X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FsimplCore%2FSimplUtils.lhs;h=34ee7d61151f4b5431acab7390848242dfc34401;hb=495ef8bd9ef30bffe50ea399b91e3ba09646b59a;hp=7997378d898ee653737df2812582f917b0063194;hpb=df10403c92440a304198b3096e65d52a1fe482ae;p=ghc-hetmet.git diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs index 7997378..34ee7d6 100644 --- a/ghc/compiler/simplCore/SimplUtils.lhs +++ b/ghc/compiler/simplCore/SimplUtils.lhs @@ -1,114 +1,381 @@ % -% (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, simplIds, + transformRhs, + mkCase, findAlt, findDefault, - floatExposesHNF, - - etaCoreExpr, mkRhsTyLam, + -- The continuation type + SimplCont(..), DupFlag(..), contIsDupable, contResultType, + pushArgs, discardCont, countValArgs, countArgs, + analyseCont, discardInline - etaExpandCount, - - simplIdWantsToBeINLINEd, - - singleConstructorType, typeOkForCase ) where -IMP_Ubiq(){-uitous-} -#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ <= 201 -IMPORT_DELOOPER(SmplLoop) -- paranoia checking -#endif +#include "HsVersions.h" import BinderInfo -import CmdLineOpts ( opt_DoEtaReduction, SimplifierSwitch(..) ) +import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge ) import CoreSyn -import CoreUnfold ( SimpleUnfolding, mkFormSummary, exprIsTrivial, FormSummary(..) ) -import Id ( idType, isBottomingId, addInlinePragma, addIdDemandInfo, - idWantsToBeINLINEd, dataConArgTys, SYN_IE(Id), - getIdArity, GenId{-instance Eq-} +import PprCore ( {- instance Outputable Expr -} ) +import CoreUnfold ( isValueUnfolding ) +import CoreFVs ( exprFreeVars ) +import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, exprEtaExpandArity, bindNonRec ) +import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, lookupIdSubst ) +import Id ( Id, idType, isId, idName, + idOccInfo, idUnfolding, + mkId, idInfo ) -import IdInfo ( ArityInfo(..), DemandInfo ) -import Maybes ( maybeToBool ) -import PrelVals ( augmentId, buildId ) -import PrimOp ( primOpIsCheap ) -import SimplEnv +import IdInfo ( arityLowerBound, setOccInfo, vanillaIdInfo ) +import Maybes ( maybeToBool, catMaybes ) +import Name ( isLocalName, setNameUnique ) import SimplMonad -import Type ( tyVarsOfType, mkForAllTys, mkTyVarTys, isPrimType, getTyVar_maybe, - maybeAppDataTyConExpandingDicts, SYN_IE(Type) +import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType, repType, + splitTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys ) -import TyCon ( isDataTyCon ) -import TyVar ( elementOfTyVarSet, - GenTyVar{-instance Eq-} ) -import Util ( isIn, panic, assertPanic ) +import TyCon ( tyConDataConsIfAvailable ) +import PprType ( {- instance Outputable Type -} ) +import DataCon ( dataConRepArity ) +import TysPrim ( statePrimTyCon ) +import Var ( setVarUnique ) +import VarSet +import VarEnv ( SubstEnv, SubstResult(..) ) +import UniqSupply ( splitUniqSupply, uniqFromSupply ) +import Util ( zipWithEqual, mapAccumL ) +import Outputable +\end{code} + + +%************************************************************************ +%* * +\subsection{The continuation data type} +%* * +%************************************************************************ + +\begin{code} +data SimplCont -- Strict contexts + = Stop OutType -- Type of the result + + | 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 -- 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 + +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") + +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 + +pushArgs :: SubstEnv -> [InExpr] -> SimplCont -> SimplCont +pushArgs se [] cont = cont +pushArgs se (arg:args) cont = ApplyTo NoDup arg se (pushArgs se args cont) + +discardCont :: SimplCont -- A continuation, expecting + -> SimplCont -- Replace the continuation with a suitable coerce +discardCont (Stop to_ty) = Stop to_ty +discardCont cont = CoerceIt to_ty (Stop 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} + + +Comment about analyseCont +~~~~~~~~~~~~~~~~~~~~~~~~~ +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. + +Previously some_benefit used to return True only if the variable was +applied to some value arguments. This didn't work: + + let x = _coerce_ (T Int) Int (I# 3) in + case _coerce_ Int (T Int) x of + I# y -> .... + +we want to inline x, but can't see that it's a constructor in a case +scrutinee position, and some_benefit is False. + +Another example: + +dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t) + +.... case dMonadST _@_ x0 of (a,b,c) -> .... + +we'd really like to inline dMonadST here, but we *don't* want to +inline if the case expression is just + case x of y { DEFAULT -> ... } + +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} +analyseCont :: InScopeSet -> SimplCont + -> ([Bool], -- Arg-info flags; one for each value argument + Bool, -- Context of the result of the call is interesting + Bool) -- There was an InlinePlease + +analyseCont in_scope cont + = case cont of + -- 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). + -- Why not? At least in the case-scrutinee situation, turning + -- case x of y -> ... + -- into + -- 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 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. + + (Stop _) -> boring_result -- Don't inline a lone variable + (Select _ _ _ _ _) -> boring_result -- Ditto + (ArgOf _ _ _) -> boring_result -- Ditto + (ApplyTo _ (Type _) _ cont) -> analyse_ty_app cont + other -> analyse_app cont + where + boring_result = ([], False, False) + + -- For now, I'm treating not treating a variable applied to types as + -- "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. + analyse_ty_app (Stop _) = boring_result + analyse_ty_app (ArgOf _ _ _) = boring_result + analyse_ty_app (Select _ _ _ _ _) = ([], True, False) -- See the $fMonadST example above + analyse_ty_app (ApplyTo _ (Type _) _ cont) = analyse_ty_app cont + analyse_ty_app cont = analyse_app cont + + analyse_app (InlinePlease cont) + = case analyse_app cont of + (infos, icont, inline) -> (infos, icont, True) + + analyse_app (ApplyTo _ arg subst cont) + | isValArg arg = case analyse_app cont of + (infos, icont, inline) -> (analyse_arg subst arg : infos, icont, inline) + | otherwise = analyse_app cont + + analyse_app cont = ([], interesting_call_context cont, False) + + -- 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. + analyse_arg :: SubstEnv -> InExpr -> Bool + analyse_arg subst (Var v) = case lookupIdSubst (mkSubst in_scope subst) v of + DoneId v' _ -> isValueUnfolding (idUnfolding v') + other -> False + analyse_arg subst (Type _) = False + analyse_arg subst (App fn (Type _)) = analyse_arg subst fn + analyse_arg subst (Note _ a) = analyse_arg subst a + analyse_arg subst other = True + + interesting_call_context (Stop ty) = canUpdateInPlace ty + interesting_call_context (InlinePlease _) = True + interesting_call_context (Select _ _ _ _ _) = True + interesting_call_context (CoerceIt _ cont) = interesting_call_context cont + interesting_call_context (ApplyTo _ (Type _) _ cont) = interesting_call_context cont + interesting_call_context (ApplyTo _ _ _ _) = True + interesting_call_context (ArgOf _ _ _) = True + -- 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. + + +discardInline :: SimplCont -> SimplCont +discardInline (InlinePlease cont) = cont +discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont) +discardInline cont = cont + +-- 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 = 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} -Floating -~~~~~~~~ -The function @floatExposesHNF@ tells whether let/case floating will -expose a head normal form. It is passed booleans indicating the -desired strategy. + +%************************************************************************ +%* * +\section{Dealing with a single binder} +%* * +%************************************************************************ + +\begin{code} +simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a +simplBinders bndrs thing_inside + = getSubst `thenSmpl` \ subst -> + let + (subst', bndrs') = substBndrs 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') = substBndr subst bndr + in + seqBndr bndr' `seq` + setSubst subst' (thing_inside bndr') + + +-- Same semantics as simplBinders, but a little less +-- plumbing and hence a little more efficient. +-- Maybe not worth the candle? +simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a +simplIds ids thing_inside + = getSubst `thenSmpl` \ subst -> + let + (subst', bndrs') = substIds subst ids + in + seqBndrs bndrs' `seq` + setSubst subst' (thing_inside bndrs') + +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} + + +%************************************************************************ +%* * +\subsection{Transform a RHS} +%* * +%************************************************************************ + +Try (a) eta expansion + (b) type-lambda swizzling \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 +transformRhs :: InExpr -> SimplM InExpr +transformRhs rhs + = tryEtaExpansion body `thenSmpl` \ body' -> + mkRhsTyLam tyvars body' 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 = case mkFormSummary other of - VarForm -> True - ValueForm -> True - other -> False - {- but *not* necessarily "BottomForm"... - - 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 + (tyvars, body) = collectTyBinders rhs \end{code} -Local tyvar-lifting -~~~~~~~~~~~~~~~~~~~ +%************************************************************************ +%* * +\subsection{Local tyvar-lifting} +%* * +%************************************************************************ + mkRhsTyLam tries this transformation, when the big lambda appears as the RHS of a let(rec) binding: @@ -124,7 +391,7 @@ This is good because it can turn things like: into letrec g' = /\a -> ... g' a ... in - let f = /\ a -> f a + let f = /\ a -> g' a which is better. In effect, it means that big lambdas don't impede let-floating. @@ -151,40 +418,112 @@ So far as the implemtation is concerned: where G = F . Let {xi = xi' tvs} -\begin{code} -mkRhsTyLam [] body = returnSmpl body +[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 } -mkRhsTyLam tyvars body +If we abstract this wrt the tyvar we then can't do the case inline +as we would normally do. + + +\begin{code} +mkRhsTyLam tyvars body -- Only does something if there's a let + | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that + = returnSmpl (mkLams tyvars body) + | otherwise = go (\x -> x) body where - tyvar_tys = mkTyVarTys tyvars + worth_it (Let _ e) = whnf_in_middle e + worth_it other = False + whnf_in_middle (Let _ e) = whnf_in_middle e + whnf_in_middle e = exprIsCheap e + + main_tyvar_set = mkVarSet tyvars go fn (Let bind@(NonRec var rhs) body) | exprIsTrivial rhs = go (fn . Let bind) body go fn (Let bind@(NonRec var rhs) body) - = mk_poly var `thenSmpl` \ (var', rhs') -> + = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') -> go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ body' -> - returnSmpl (Let (NonRec var' (mkTyLam tyvars (fn rhs))) body') + returnSmpl (Let (NonRec var' (mkLams tyvars_here (fn rhs))) body') + where + tyvars_here = tyvars + -- 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 + + var_ty = idType var go fn (Let (Rec prs) body) - = mapAndUnzipSmpl mk_poly vars `thenSmpl` \ (vars', rhss') -> + = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') -> let gn body = fn $ foldr Let body (zipWith mk_silly_bind vars rhss') in go gn body `thenSmpl` \ body' -> - returnSmpl (Let (Rec (vars' `zip` [mkTyLam tyvars (gn rhs) | rhs <- rhss])) body') + returnSmpl (Let (Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])) body') where (vars,rhss) = unzip prs + tyvars_here = tyvars + -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys) + -- See notes with tyvars_here above + + var_tys = map idType vars + + go fn body = returnSmpl (mkLams tyvars (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 + + -- It's crucial to copy the occInfo of the original var, because + -- we're looking at occurrence-analysed but as yet unsimplified code! + -- In particular, we mustn't lose the loop breakers. + -- + -- 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 E. 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 originaly + -- pinned on x. + poly_info = vanillaIdInfo `setOccInfo` idOccInfo var + + poly_id = mkId poly_name poly_ty poly_info + in + returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here)) - go fn body = returnSmpl (mkTyLam tyvars (fn body)) - - mk_poly var - = newId (mkForAllTys tyvars (idType var)) `thenSmpl` \ poly_id -> - returnSmpl (poly_id, mkTyApp (Var poly_id) tyvar_tys) - - mk_silly_bind var rhs = NonRec (addInlinePragma var) rhs - -- The addInlinePragma is really important! If we don't say + mk_silly_bind var rhs = NonRec var rhs + -- The Inline note is really important! If we don't say -- INLINE on these silly little bindings then look what happens! -- Suppose we start with: -- @@ -196,279 +535,218 @@ mkRhsTyLam tyvars body -- * but then it gets inlined into the rhs of g* -- * then the binding for g* is floated out of the /\b -- * so we're back to square one - -- The silly binding for g* must be INLINE, so that no inlining - -- will happen in its RHS. + -- The silly binding for g* must be INLINEd, so that + -- we simply substitute for g* throughout. \end{code} -Eta reduction -~~~~~~~~~~~~~ -@etaCoreExpr@ trys an eta reduction at the top level of a Core Expr. - -e.g. \ x y -> f x y ===> f - -It is used - a) Before constructing an Unfolding, to - try to make the unfolding smaller; - b) In tidyCoreExpr, which is done just before converting to STG. - -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. - -Doing eta on type lambdas is useful too: - - /\a -> a ===> - -where doesn't mention a. -This is sometimes quite useful, because we can get the sequence: - f ab d = let d1 = ...d... in - letrec f' b x = ...d...