X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FsimplCore%2FSimplUtils.lhs;h=7e9a010051ee64822a2604bf0d628125e5b5df63;hb=6cec61d14a324285dbb8ce73d4c7215f1f8d6766;hp=1fb04febf1aeae0e022292753238de01b4b35aa6;hpb=fb982282ff6307b342d8fbc09b58a990d76c68fb;p=ghc-hetmet.git diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs index 1fb04fe..7e9a010 100644 --- a/compiler/simplCore/SimplUtils.lhs +++ b/compiler/simplCore/SimplUtils.lhs @@ -6,12 +6,13 @@ \begin{code} module SimplUtils ( -- Rebuilding - mkLam, mkCase, prepareAlts, + mkLam, mkCase, prepareAlts, tryEtaExpand, -- Inlining, preInlineUnconditionally, postInlineUnconditionally, - activeUnfolding, activeUnfInRule, activeRule, - simplEnvForGHCi, simplEnvForRules, updModeForInlineRules, + activeUnfolding, activeRule, + getUnfoldingInRuleMatch, + simplEnvForGHCi, updModeForInlineRules, -- The continuation type SimplCont(..), DupFlag(..), ArgInfo(..), @@ -29,7 +30,7 @@ module SimplUtils ( #include "HsVersions.h" import SimplEnv -import CoreMonad ( SimplifierMode(..), Tick(..) ) +import CoreMonad ( SimplifierMode(..), Tick(..) ) import DynFlags import StaticFlags import CoreSyn @@ -41,9 +42,10 @@ import CoreArity import CoreUnfold import Name import Id -import Var ( Var, isCoVar ) +import Var import Demand import SimplMonad +import TcType ( isDictLikeTy ) import Type hiding( substTy ) import Coercion ( coercionKind ) import TyCon @@ -452,33 +454,55 @@ interestingArgContext rules call_cont \end{code} - %************************************************************************ %* * -\subsection{Decisions about inlining} + SimplifierMode %* * %************************************************************************ -Inlining is controlled partly by the SimplifierMode switch. This has two -settings - - SimplGently (a) Simplifying before specialiser/full laziness - (b) Simplifiying inside InlineRules - (c) Simplifying the LHS of a rule - (d) Simplifying a GHCi expression or Template - Haskell splice - - SimplPhase n _ Used at all other times - -Note [Gentle mode] -~~~~~~~~~~~~~~~~~~ -Gentle mode has a separate boolean flag to control - a) inlining (sm_inline flag) - b) rules (sm_rules flag) -A key invariant about Gentle mode is that it is treated as the EARLIEST -phase. Something is inlined if the sm_inline flag is on AND the thing -is inlinable in the earliest phase. This is important. Example +The SimplifierMode controls several switches; see its definition in +CoreMonad + sm_rules :: Bool -- Whether RULES are enabled + sm_inline :: Bool -- Whether inlining is enabled + sm_case_case :: Bool -- Whether case-of-case is enabled + sm_eta_expand :: Bool -- Whether eta-expansion is enabled + +\begin{code} +simplEnvForGHCi :: DynFlags -> SimplEnv +simplEnvForGHCi dflags + = mkSimplEnv $ SimplMode { sm_names = ["GHCi"] + , sm_phase = InitialPhase + , sm_rules = rules_on + , sm_inline = False + , sm_eta_expand = eta_expand_on + , sm_case_case = True } + where + rules_on = dopt Opt_EnableRewriteRules dflags + eta_expand_on = dopt Opt_DoLambdaEtaExpansion dflags + -- Do not do any inlining, in case we expose some unboxed + -- tuple stuff that confuses the bytecode interpreter + +updModeForInlineRules :: Activation -> SimplifierMode -> SimplifierMode +-- See Note [Simplifying inside InlineRules] +updModeForInlineRules inline_rule_act current_mode + = current_mode { sm_phase = phaseFromActivation inline_rule_act + , sm_inline = True + , sm_eta_expand = False } + -- For sm_rules, just inherit; sm_rules might be "off" + -- becuase of -fno-enable-rewrite-rules + where + phaseFromActivation (ActiveAfter n) = Phase n + phaseFromActivation _ = InitialPhase +\end{code} + +Note [Inlining in gentle mode] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Something is inlined if + (i) the sm_inline flag is on, AND + (ii) the thing has an INLINE pragma, AND + (iii) the thing is inlinable in the earliest phase. +Example of why (iii) is important: {-# INLINE [~1] g #-} g = ... @@ -508,22 +532,6 @@ running it, we don't want to use -O2. Indeed, we don't want to inline anything, because the byte-code interpreter might get confused about unboxed tuples and suchlike. -Note [RULEs enabled in SimplGently] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -RULES are enabled when doing "gentle" simplification. Two reasons: - - * We really want the class-op cancellation to happen: - op (df d1 d2) --> $cop3 d1 d2 - because this breaks the mutual recursion between 'op' and 'df' - - * I wanted the RULE - lift String ===> ... - to work in Template Haskell when simplifying - splices, so we get simpler code for literal strings - -But watch out: list fusion can prevent floating. So use phase control -to switch off those rules until after floating. - Note [Simplifying inside InlineRules] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We must take care with simplification inside InlineRules (which come from @@ -542,8 +550,9 @@ one; see OccurAnal.addRuleUsage. Second, we do want *do* to some modest rules/inlining stuff in InlineRules, partly to eliminate senseless crap, and partly to break the recursive knots -generated by instance declarations. To keep things simple, we always set -the phase to 'gentle' when processing InlineRules. OK, so suppose we have +generated by instance declarations. + +However, suppose we have {-# INLINE f #-} f = meaning "inline f in phases p where activation (p) holds". @@ -553,12 +562,8 @@ f when it is inlined. So our conservative plan (implemented by updModeForInlineRules) is this: ------------------------------------------------------------- - When simplifying the RHS of an InlineRule, - If the InlineRule becomes active in phase p, then - if the current phase is *earlier than* p, - make no inlinings or rules active when simplifying the RHS - otherwise - set the phase to p when simplifying the RHS + When simplifying the RHS of an InlineRule, set the phase to the + phase in which the InlineRule first becomes active ------------------------------------------------------------- That ensures that @@ -572,13 +577,13 @@ That ensures that inlining the *original* rhs in phase p. For example, - {-# INLINE f #-} - f x = ...g... + {-# INLINE f #-} + f x = ...g... - {-# NOINLINE [1] g #-} - g y = ... + {-# NOINLINE [1] g #-} + g y = ... - {-# RULE h g = ... #-} + {-# RULE h g = ... #-} Here we must not inline g into f's RHS, even when we get to phase 0, because when f is later inlined into some other module we want the rule for h to fire. @@ -607,42 +612,75 @@ mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf continuation. \begin{code} -simplEnvForGHCi :: SimplEnv -simplEnvForGHCi = mkSimplEnv allOffSwitchChecker $ - SimplGently { sm_rules = False, sm_inline = False } - -- Do not do any inlining, in case we expose some unboxed - -- tuple stuff that confuses the bytecode interpreter - -simplEnvForRules :: SimplEnv -simplEnvForRules = mkSimplEnv allOffSwitchChecker $ - SimplGently { sm_rules = True, sm_inline = False } +activeUnfolding :: SimplEnv -> Id -> Bool +activeUnfolding env + | not (sm_inline mode) = active_unfolding_minimal + | otherwise = case sm_phase mode of + InitialPhase -> active_unfolding_gentle + Phase n -> active_unfolding n + where + mode = getMode env -updModeForInlineRules :: Activation -> SimplifierMode -> SimplifierMode --- See Note [Simplifying inside InlineRules] --- Treat Gentle as phase "infinity" --- If current_phase `earlier than` inline_rule_start_phase --- then no_op --- else --- if current_phase `same phase` inline_rule_start_phase --- then current_phase (keep gentle flags) --- else inline_rule_start_phase -updModeForInlineRules inline_rule_act current_mode - = case inline_rule_act of - NeverActive -> no_op - AlwaysActive -> mk_gentle current_mode - ActiveBefore {} -> mk_gentle current_mode - ActiveAfter n -> mk_phase n current_mode +getUnfoldingInRuleMatch :: SimplEnv -> IdUnfoldingFun +-- When matching in RULE, we want to "look through" an unfolding +-- (to see a constructor) if *rules* are on, even if *inlinings* +-- are not. A notable example is DFuns, which