keener.) + + | CoerceIt OutType -- The To-type, simplified + SimplCont + + | InlinePlease -- This continuation makes a function very + SimplCont -- keen to inline itelf + + | ApplyTo DupFlag + InExpr SubstEnv -- The argument, as yet unsimplified, + SimplCont -- and its subst-env + + | Select DupFlag + InId [InAlt] SubstEnv -- The case binder, alts, and subst-env + SimplCont + + | ArgOf DupFlag -- An arbitrary strict context: the argument + -- of a strict function, or a primitive-arg fn + -- or a PrimOp + OutType -- cont_ty: the type of the expression being sought by the context + -- f (error "foo") ==> coerce t (error "foo") + -- when f is strict + -- We need to know the type t, to which to coerce. + (OutExpr -> SimplM OutExprStuff) -- What to do with the result + -- The result expression in the OutExprStuff has type cont_ty + +instance Outputable SimplCont where + ppr (Stop _ _) = ptext SLIT("Stop") + ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont + ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup + ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$ + (nest 4 (ppr alts)) $$ ppr cont + ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont + ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont + +data DupFlag = OkToDup | NoDup + +instance Outputable DupFlag where + ppr OkToDup = ptext SLIT("ok") + ppr NoDup = ptext SLIT("nodup") + + +------------------- +mkRhsStop, mkStop :: OutType -> SimplCont +mkStop ty = Stop ty False +mkRhsStop ty = Stop ty (canUpdateInPlace ty) + + +------------------- +contIsDupable :: SimplCont -> Bool +contIsDupable (Stop _ _) = True +contIsDupable (ApplyTo OkToDup _ _ _) = True +contIsDupable (ArgOf OkToDup _ _) = True +contIsDupable (Select OkToDup _ _ _ _) = True +contIsDupable (CoerceIt _ cont) = contIsDupable cont +contIsDupable (InlinePlease cont) = contIsDupable cont +contIsDupable other = False + +------------------- +discardInline :: SimplCont -> SimplCont +discardInline (InlinePlease cont) = cont +discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont) +discardInline cont = cont + +------------------- +discardableCont :: SimplCont -> Bool +discardableCont (Stop _ _) = False +discardableCont (CoerceIt _ cont) = discardableCont cont +discardableCont (InlinePlease cont) = discardableCont cont +discardableCont other = True + +discardCont :: SimplCont -- A continuation, expecting + -> SimplCont -- Replace the continuation with a suitable coerce +discardCont cont = case cont of + Stop to_ty _ -> cont + other -> CoerceIt to_ty (mkStop to_ty) + where + to_ty = contResultType cont + +------------------- +contResultType :: SimplCont -> OutType +contResultType (Stop to_ty _) = to_ty +contResultType (ArgOf _ to_ty _) = to_ty +contResultType (ApplyTo _ _ _ cont) = contResultType cont +contResultType (CoerceIt _ cont) = contResultType cont +contResultType (InlinePlease cont) = contResultType cont +contResultType (Select _ _ _ _ cont) = contResultType cont + +------------------- +countValArgs :: SimplCont -> Int +countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont +countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont +countValArgs other = 0 + +countArgs :: SimplCont -> Int +countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont +countArgs other = 0 \end{code} -Eta reduction on ordinary lambdas -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -We have a go at doing - - \ x y -> f x y ===> f - -But we only do this if it gets rid of a whole lambda, not part. -The idea is that lambdas are often quite helpful: they indicate -head normal forms, so we don't want to chuck them away lightly. -But if they expose a simple variable then we definitely win. Even -if they expose a type application we win. So we check for this special -case. - -It does arise: - - f xs = [y | (y,_) <- xs] - -gives rise to a recursive function for the list comprehension, and -f turns out to be just a single call to this recursive function. - \begin{code} -mkValLamTryingEta :: [Id] -- Args to the lambda - -> CoreExpr -- Lambda body - -> CoreExpr - -mkValLamTryingEta [] body = body - -mkValLamTryingEta orig_ids body - = reduce_it (reverse orig_ids) body +getContArgs :: OutId -> SimplCont + -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args + SimplCont, -- Remaining continuation + Bool) -- Whether we came across an InlineCall +-- getContArgs id k = (args, k', inl) +-- args are the leading ApplyTo items in k +-- (i.e. outermost comes first) +-- augmented with demand info from the functionn +getContArgs fun orig_cont + = getSwitchChecker `thenSmpl` \ chkr -> + let + -- Ignore strictness info if the no-case-of-case + -- flag is on. Strictness changes evaluation order + -- and that can change full laziness + stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts + | otherwise = computed_stricts + in + go [] stricts False orig_cont where - bale_out = mkValLam orig_ids body - - reduce_it [] residual - | residual_ok residual = residual - | otherwise = bale_out + ---------------------------- + + -- Type argument + go acc ss inl (ApplyTo _ arg@(Type _) se cont) + = go ((arg,se,False) : acc) ss inl cont + -- NB: don't bother to instantiate the function type + + -- Value argument + go acc (s:ss) inl (ApplyTo _ arg se cont) + = go ((arg,se,s) : acc) ss inl cont + + -- An Inline continuation + go acc ss inl (InlinePlease cont) + = go acc ss True cont + + -- We're run out of arguments, or else we've run out of demands + -- The latter only happens if the result is guaranteed bottom + -- This is the case for + -- * case (error "hello") of { ... } + -- * (error "Hello") arg + -- * f (error "Hello") where f is strict + -- etc + go acc ss inl cont + | null ss && discardableCont cont = tick BottomFound `thenSmpl_` + returnSmpl (reverse acc, discardCont cont, inl) + | otherwise = returnSmpl (reverse acc, cont, inl) + + ---------------------------- + vanilla_stricts, computed_stricts :: [Bool] + vanilla_stricts = repeat False + computed_stricts = zipWith (||) fun_stricts arg_stricts + + ---------------------------- + (val_arg_tys, _) = splitRepFunTys (idType fun) + arg_stricts = map isStrictType val_arg_tys ++ repeat False + -- These argument types are used as a cheap and cheerful way to find + -- unboxed arguments, which must be strict. But it's an InType + -- and so there might be a type variable where we expect a function + -- type (the substitution hasn't happened yet). And we don't bother + -- doing the type applications for a polymorphic function. + -- Hence the split*Rep*FunTys + + ---------------------------- + -- If fun_stricts is finite, it means the function returns bottom + -- after that number of value args have been consumed + -- Otherwise it's infinite, extended with False + fun_stricts + = case idStrictness fun of + StrictnessInfo demands result_bot + | not (demands `lengthExceeds` countValArgs orig_cont) + -> -- Enough args, use the strictness given. + -- For bottoming functions we used to pretend that the arg + -- is lazy, so that we don't treat the arg as an + -- interesting context. This avoids substituting + -- top-level bindings for (say) strings into + -- calls to error. But now we are more careful about + -- inlining lone variables, so its ok (see SimplUtils.analyseCont) + if result_bot then + map isStrict demands -- Finite => result is bottom + else + map isStrict demands ++ vanilla_stricts + + other -> vanilla_stricts -- Not enough args, or no strictness + + +------------------- +isStrictType :: Type -> Bool + -- isStrictType computes whether an argument (or let RHS) should + -- be computed strictly or lazily, based only on its type +isStrictType ty + | isUnLiftedType ty = True + | opt_DictsStrict && isDictTy ty && isDataType ty = True + | otherwise = False + -- Return true only for dictionary types where the dictionary + -- has more than one component (else we risk poking on the component + -- of a newtype dictionary) + +------------------- +interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool + -- An argument is interesting if it has *some* structure + -- We are here trying to avoid unfolding a function that + -- is applied only to variables that have no unfolding + -- (i.e. they are probably lambda bound): f x y z + -- There is little point in inlining f here. +interestingArg in_scope arg subst + = analyse (substExpr (mkSubst in_scope