(f' b)... in - f' b -specialise ==> +%************************************************************************ +%* * +\subsection{Eta expansion} +%* * +%************************************************************************ - f.Int b = letrec f' b x = ...dInt...(f' b)... in - f' b + Try eta expansion for RHSs -float ==> +We go for: + \x1..xn -> N ==> \x1..xn y1..ym -> N y1..ym + AND + N E1..En ==> let z1=E1 .. zn=En in \y1..ym -> N z1..zn y1..ym - f' b x = ...dInt...(f' b)... - f.Int b = f' b +where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere) +wanting a suitable number of extra args. -Now we really want to simplify to +NB: the Ei may have unlifted type, but the simplifier (which is applied +to the result) deals OK with this. - f.Int = f' - -and then replace all the f's with f.Ints. - -N.B. We are careful not to partially eta-reduce a sequence of type -applications since this breaks the specialiser: - - /\ a -> f Char# a =NO=> f Char# +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. \begin{code} -etaCoreExpr :: CoreExpr -> CoreExpr - - -etaCoreExpr expr@(Lam bndr body) - | opt_DoEtaReduction - = case etaCoreExpr body of - App fun arg | eta_match bndr arg && - residual_ok fun - -> fun -- Eta - other -> expr -- Can't eliminate it, so do nothing at all +tryEtaExpansion :: InExpr -> SimplM InExpr +tryEtaExpansion rhs + | not opt_SimplDoLambdaEtaExpansion + || exprIsTrivial rhs -- Don't eta-expand a trival RHS + || null y_tys -- No useful expansion + || not (null x_bndrs || and trivial_args) -- Not (no x-binders or no z-binds) + = returnSmpl rhs + + | otherwise -- Consider eta expansion + = newIds SLIT("y") y_tys $ ( \ y_bndrs -> + tick (EtaExpansion (head y_bndrs)) `thenSmpl_` + mapAndUnzipSmpl bind_z_arg (args `zip` trivial_args) `thenSmpl` (\ (maybe_z_binds, z_args) -> + returnSmpl (mkLams x_bndrs $ + mkLets (catMaybes maybe_z_binds) $ + mkLams y_bndrs $ + mkApps (mkApps fun z_args) (map Var y_bndrs)))) where - eta_match (ValBinder v) (VarArg v') = v == v' - eta_match (TyBinder tv) (TyArg ty) = case getTyVar_maybe ty of - Nothing -> False - Just tv' -> tv == tv' - eta_match bndr arg = False - - residual_ok :: CoreExpr -> Bool -- Checks for type application - -- and function not one of the - -- bound vars - - (VarArg v) `mentions` (ValBinder v') = v == v' - (TyArg ty) `mentions` (TyBinder tv) = tv `elementOfTyVarSet` tyVarsOfType ty - bndr `mentions` arg = False - - residual_ok (Var v) - = not (VarArg v `mentions` bndr) - residual_ok (App fun arg) - | arg `mentions` bndr = False - | otherwise = residual_ok fun - residual_ok (Coerce coercion ty body) - | TyArg ty `mentions` bndr = False - | otherwise = residual_ok body - - residual_ok other = False -- Safe answer - -- This last clause may seem conservative, but consider: - -- primops, constructors, and literals, are impossible here - -- let and case are unlikely (the argument would have been floated inside) - -- SCCs we probably want to be conservative about (not sure, but it's safe to be) + (x_bndrs, body) = collectValBinders rhs + (fun, args) = collectArgs body + trivial_args = map exprIsTrivial args + fun_arity = exprEtaExpandArity fun + + bind_z_arg (arg, trivial_arg) + | trivial_arg = returnSmpl (Nothing, arg) + | otherwise = newId SLIT("z") (exprType arg) $ \ z -> + returnSmpl (Just (NonRec z arg), Var z) + + -- Note: I used to try to avoid the exprType call by using + -- the type of the binder. But this type doesn't necessarily + -- belong to the same substitution environment as this rhs; + -- and we are going to make extra term binders (y_bndrs) from the type + -- which will be processed with the rhs substitution environment. + -- This only went wrong in a mind bendingly complicated case. + (potential_extra_arg_tys, inner_ty) = splitFunTys (exprType body) -etaCoreExpr expr = expr -- The common case -\end{code} + y_tys :: [InType] + y_tys = take no_extras_wanted potential_extra_arg_tys + no_extras_wanted :: Int + no_extras_wanted = 0 `max` + + -- We used to expand the arity to the previous arity fo the + -- function; but this is pretty dangerous. Consdier + -- f = \xy -> e + -- so that f has arity 2. Now float something into f's RHS: + -- f = let z = BIG in \xy -> e + -- The last thing we want to do now is to put some lambdas + -- outside, to get + -- f = \xy -> let z = BIG in e + -- + -- (bndr_arity - no_of_xs) `max` + + -- See if the body could obviously do with more args + (fun_arity - valArgCount args) + +-- This case is now deal with by exprEtaExpandArity + -- Finally, see if it's a state transformer, and xs is non-null + -- (so it's also a function not a thunk) in which + -- case we eta-expand on principle! This can waste work, + -- but usually doesn't. + -- I originally checked for a singleton type [ty] in this case + -- but then I found a situation in which I had + -- \ x -> let {..