really we want to +-- match in rules like (op dfun) in gentle mode. Another example +-- is 'otherwise' which we want exprIsConApp_maybe to be able to +-- see very early on +getUnfoldingInRuleMatch env id + | unf_is_active = idUnfolding id + | otherwise = NoUnfolding where - no_op = SimplGently { sm_rules = False, sm_inline = False } + mode = getMode env + unf_is_active + | not (sm_rules mode) = active_unfolding_minimal id + | otherwise = isActive (sm_phase mode) (idInlineActivation id) - mk_gentle (SimplGently {}) = current_mode - mk_gentle _ = SimplGently { sm_rules = True, sm_inline = True } +active_unfolding_minimal :: Id -> Bool +-- Compuslory unfoldings only +-- Ignore SimplGently, because we want to inline regardless; +-- the Id has no top-level binding at all +-- +-- NB: we used to have a second exception, for data con wrappers. +-- On the grounds that we use gentle mode for rule LHSs, and +-- they match better when data con wrappers are inlined. +-- But that only really applies to the trivial wrappers (like (:)), +-- and they are now constructed as Compulsory unfoldings (in MkId) +-- so they'll happen anyway. +active_unfolding_minimal id = isCompulsoryUnfolding (realIdUnfolding id) + +active_unfolding :: PhaseNum -> Id -> Bool +active_unfolding n id = isActiveIn n (idInlineActivation id) + +active_unfolding_gentle :: Id -> Bool +-- Anything that is early-active +-- See Note [Gentle mode] +active_unfolding_gentle id + = isInlinePragma prag + && isEarlyActive (inlinePragmaActivation prag) + -- NB: wrappers are not early-active + where + prag = idInlinePragma id - mk_phase n (SimplPhase _ ss) = SimplPhase n ss - mk_phase n (SimplGently {}) = SimplPhase n ["gentle-rules"] +---------------------- +activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool) +-- Nothing => No rules at all +activeRule _dflags env + | not (sm_rules mode) = Nothing -- Rewriting is off + | otherwise = Just (isActive (sm_phase mode)) + where + mode = getMode env \end{code} + +%************************************************************************ +%* * + preInlineUnconditionally +%* * +%************************************************************************ + preInlineUnconditionally ~~~~~~~~~~~~~~~~~~~~~~~~ @preInlineUnconditionally@ examines a bndr to see if it is used just @@ -758,11 +796,9 @@ preInlineUnconditionally env top_lvl bndr rhs OneOcc in_lam True int_cxt -> try_once in_lam int_cxt _ -> False where - phase = getMode env - active = case phase of - SimplGently {} -> isEarlyActive act - -- See Note [pre/postInlineUnconditionally in gentle mode] - SimplPhase n _ -> isActive n act + mode = getMode env + active = isActive (sm_phase mode) act + -- See Note [pre/postInlineUnconditionally in gentle mode] act = idInlineActivation bndr try_once in_lam int_cxt -- There's one textual occurrence | not in_lam = isNotTopLevel top_lvl || early_phase @@ -794,9 +830,9 @@ preInlineUnconditionally env top_lvl bndr rhs canInlineInLam (Note _ e) = canInlineInLam e canInlineInLam _ = False - early_phase = case phase of - SimplPhase 0 _ -> False - _ -> True + early_phase = case sm_phase mode of + Phase 0 -> False + _ -> True -- If we don't have this early_phase test, consider -- x = length [1,2,3] -- The full laziness pass carefully floats all the cons cells to @@ -814,6 +850,12 @@ preInlineUnconditionally env top_lvl bndr rhs \end{code} +%************************************************************************ +%* * + postInlineUnconditionally +%* * +%************************************************************************ + postInlineUnconditionally ~~~~~~~~~~~~~~~~~~~~~~~~~ @postInlineUnconditionally@ decides whether to unconditionally inline @@ -821,7 +863,7 @@ a thing based on the form of its RHS; in particular if it has a trivial RHS. If so, we can inline and discard the binding altogether. NB: a loop breaker has must_keep_binding = True and non-loop-breakers -only have *forward* references Hence, it's safe to discard the binding +only have *forward* references. Hence, it's safe