subst) arg) + -- 'analyse' only looks at the top part of the result + -- and substExpr is lazy, so this isn't nearly as brutal + -- as it looks. + where + analyse (Var v) = hasSomeUnfolding (idUnfolding v) + -- Was: isValueUnfolding (idUnfolding v') + -- But that seems over-pessimistic + analyse (Type _) = False + analyse (App fn (Type _)) = analyse fn + analyse (Note _ a) = analyse a + analyse other = True + -- Consider let x = 3 in f x + -- The substitution will contain (x -> ContEx 3), and we want to + -- to say that x is an interesting argument. + -- But consider also (\x. f x y) y + -- The substitution will contain (x -> ContEx y), and we want to say + -- that x is not interesting (assuming y has no unfolding) +\end{code} - reduce_it (id:ids) (App fun (VarArg arg)) - | id == arg - && not (idType id `eqTy` realWorldStateTy) - -- *never* eta-reduce away a PrimIO state token! (WDP 94/11) - = reduce_it ids fun +Comment about interestingCallContext +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We want to avoid inlining an expression where there can't possibly be +any gain, such as in an argument position. Hence, if the continuation +is interesting (eg. a case scrutinee, application etc.) then we +inline, otherwise we don't. - reduce_it ids other = bale_out +Previously some_benefit used to return True only if the variable was +applied to some value arguments. This didn't work: - is_elem = isIn "mkValLamTryingEta" + let x = _coerce_ (T Int) Int (I# 3) in + case _coerce_ Int (T Int) x of + I# y -> .... - ----------- - residual_ok :: CoreExpr -> Bool -- Checks for type application - -- and function not one of the - -- bound vars +we want to inline x, but can't see that it's a constructor in a case +scrutinee position, and some_benefit is False. - residual_ok (Var v) = not (v `is_elem` orig_ids) - -- Fun mustn't be one of the bound ids - residual_ok (App fun arg) - | notValArg arg = residual_ok fun - residual_ok other = False -\end{code} +Another example: -Eta expansion -~~~~~~~~~~~~~ -@etaExpandCount@ takes an expression, E, and returns an integer n, -such that +dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t) - E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn) +.... case dMonadST _@_ x0 of (a,b,c) -> .... -is a safe transformation. In particular, the transformation should -not cause work to be duplicated, unless it is ``cheap'' (see -@manifestlyCheap@ below). +we'd really like to inline dMonadST here, but we *don't* want to +inline if the case expression is just -@etaExpandCount@ errs on the conservative side. It is always safe to -return 0. + case x of y { DEFAULT -> ... } -An application of @error@ is special, because it can absorb as many -arguments as you care to give it. For this special case we return -100, to represent "infinity", which is a bit of a hack. +since we can just eliminate this case instead (x is in WHNF). Similar +applies when x is bound to a lambda expression. Hence +contIsInteresting looks for case expressions with just a single +default case. \begin{code} -etaExpandCount :: GenCoreExpr bdr Id tyvar uvar - -> Int -- Number of extra args you can safely abstract - -etaExpandCount (Lam (ValBinder _) body) - = 1 + etaExpandCount body - -etaExpandCount (Let bind body) - | all manifestlyCheap (rhssOfBind bind) - = etaExpandCount body - -etaExpandCount (Case scrut alts) - | manifestlyCheap scrut - = minimum [etaExpandCount rhs | rhs <- rhssOfAlts alts] - -etaExpandCount fun@(Var _) = eta_fun fun -etaExpandCount (App fun arg) - | notValArg arg = eta_fun fun - | otherwise = case etaExpandCount fun of - 0 -> 0 - n -> n-1 -- Knock off one - -etaExpandCount other = 0 -- Give up - -- Lit, Con, Prim, - -- non-val Lam, - -- Scc (pessimistic; ToDo), - -- Let with non-whnf rhs(s), - -- Case with non-whnf scrutinee - ------------------------------ -eta_fun :: GenCoreExpr bdr Id tv uv -- The function - -> Int -- How many args it can safely be applied to - -eta_fun (App fun arg) | notValArg arg = eta_fun fun - -eta_fun expr@(Var v) - | isBottomingId v -- Bottoming ids have "infinite arity" - = 10000 -- Blargh. Infinite enough! - -eta_fun expr@(Var v) - | maybeToBool arity_maybe -- We know