} in \ s -> f (...) s + -- AND f RETURNED A FUNCTION. That is, 's' wasn't the only + -- potential extra arg. +-- case (x_bndrs, potential_extra_arg_tys) of +-- (_:_, ty:_) -> case splitTyConApp_maybe ty of +-- Just (tycon,_) | tycon == statePrimTyCon -> 1 +-- other -> 0 +-- other -> 0 +\end{code} -Eta expansion -~~~~~~~~~~~~~ -@etaExpandCount@ takes an expression, E, and returns an integer n, -such that - - E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn) - -is a safe transformation. In particular, the transformation should -not cause work to be duplicated, unless it is ``cheap'' (see -@manifestlyCheap@ below). - -@etaExpandCount@ errs on the conservative side. It is always safe to -return 0. -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. +%************************************************************************ +%* * +\subsection{Case absorption and identity-case elimination} +%* * +%************************************************************************ \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) = idMinArity v - -eta_fun other = 0 -- Give up +mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr \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: +@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. - * case e of - pi -> ei - - where e, and all the ei are cheap; and - - * let x = e - in b +\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} - where e and b are cheap; and +Now the identity-case transformation: - * op x1 ... xn + case e of ===> e + True -> True; + False -> False - where op is a cheap primitive operator +and similar friends. \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 - num_val_args < idMinArity f - - _ -> False - } +mkCase scrut case_bndr alts + | all identity_alt alts + = tick (CaseIdentity case_bndr) `thenSmpl_` + returnSmpl scrut + where + identity_alt (DEFAULT, [], Var v) = v == case_bndr + identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs + (mkConApp con (map Type arg_tys ++ map varToCoreExpr args)) + identity_alt other = False + arg_tys = case splitTyConApp_maybe (idType case_bndr) of + Just (tycon, arg_tys) -> arg_tys \end{code} +The catch-all case \begin{code} -simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool +mkCase other_scrut case_bndr other_alts + = returnSmpl (Case other_scrut case_bndr other_alts) +\end{code} -simplIdWantsToBeINLINEd id env - = {- We used to arrange that in the final simplification pass we'd switch - off all INLINE pragmas, so that we'd inline workers back into the - body of their wrapper if the wrapper hadn't itself been inlined by then. - This occurred especially for methods in dictionaries. - We no longer do this: - a) there's a good chance that the exported wrapper will get - inlined in some importing scope, in which case we don't - want to lose the w/w idea. +\begin{code} +findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr) +findDefault [] = ([], Nothing) +findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args ) + ([], Just rhs) +findDefault (alt : alts) = case findDefault alts of + (alts', deflt) -> (alt : alts', deflt) + +findAlt :: AltCon -> [CoreAlt] -> CoreAlt +findAlt con alts + = go alts + where + go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts)) + go (alt : alts) | matches alt = alt + | otherwise = go alts - b) The occurrence analyser must agree about what has an - INLINE pragma. Not hard, but delicate. - - c) if the worker gets inlined we have to tell the wrapepr - that it's no longer a wrapper, else the interface file stuff - asks for a worker that no longer exists. - - if switchIsSet env IgnoreINLINEPragma - then False - else - -} - - idWantsToBeINLINEd id - -idMinArity id = case getIdArity id of - UnknownArity -> 0 - ArityAtLeast n -> n - ArityExactly n -> n - -singleConstructorType :: Type -> Bool -singleConstructorType ty - = case (maybeAppDataTyConExpandingDicts ty) of - Just (tycon, ty_args, [con]) | isDataTyCon tycon -> True - other -> False - -typeOkForCase :: Type -> Bool -typeOkForCase ty - = case (maybeAppDataTyConExpandingDicts ty) of - Just (tycon, ty_args, []) -> False - Just (tycon, ty_args, non_null_data_cons) | isDataTyCon tycon -> True - other -> False - -- Null data cons => type is abstract, which code gen can't - -- currently handle. (ToDo: when return-in-heap is universal we - -- don't need to worry about this.) + matches (DEFAULT, _, _) = True + matches (con1, _, _) = con == con1 \end{code}