to discard the binding NOTE: This isn't our last opportunity to inline. We're at the binding site right now, and we'll get another opportunity when we get to the @@ -856,8 +898,8 @@ postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding -- because it might be referred to "earlier" | isExportedId bndr = False | isStableUnfolding unfolding = False -- Note [InlineRule and postInlineUnconditionally] - | exprIsTrivial rhs = True | isTopLevel top_lvl = False -- Note [Top level and postInlineUnconditionally] + | exprIsTrivial rhs = True | otherwise = case occ_info of -- The point of examining occ_info here is that for *non-values* @@ -918,86 +960,35 @@ postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding -- Alas! where - active = case getMode env of - SimplGently {} -> isEarlyActive act - -- See Note [pre/postInlineUnconditionally in gentle mode] - SimplPhase n _ -> isActive n act - act = idInlineActivation bndr - -activeUnfolding :: SimplEnv -> IdUnfoldingFun -activeUnfolding env - = case getMode env of - SimplGently { sm_inline = False } -> active_unfolding_minimal - SimplGently { sm_inline = True } -> active_unfolding_gentle - SimplPhase n _ -> active_unfolding n - -activeUnfInRule :: SimplEnv -> IdUnfoldingFun --- When matching in RULE, we want to "look through" an unfolding --- if *rules* are on, even if *inlinings* are not. A notable example --- is DFuns, which really we want to match in rules like (op dfun) --- in gentle mode. -activeUnfInRule env - = case getMode env of - SimplGently { sm_rules = False } -> active_unfolding_minimal - SimplGently { sm_rules = True } -> active_unfolding_gentle - SimplPhase n _ -> active_unfolding n - -active_unfolding_minimal :: IdUnfoldingFun --- Compuslory unfoldings only --- Ignore SimplGently, because we want to inline regardless; --- the Id has no top-level binding at all --- --- NB: we used to have a second exception, for data con wrappers. --- On the grounds that we use gentle mode for rule LHSs, and --- they match better when data con wrappers are inlined. --- But that only really applies to the trivial wrappers (like (:)), --- and they are now constructed as Compulsory unfoldings (in MkId) --- so they'll happen anyway. -active_unfolding_minimal id - | isCompulsoryUnfolding unf = unf - | otherwise = NoUnfolding - where - unf = realIdUnfolding id -- Never a loop breaker - -active_unfolding_gentle :: IdUnfoldingFun --- Anything that is early-active --- See Note [Gentle mode] -active_unfolding_gentle id - | isEarlyActive (idInlineActivation id) = idUnfolding id - | otherwise = NoUnfolding - -- idUnfolding checks for loop-breakers - -- Things with an INLINE pragma may have - -- an unfolding *and* be a loop breaker - -- (maybe the knot is not yet untied) - -active_unfolding :: CompilerPhase -> IdUnfoldingFun -active_unfolding n id - | isActive n (idInlineActivation id) = idUnfolding id - | otherwise = NoUnfolding - -activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool) --- Nothing => No rules at all -activeRule dflags env - | not (dopt Opt_EnableRewriteRules dflags) - = Nothing -- Rewriting is off - | otherwise - = case getMode env of - SimplGently { sm_rules = rules_on } - | rules_on -> Just isEarlyActive -- Note [RULEs enabled in SimplGently] - | otherwise -> Nothing - SimplPhase n _ -> Just (isActive n) + active = isActive (sm_phase (getMode env)) (idInlineActivation bndr) + -- See Note [pre/postInlineUnconditionally in gentle mode] \end{code} Note [Top level and postInlineUnconditionally] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -We don't do postInlineUnconditionally for top-level things (exept ones that -are trivial): - * There is no point, because the main goal is to get rid of local - bindings used in multiple case branches. +We don't do postInlineUnconditionally for top-level things (even for +ones that are trivial): + * Doing so will inline top-level error expressions that have been carefully floated out by FloatOut. More generally, it might replace static allocation with dynamic. + * Even