the arity - = arity +interestingCallContext :: Bool -- False <=> no args at all + -> Bool -- False <=> no value args + -> SimplCont -> Bool + -- The "lone-variable" case is important. I spent ages + -- messing about with unsatisfactory varaints, but this is nice. + -- The idea is that if a variable appear all alone + -- as an arg of lazy fn, or rhs Stop + -- as scrutinee of a case Select + -- as arg of a strict fn ArgOf + -- then we should not inline it (unless there is some other reason, + -- e.g. is is the sole occurrence). We achieve this by making + -- interestingCallContext return False for a lone variable. + -- + -- Why? At least in the case-scrutinee situation, turning + -- let x = (a,b) in case x of y -> ... + -- into + -- let x = (a,b) in case (a,b) of y -> ... + -- and thence to + -- let x = (a,b) in let y = (a,b) in ... + -- is bad if the binding for x will remain. + -- + -- Another example: I discovered that strings + -- were getting inlined straight back into applications of 'error' + -- because the latter is strict. + -- s = "foo" + -- f = \x -> ...(error s)... + + -- Fundamentally such contexts should not ecourage inlining becuase + -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE) + -- so there's no gain. + -- + -- However, even a type application or coercion isn't a lone variable. + -- Consider + -- case $fMonadST @ RealWorld of { :DMonad a b c -> c } + -- We had better inline that sucker! The case won't see through it. + -- + -- For now, I'm treating treating a variable applied to types + -- in a *lazy* context "lone". The motivating example was + -- f = /\a. \x. BIG + -- g = /\a. \y. h (f a) + -- There's no advantage in inlining f here, and perhaps + -- a significant disadvantage. Hence some_val_args in the Stop case + +interestingCallContext some_args some_val_args cont + = interesting cont where - arity_maybe = arityMaybe (getIdArity v) - arity = case arity_maybe of { Just arity -> arity } - -eta_fun other = 0 -- Give up + interesting (InlinePlease _) = True + interesting (Select _ _ _ _ _) = some_args + interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y + -- Perhaps True is a bit over-keen, but I've + -- seen (coerce f) x, where f has an INLINE prag, + -- So we have to give some motivaiton for inlining it + interesting (ArgOf _ _ _) = some_val_args + interesting (Stop ty upd_in_place) = some_val_args && upd_in_place + 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. + -- x + (y * z) + -- Here the contIsInteresting makes the '*' keener to inline, + -- which in turn exposes a constructor which makes the '+' inline. + -- Assuming that +,* aren't small enough to inline regardless. + -- + -- It's also very important to inline in a strict context for things + -- like + -- foldr k z (f x) + -- Here, the context of (f x) is strict, and if f's unfolding is + -- a build it's *great* to inline it here. So we must ensure that + -- the context for (f x) is not totally uninteresting. + + +------------------- +canUpdateInPlace :: Type -> Bool +-- Consider let x = in ... +-- If returns an explicit constructor, we might be able +-- to do update in place. So we treat even a thunk RHS context +-- as interesting if update in place is possible. We approximate +-- this by seeing if the type has a single constructor with a +-- small arity. But arity zero isn't good -- we share the single copy +-- for that case, so no point in sharing. + +-- Note the repType: we want to look through newtypes for this purpose + +canUpdateInPlace ty + | not opt_UF_UpdateInPlace = False + | otherwise + = case splitTyConApp_maybe (repType ty) of { + Nothing -> False ; + Just (tycon, _) -> + + case tyConDataConsIfAvailable tycon of + [dc] -> arity == 1 || arity == 2 + where + arity = dataConRepArity dc + other -> False + } \end{code} -@manifestlyCheap@ looks at a Core expression and returns \tr{True} if -it is obviously in weak head normal form, or is cheap to get to WHNF. -By ``cheap'' we mean a computation we're willing to duplicate in order -to bring a couple of lambdas together. The main examples of things -which aren't WHNF but are ``cheap'' are: - * case e of - pi -> ei - where e, and all the ei are