for trivial expressions there's a problem. Consider + {-# RULE "foo" forall (xs::[T]). reverse xs = ruggle xs #-} + blah xs = reverse xs + ruggle = sort + In one simplifier pass we might fire the rule, getting + blah xs = ruggle xs + but in *that* simplifier pass we must not do postInlineUnconditionally + on 'ruggle' because then we'll have an unbound occurrence of 'ruggle' + + If the rhs is trivial it'll be inlined by callSiteInline, and then + the binding will be dead and discarded by the next use of OccurAnal + + * There is less point, because the main goal is to get rid of local + bindings used in multiple case branches. + + Note [InlineRule and postInlineUnconditionally] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Do not do postInlineUnconditionally if the Id has an InlineRule, otherwise @@ -1057,40 +1048,10 @@ mkLam _env bndrs body = do { tick (EtaReduction (head bndrs)) ; return etad_lam } - | dopt Opt_DoLambdaEtaExpansion dflags - , any ok_to_expand bndrs - = do { let body' = etaExpand fun_arity body - fun_arity = exprEtaExpandArity dflags body - ; return (mkLams bndrs body') } - | otherwise = return (mkLams bndrs body) - - ok_to_expand :: Var -> Bool -- Note [When to eta expand] - ok_to_expand bndr = isId bndr && not (isDictId bndr) \end{code} -Note [When to eta expand] -~~~~~~~~~~~~~~~~~~~~~~~~~ -We only eta expand if there is at least one non-tyvar, non-dict -binder. The proximate cause for not eta-expanding dictionary lambdas -was this example: - genMap :: C a => ... - {-# INLINE genMap #-} - genMap f xs = ... - - myMap :: D a => ... - {-# INLINE myMap #-} - myMap = genMap - -Notice that 'genMap' should only inline if applied to two arguments. -In the InlineRule for myMap we'll have the unfolding - (\d -> genMap Int (..d..)) -We do not want to eta-expand to - (\d f xs -> genMap Int (..d..) f xs) -because then 'genMap' will inline, and it really shouldn't: at least -as far as the programmer is concerned, it's not applied to two -arguments! Note [Casts and lambdas] ~~~~~~~~~~~~~~~~~~~~~~~~ @@ -1124,20 +1085,155 @@ It does not make sense to transform /\g. e `cast` g ==> (/\g.e) `cast` (/\g.g) because the latter is not well-kinded. --- c) floating lets out through big lambdas --- [only if all tyvar lambdas, and only if this lambda --- is the RHS of a let] - -{- Sept 01: I'm experimenting with getting the - full laziness pass to float out past big lambdsa - | all isTyCoVar bndrs, -- Only for big lambdas - contIsRhs cont -- Only try the rhs type-lambda floating - -- if this is indeed a right-hand side; otherwise - -- we end up floating the thing out, only for float-in - -- to float it right back in again! - = do (floats, body') <- tryRhsTyLam env bndrs body - return (floats, mkLams bndrs body') --} +%************************************************************************ +%* * + Eta expansion +%* * +%************************************************************************ + +When we meet a let-binding we try eta-expansion. To find the +arity of the RHS we use a little fixpoint analysis; see Note [Arity analysis] + +\begin{code} +tryEtaExpand :: SimplEnv -> OutId -> OutExpr -> SimplM (Arity, OutExpr) +-- See Note [Eta-expanding at let bindings] +tryEtaExpand env bndr rhs + = do { dflags <- getDOptsSmpl + ; (new_arity, new_rhs) <- try_expand dflags + + ; WARN( new_arity < old_arity || new_arity < _dmd_arity, + (ptext (sLit "Arity decrease:") <+> (ppr bndr <+> ppr old_arity + <+> ppr new_arity <+> ppr _dmd_arity) $$ ppr new_rhs) ) + -- Note [Arity decrease] + return (new_arity, new_rhs) } + where + try_expand dflags + | sm_eta_expand (getMode env) -- Provided eta-expansion is on + , not (exprIsTrivial rhs) + , let dicts_cheap = dopt Opt_DictsCheap dflags + new_arity = findArity dicts_cheap bndr rhs old_arity + , new_arity > rhs_arity + = do { tick (EtaExpansion bndr) + ; return (new_arity, etaExpand new_arity rhs) } + | otherwise + = return (rhs_arity, rhs) + + rhs_arity = exprArity rhs + old_arity = idArity bndr + _dmd_arity = length $ fst $ splitStrictSig $ idStrictness bndr + +findArity :: Bool -> Id -> CoreExpr -> Arity -> Arity +-- This implements the fixpoint loop for arity analysis +-- See Note [Arity analysis] +findArity dicts_cheap bndr rhs old_arity + = go (exprEtaExpandArity (mk_cheap_fn dicts_cheap init_cheap_app) rhs) + -- We always call exprEtaExpandArity once, but usually + -- that produces a result equal to old_arity, and then + -- we stop right away (since arities should not decrease) + -- Result: the common case is that there is just one iteration + where + go :: Arity -> Arity + go cur_arity + | cur_arity <= old_arity = cur_arity + | new_arity == cur_arity = cur_arity + | otherwise = ASSERT( new_arity < cur_arity ) + pprTrace "Exciting arity" + (vcat [ ppr bndr <+> ppr cur_arity <+> ppr new_arity + , ppr rhs]) + go new_arity + where + new_arity = exprEtaExpandArity (mk_cheap_fn dicts_cheap cheap_app) rhs + + cheap_app :: CheapAppFun + cheap_app fn n_val_args + | fn == bndr = n_val_args < cur_arity + | otherwise = isCheapApp fn n_val_args + + init_cheap_app :: CheapAppFun + init_cheap_app fn n_val_args + | fn == bndr = True + | otherwise = isCheapApp fn n_val_args + +mk_cheap_fn :: Bool -> CheapAppFun -> CheapFun +mk_cheap_fn dicts_cheap cheap_app + | not dicts_cheap + = \e _ -> exprIsCheap' cheap_app e + | otherwise + = \e mb_ty -> exprIsCheap' cheap_app e + || case mb_ty of + Nothing -> False + Just ty -> isDictLikeTy ty + -- If the experimental -fdicts-cheap flag is on, we eta-expand through + -- dictionary bindings. This improves arities. Thereby, it also + -- means that full laziness is less prone to floating out the + -- application of a function to its dictionary arguments, which + -- can thereby lose opportunities for fusion. Example: + -- foo :: Ord a => a -> ... + -- foo = /\a \(d:Ord a). let d' = ...d... in \(x:a). .... + -- -- So foo has arity 1 + -- + -- f = \x. foo dInt $ bar x + -- + -- The (foo DInt) is floated out, and makes ineffective a RULE + -- foo (bar x) = ... + -- + -- One could go further and make exprIsCheap reply True to any + -- dictionary-typed expression, but that's more work. + -- + -- See Note [Dictionary-like types] in TcType.lhs for why we use + -- isDictLikeTy here rather than isDictTy +\end{code} + +Note [Eta-expanding at let bindings] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We now eta expand at let-bindings, which is where the payoff +comes. + +One useful consequence is this example: + genMap :: C a => ... + {-# INLINE genMap #-} + genMap f xs = ... + + myMap :: D a => ... + {-# INLINE myMap #-} + myMap = genMap + +Notice that 'genMap' should only inline if applied to two arguments. +In the InlineRule for myMap we'll have the unfolding + (\d -> genMap Int (..d..)) +We do not want to eta-expand to + (\d f xs -> genMap Int (..d..) f xs) +because then 'genMap' will inline, and it really shouldn't: at least +as far as the programmer is concerned, it's not applied to two +arguments! + +Note [Arity analysis] +~~~~~~~~~~~~~~~~~~~~~ +The motivating example for arity analysis is this: + + f = \x. let g = f (x+1) + in \y. ...g... + +What arity does f have? Really it should have arity 2, but a naive +look at the RHS won't see that. You need a fixpoint analysis which +says it has arity "infinity" the first time round. + +This example happens a lot; it first showed up in Andy Gill's thesis, +fifteen years ago! It also shows up in the code for 'rnf' on lists +in Trac #4138. + +The analysis is easy to achieve because exprEtaExpandArity takes an +argument + type CheapFun = CoreExpr -> Maybe Type -> Bool +used to decide if an expression is cheap enough to push inside a +lambda. And exprIsCheap' in turn takes an argument + type CheapAppFun = Id -> Int -> Bool +which tells when an application is cheap. This makes it easy to +write the analysis loop. + +The analysis is cheap-and-cheerful because it doesn't deal with +mutual recursion. But the self-recursive case is the important one. + %************************************************************************ %* *