cheap; and +%************************************************************************ +%* * +\section{Dealing with a single binder} +%* * +%************************************************************************ - * let x = e - in b +\begin{code} +simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a +simplBinders bndrs thing_inside + = getSubst `thenSmpl` \ subst -> + let + (subst', bndrs') = Subst.simplBndrs subst bndrs + in + seqBndrs bndrs' `seq` + setSubst subst' (thing_inside bndrs') + +simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a +simplBinder bndr thing_inside + = getSubst `thenSmpl` \ subst -> + let + (subst', bndr') = Subst.simplBndr subst bndr + in + seqBndr bndr' `seq` + setSubst subst' (thing_inside bndr') + + +simplRecIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a +simplRecIds ids thing_inside + = getSubst `thenSmpl` \ subst -> + let + (subst', ids') = mapAccumL Subst.simplLetId subst ids + in + seqBndrs ids' `seq` + setSubst subst' (thing_inside ids') + +simplLetId :: InBinder -> (OutBinder -> SimplM a) -> SimplM a +simplLetId id thing_inside + = getSubst `thenSmpl` \ subst -> + let + (subst', id') = Subst.simplLetId subst id + in + seqBndr id' `seq` + setSubst subst' (thing_inside id') + +seqBndrs [] = () +seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs + +seqBndr b | isTyVar b = b `seq` () + | otherwise = seqType (idType b) `seq` + idInfo b `seq` + () +\end{code} - where e and b are cheap; and - * op x1 ... xn +%************************************************************************ +%* * +\subsection{Local tyvar-lifting} +%* * +%************************************************************************ + +mkRhsTyLam tries this transformation, when the big lambda appears as +the RHS of a let(rec) binding: + + /\abc -> let(rec) x = e in b + ==> + let(rec) x' = /\abc -> let x = x' a b c in e + in + /\abc -> let x = x' a b c in b + +This is good because it can turn things like: + + let f = /\a -> letrec g = ... g ... in g +into + letrec g' = /\a -> ... g' a ... + in + let f = /\ a -> g' a + +which is better. In effect, it means that big lambdas don't impede +let-floating. + +This optimisation is CRUCIAL in eliminating the junk introduced by +desugaring mutually recursive definitions. Don't eliminate it lightly! + +So far as the implementation is concerned: + + Invariant: go F e = /\tvs -> F e + + Equalities: + go F (Let x=e in b) + = Let x' = /\tvs -> F e + in + go G b + where + G = F . Let x = x' tvs + + go F (Letrec xi=ei in b) + = Letrec {xi' = /\tvs -> G ei} + in + go G b + where + G = F . Let {xi = xi' tvs} + +[May 1999] If we do this transformation *regardless* then we can +end up with some pretty silly stuff. For example, + + let + st = /\ s -> let { x1=r1 ; x2=r2 } in ... + in .. +becomes + let y1 = /\s -> r1 + y2 = /\s -> r2 + st = /\s -> ...[y1 s/x1, y2 s/x2] + in .. + +Unless the "..." is a WHNF there is really no point in doing this. +Indeed it can make things worse. Suppose x1 is used strictly, +and is of the form + + x1* = case f y of { (a,b) -> e } + +If we abstract this wrt the tyvar we then can't do the case inline +as we would normally do. - where op is a cheap primitive operator \begin{code} -manifestlyCheap :: GenCoreExpr bndr Id tv uv -> Bool - -manifestlyCheap (Var _) = True -manifestlyCheap (Lit _) = True -manifestlyCheap (Con _ _) = True -manifestlyCheap (SCC _ e) = manifestlyCheap e -manifestlyCheap (Coerce _ _ e) = manifestlyCheap e -manifestlyCheap (Lam x e) = if isValBinder x then True else manifestlyCheap e -manifestlyCheap (Prim op _) = primOpIsCheap op - -manifestlyCheap (Let bind body) - = manifestlyCheap body && all manifestlyCheap (rhssOfBind bind) - -manifestlyCheap (Case scrut alts) - = manifestlyCheap scrut && all manifestlyCheap (rhssOfAlts alts) - -manifestlyCheap other_expr -- look for manifest partial application - = case (collectArgs other_expr) of { (fun, _, _, vargs) -> - case fun of - - Var f | isBottomingId f -> True -- Application of a function which - -- always gives bottom; we treat this as - -- a WHNF, because it certainly doesn't - -- need to be shared! - - Var f -> let - num_val_args = length vargs - in - num_val_args == 0 || -- Just a type application of - -- a variable (f t1 t2 t3) - -- counts as WHNF - case (arityMaybe (getIdArity f)) of - Nothing -> False - Just arity -> num_val_args < arity - - _ -> False - } -\end{code} +tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr) -Eta reduction on type lambdas -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -We have a go at doing +tryRhsTyLam rhs -- Only does something if there's a let + | null tyvars || not (worth_it body) -- inside a type lambda, + = returnSmpl ([], rhs) -- and a WHNF inside that - /\a -> a ===> + | otherwise + = go (\x -> x) body `thenSmpl` \ (binds, body') -> + returnSmpl (binds, mkLams tyvars body') -where doesn't mention a. -This is sometimes quite useful, because we can get the sequence: + where + (tyvars, body) = collectTyBinders rhs + + worth_it e@(Let _ _) = whnf_in_middle e + worth_it e = False + + whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False + whnf_in_middle (Let _ e) = whnf_in_middle e + whnf_in_middle e = exprIsCheap e + + go fn (Let bind@(NonRec var rhs) body) + | exprIsTrivial rhs + = go (fn . Let bind) body + + go fn (Let (NonRec var rhs) body) + = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') -> + go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') -> + returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body') + + where + tyvars_here = tyvars + -- main_tyvar_set = mkVarSet tyvars + -- var_ty = idType var + -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty) + -- tyvars_here was an attempt to reduce the number of tyvars + -- wrt which the new binding is abstracted. But the naive + -- approach of abstract wrt the tyvars free in the Id's type + -- fails. Consider: + -- /\ a b -> let t :: (a,b) = (e1, e2) + -- x :: a = fst t + -- in ... + -- Here, b isn't free in x's type, but we must nevertheless + -- abstract wrt b as well, because t's type mentions b. + -- Since t is floated too, we'd end up with the bogus: + -- poly_t = /\ a b -> (e1, e2) + -- poly_x = /\ a -> fst (poly_t a *b*) + -- So for now we adopt the even more naive approach of + -- abstracting wrt *all* the tyvars. We'll see if that + -- gives rise to problems. SLPJ June 98 + + go fn (Let (Rec prs) body) + = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') -> + let + gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss')) + new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]) + in + go gn body `thenSmpl` \ (binds, body') -> + returnSmpl (new_bind : binds, body') + where + (vars,rhss) = unzip prs + tyvars_here = tyvars + -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys) + -- var_tys = map idType vars + -- See notes with tyvars_here above + + go fn body = returnSmpl ([], fn body) + + mk_poly tyvars_here var + = getUniqueSmpl `thenSmpl` \ uniq -> + let + poly_name = setNameUnique (idName var) uniq -- Keep same name + poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course + poly_id = mkLocalId poly_name poly_ty + + -- In the olden days, it was crucial to copy the occInfo of the original var, + -- because we were looking at occurrence-analysed but as yet unsimplified code! + -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking + -- at already simplified code, so it doesn't matter + -- + -- It's even right to retain single-occurrence or dead-var info: + -- Suppose we started with /\a -> let x = E in B + -- where x occurs once in B. Then we transform to: + -- let x' = /\a -> E in /\a -> let x* = x' a in B + -- where x* has an INLINE prag on it. Now, once x* is inlined, + -- the occurrences of x' will be just the occurrences originally + -- pinned on x. + in + returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here)) + + mk_silly_bind var rhs = NonRec var (Note InlineMe rhs) + -- Suppose we start with: + -- + -- x = /\ a -> let g = G in E + -- + -- Then we'll float to get + -- + -- x = let poly_g = /\ a -> G + -- in /\ a -> let g = poly_g a in E + -- + -- But now the occurrence analyser will see just one occurrence + -- of poly_g, not inside a lambda, so the simplifier will + -- PreInlineUnconditionally poly_g back into g! Badk to square 1! + -- (I used to think that the "don't inline lone occurrences" stuff + -- would stop this happening, but since it's the *only* occurrence, + -- PreInlineUnconditionally kicks in first!) + -- + -- Solution: put an INLINE note on g's RHS, so that poly_g seems + -- to appear many times. (NB: mkInlineMe eliminates + -- such notes on trivial RHSs, so do it manually.) +\end{code} - f ab d = let d1 = ...d... in - letrec f' b x = ...d...(f' b)... in - f' b -specialise ==> - f.Int b = letrec f' b x = ...dInt...(f' b)... in - f' b +%************************************************************************ +%* * +\subsection{Eta expansion} +%* * +%************************************************************************ -float ==> + Try eta expansion for RHSs - f' b x = ...dInt...(f' b)... - f.Int b = f' b +We go for: + Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym + (n >= 0) + OR + Case 2 f = N E1..En ==> z1=E1 + (n > 0) .. + zn=En + f = \y1..ym -> N z1..zn y1..ym -Now we really want to simplify to +where (in both cases) - f.Int = f' + * The xi can include type variables -and then replace all the f's with f.Ints. + * The yi are all value variables -N.B. We are careful not to partially eta-reduce a sequence of type -applications since this breaks the specialiser: + * N is a NORMAL FORM (i.e. no redexes anywhere) + wanting a suitable number of extra args. - /\ a -> f Char# a =NO=> f Char# + * the Ei must not have unlifted type -\begin{code} -mkTyLamTryingEta :: [TyVar] -> CoreExpr -> CoreExpr - -mkTyLamTryingEta tyvars tylam_body - = if - tyvars == tyvar_args && -- Same args in same order - check_fun fun -- Function left is ok - then - -- Eta reduction worked - fun - else - -- The vastly common case - mkTyLam tyvars tylam_body - where - (tyvar_args, fun) = strip_tyvar_args [] tylam_body - - strip_tyvar_args args_so_far tyapp@(App fun (TyArg ty)) - = case getTyVar_maybe ty of - Just tyvar_arg -> strip_tyvar_args (tyvar_arg:args_so_far) fun - Nothing -> (args_so_far, tyapp) +There is no point in looking for a combination of the two, because +that would leave use with some lets sandwiched between lambdas; that's +what the final test in the first equation is for. - strip_tyvar_args args_so_far (App _ (UsageArg _)) - = panic "SimplUtils.mkTyLamTryingEta: strip_tyvar_args UsageArg" +In Case 1, we may have to sandwich some coerces between the lambdas +to make the types work. exprEtaExpandArity looks through coerces +when computing arity; and etaExpand adds the coerces as necessary when +actually computing the expansion. - strip_tyvar_args args_so_far fun - = (args_so_far, fun) +\begin{code} +tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr) +tryEtaExpansion rhs rhs_ty + | not opt_SimplDoLambdaEtaExpansion -- Not if switched off + || exprIsTrivial rhs -- Not if RHS is trivial + || final_arity == 0 -- Not if arity is zero + = returnSmpl ([], rhs) + + | n_val_args == 0 && not arity_is_manifest + = -- Some lambdas but not enough: case 1 + getUniqSupplySmpl `thenSmpl` \ us -> + returnSmpl ([], etaExpand final_arity us rhs rhs_ty) + + | n_val_args > 0 && not (any cant_bind arg_infos) + = -- Partial application: case 2 + mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) -> + getUniqSupplySmpl `thenSmpl` \ us -> + returnSmpl (catMaybes maybe_z_binds, + etaExpand final_arity us (mkApps fun z_args) rhs_ty) - check_fun (Var f) = True -- Claim: tyvars not mentioned by type of f - check_fun other = False + | otherwise + = returnSmpl ([], rhs) + where + (fun, args) = collectArgs rhs + n_val_args = valArgCount args + (fun_arity, arity_is_manifest) = exprEtaExpandArity fun + final_arity = 0 `max` (fun_arity - n_val_args) + arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args] + cant_bind (_, ty, triv) = not triv && isUnLiftedType ty + + bind_z_arg (arg, arg_ty, trivial_arg) + | trivial_arg = returnSmpl (Nothing, arg) + | otherwise = newId SLIT("z") arg_ty $ \ z -> + returnSmpl (Just (NonRec z arg), Var z) \end{code} -Let to case -~~~~~~~~~~~ - -Given a type generate the case alternatives - C a b -> C a b +%************************************************************************ +%* * +\subsection{Case absorption and identity-case elimination} +%* * +%************************************************************************ -if there's one constructor, or +\begin{code} +mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr +\end{code} - x -> x +@mkCase@ tries the following transformation (if possible): + +case e of b { ==> case e of b { + p1 -> rhs1 p1 -> rhs1 + ... ... + pm -> rhsm pm -> rhsm + _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn} + {or (prim) case b of b' { _ -> rhsn}} + pn -> rhsn ... + ... po -> rhso[b/b'] + po -> rhso _ -> rhsd[b/b'] {or 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. -if there's many, or if it's a primitive type. +\begin{code} +mkCase scrut outer_bndr outer_alts + | opt_SimplCaseMerge + && maybeToBool maybe_case_in_default + + = tick (CaseMerge outer_bndr) `thenSmpl_` + returnSmpl (Case scrut outer_bndr new_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 bindNonRec + -- in munge_rhs puts a case into the DEFAULT branch! + where + new_alts = outer_alts_without_deflt ++ munged_inner_alts + maybe_case_in_default = case findDefault outer_alts of + (outer_alts_without_default, + Just (Case (Var scrut_var) inner_bndr inner_alts)) + + | outer_bndr == scrut_var + -> Just (outer_alts_without_default, inner_bndr, inner_alts) + other -> Nothing + + Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default + + -- Eliminate any inner alts which are shadowed by the outer ones + outer_cons = [con | (con,_,_) <- outer_alts_without_deflt] + + munged_inner_alts = [ (con, args, munge_rhs rhs) + | (con, args, rhs) <- inner_alts, + not (con `elem` outer_cons) -- Eliminate shadowed inner alts + ] + munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs +\end{code} +Now the identity-case transformation: -\begin{code} -mkIdentityAlts - :: Type -- type of RHS - -> SmplM InAlts -- result + case e of ===> e + True -> True; + False -> False -mkIdentityAlts rhs_ty - | isPrimType rhs_ty - = newId rhs_ty `thenSmpl` \ binder -> - returnSmpl (PrimAlts [] (BindDefault (binder, bad_occ_info) (Var binder))) +and similar friends. - | otherwise - = case (maybeAppDataTyConExpandingDicts rhs_ty) of - Just (tycon, ty_args, [data_con]) -> -- algebraic type suitable for unpacking - let - inst_con_arg_tys = dataConArgTys data_con ty_args - in - newIds inst_con_arg_tys `thenSmpl` \ new_bindees -> - let - new_binders = [ (b, bad_occ_info) | b <- new_bindees ] - in - returnSmpl ( - AlgAlts - [(data_con, new_binders, mkCon data_con [] ty_args (map VarArg new_bindees))] - NoDefault - ) - - _ -> -- Multi-constructor or abstract algebraic type - newId rhs_ty `thenSmpl` \ binder -> - returnSmpl (AlgAlts [] (BindDefault (binder,bad_occ_info) (Var binder))) +\begin{code} +mkCase scrut case_bndr alts + | all identity_alt alts + = tick (CaseIdentity case_bndr) `thenSmpl_` + returnSmpl (re_note scrut) where - bad_occ_info = ManyOcc 0 -- Non-committal! + identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args + + identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args) + identity_rhs (LitAlt lit) _ = Lit lit + identity_rhs DEFAULT _ = Var case_bndr + + arg_tys = map Type (tyConAppArgs (idType case_bndr)) + + -- We've seen this: + -- case coerce T e of x { _ -> coerce T' x } + -- And we definitely want to eliminate this case! + -- So we throw away notes from the RHS, and reconstruct + -- (at least an approximation) at the other end + de_note (Note _ e) = de_note e + de_note e = e + + -- re_note wraps a coerce if it might be necessary + re_note scrut = case head alts of + (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut + other -> scrut \end{code} -\begin{code} -simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool +The catch-all case -simplIdWantsToBeINLINEd id env - = if switchIsSet env IgnoreINLINEPragma - then False - else idWantsToBeINLINEd id +\begin{code} +mkCase other_scrut case_bndr other_alts + = returnSmpl (Case other_scrut case_bndr other_alts) +\end{code} -type_ok_for_let_to_case :: Type -> Bool -type_ok_for_let_to_case ty - = case (maybeAppDataTyConExpandingDicts ty) of - Nothing -> False - Just (tycon, ty_args, []) -> False - Just (tycon, ty_args, non_null_data_cons) -> True - -- Null data cons => type is abstract -\end{code}