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diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs new file mode 100644 index 0000000000..9e616b5df1 --- /dev/null +++ b/compiler/simplCore/SimplUtils.lhs @@ -0,0 +1,1592 @@ +% +% (c) The AQUA Project, Glasgow University, 1993-1998 +% +\section[SimplUtils]{The simplifier utilities} + +\begin{code} +module SimplUtils ( + mkLam, prepareAlts, mkCase, + + -- Inlining, + preInlineUnconditionally, postInlineUnconditionally, activeInline, activeRule, + inlineMode, + + -- The continuation type + SimplCont(..), DupFlag(..), LetRhsFlag(..), + contIsDupable, contResultType, + countValArgs, countArgs, pushContArgs, + mkBoringStop, mkRhsStop, contIsRhs, contIsRhsOrArg, + getContArgs, interestingCallContext, interestingArg, isStrictType + + ) where + +#include "HsVersions.h" + +import SimplEnv +import DynFlags ( SimplifierSwitch(..), SimplifierMode(..), + DynFlag(..), dopt ) +import StaticFlags ( opt_UF_UpdateInPlace, opt_SimplNoPreInlining, + opt_RulesOff ) +import CoreSyn +import CoreFVs ( exprFreeVars ) +import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial, exprIsCheap, + etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2, + findDefault, exprOkForSpeculation, exprIsHNF + ) +import Literal ( mkStringLit ) +import CoreUnfold ( smallEnoughToInline ) +import MkId ( eRROR_ID ) +import Id ( idType, isDataConWorkId, idOccInfo, isDictId, + mkSysLocal, isDeadBinder, idNewDemandInfo, isExportedId, + idUnfolding, idNewStrictness, idInlinePragma, + ) +import NewDemand ( isStrictDmd, isBotRes, splitStrictSig ) +import SimplMonad +import Type ( Type, splitFunTys, dropForAlls, isStrictType, + splitTyConApp_maybe, tyConAppArgs, mkTyVarTys + ) +import Name ( mkSysTvName ) +import TyCon ( tyConDataCons_maybe, isAlgTyCon, isNewTyCon ) +import DataCon ( dataConRepArity, dataConTyVars, dataConInstArgTys, isVanillaDataCon ) +import Var ( tyVarKind, mkTyVar ) +import VarSet +import BasicTypes ( TopLevelFlag(..), isNotTopLevel, OccInfo(..), isLoopBreaker, isOneOcc, + Activation, isAlwaysActive, isActive ) +import Util ( lengthExceeds ) +import Outputable +\end{code} + + +%************************************************************************ +%* * +\subsection{The continuation data type} +%* * +%************************************************************************ + +\begin{code} +data SimplCont -- Strict contexts + = Stop OutType -- Type of the result + LetRhsFlag + 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 SimplEnv -- The argument, as yet unsimplified, + SimplCont -- and its environment + + | Select DupFlag + InId [InAlt] SimplEnv -- The case binder, alts, and subst-env + SimplCont + + | ArgOf LetRhsFlag -- An arbitrary strict context: the argument + -- of a strict function, or a primitive-arg fn + -- or a PrimOp + -- No DupFlag because we never duplicate it + OutType -- arg_ty: type of the argument itself + 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. + + (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result + -- The result expression in the OutExprStuff has type cont_ty + +data LetRhsFlag = AnArg -- It's just an argument not a let RHS + | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas) + +instance Outputable LetRhsFlag where + ppr AnArg = ptext SLIT("arg") + ppr AnRhs = ptext SLIT("rhs") + +instance Outputable SimplCont where + ppr (Stop ty is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs) <+> ppr ty + ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont + ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...") + ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$ + (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") + + +------------------- +mkBoringStop, mkRhsStop :: OutType -> SimplCont +mkBoringStop ty = Stop ty AnArg (canUpdateInPlace ty) +mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty) + +contIsRhs :: SimplCont -> Bool +contIsRhs (Stop _ AnRhs _) = True +contIsRhs (ArgOf AnRhs _ _ _) = True +contIsRhs other = False + +contIsRhsOrArg (Stop _ _ _) = True +contIsRhsOrArg (ArgOf _ _ _ _) = True +contIsRhsOrArg other = False + +------------------- +contIsDupable :: SimplCont -> Bool +contIsDupable (Stop _ _ _) = True +contIsDupable (ApplyTo OkToDup _ _ _) = True +contIsDupable (Select OkToDup _ _ _ _) = True +contIsDupable (CoerceIt _ cont) = contIsDupable cont +contIsDupable (InlinePlease cont) = contIsDupable cont +contIsDupable other = False + +------------------- +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 is_rhs _ -> cont + other -> CoerceIt to_ty (mkBoringStop 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 + +------------------- +pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont +-- Pushes args with the specified environment +pushContArgs env [] cont = cont +pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont) +\end{code} + + +\begin{code} +getContArgs :: SwitchChecker + -> OutId -> SimplCont + -> ([(InExpr, SimplEnv, 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 chkr fun orig_cont + = 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 + ---------------------------- + + -- 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 + -- Then, especially in the first of these cases, we'd like to discard + -- the continuation, leaving just the bottoming expression. But the + -- type might not be right, so we may have to add a coerce. + go acc ss inl cont + | null ss && discardableCont cont = (reverse acc, discardCont cont, inl) + | otherwise = (reverse acc, cont, inl) + + ---------------------------- + vanilla_stricts, computed_stricts :: [Bool] + vanilla_stricts = repeat False + computed_stricts = zipWith (||) fun_stricts arg_stricts + + ---------------------------- + (val_arg_tys, _) = splitFunTys (dropForAlls (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 splitFunTys*IgnoringForAlls* + + ---------------------------- + -- 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 splitStrictSig (idNewStrictness fun) of + (demands, result_info) + | 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 isBotRes result_info then + map isStrictDmd demands -- Finite => result is bottom + else + map isStrictDmd demands ++ vanilla_stricts + + other -> vanilla_stricts -- Not enough args, or no strictness + +------------------- +interestingArg :: OutExpr -> 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 (Var v) = hasSomeUnfolding (idUnfolding v) + -- Was: isValueUnfolding (idUnfolding v') + -- But that seems over-pessimistic + || isDataConWorkId v + -- This accounts for an argument like + -- () or [], which is definitely interesting +interestingArg (Type _) = False +interestingArg (App fn (Type _)) = interestingArg fn +interestingArg (Note _ a) = interestingArg a +interestingArg 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} + +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. + +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} +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 because + -- 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 + 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 = <wurble> in ... +-- If <wurble> 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. + +canUpdateInPlace ty + | not opt_UF_UpdateInPlace = False + | otherwise + = case splitTyConApp_maybe ty of + Nothing -> False + Just (tycon, _) -> case tyConDataCons_maybe tycon of + Just [dc] -> arity == 1 || arity == 2 + where + arity = dataConRepArity dc + other -> False +\end{code} + + + +%************************************************************************ +%* * +\subsection{Decisions about inlining} +%* * +%************************************************************************ + +Inlining is controlled partly by the SimplifierMode switch. This has two +settings: + + SimplGently (a) Simplifying before specialiser/full laziness + (b) Simplifiying inside INLINE pragma + (c) Simplifying the LHS of a rule + (d) Simplifying a GHCi expression or Template + Haskell splice + + SimplPhase n Used at all other times + +The key thing about SimplGently is that it does no call-site inlining. +Before full laziness we must be careful not to inline wrappers, +because doing so inhibits floating + e.g. ...(case f x of ...)... + ==> ...(case (case x of I# x# -> fw x#) of ...)... + ==> ...(case x of I# x# -> case fw x# of ...)... +and now the redex (f x) isn't floatable any more. + +The no-inling thing is also important for Template Haskell. You might be +compiling in one-shot mode with -O2; but when TH compiles a splice before +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. + +INLINE pragmas +~~~~~~~~~~~~~~ +SimplGently is also used as the mode to simplify inside an InlineMe note. + +\begin{code} +inlineMode :: SimplifierMode +inlineMode = SimplGently +\end{code} + +It really is important to switch off inlinings inside such +expressions. Consider the following example + + let f = \pq -> BIG + in + let g = \y -> f y y + {-# INLINE g #-} + in ...g...g...g...g...g... + +Now, if that's the ONLY occurrence of f, it will be inlined inside g, +and thence copied multiple times when g is inlined. + + +This function may be inlinined in other modules, so we +don't want to remove (by inlining) calls to functions that have +specialisations, or that may have transformation rules in an importing +scope. + +E.g. {-# INLINE f #-} + f x = ...g... + +and suppose that g is strict *and* has specialisations. If we inline +g's wrapper, we deny f the chance of getting the specialised version +of g when f is inlined at some call site (perhaps in some other +module). + +It's also important not to inline a worker back into a wrapper. +A wrapper looks like + wraper = inline_me (\x -> ...worker... ) +Normally, the inline_me prevents the worker getting inlined into +the wrapper (initially, the worker's only call site!). But, +if the wrapper is sure to be called, the strictness analyser will +mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf +continuation. That's why the keep_inline predicate returns True for +ArgOf continuations. It shouldn't do any harm not to dissolve the +inline-me note under these circumstances. + +Note that the result is that we do very little simplification +inside an InlineMe. + + all xs = foldr (&&) True xs + any p = all . map p {-# INLINE any #-} + +Problem: any won't get deforested, and so if it's exported and the +importer doesn't use the inlining, (eg passes it as an arg) then we +won't get deforestation at all. We havn't solved this problem yet! + + +preInlineUnconditionally +~~~~~~~~~~~~~~~~~~~~~~~~ +@preInlineUnconditionally@ examines a bndr to see if it is used just +once in a completely safe way, so that it is safe to discard the +binding inline its RHS at the (unique) usage site, REGARDLESS of how +big the RHS might be. If this is the case we don't simplify the RHS +first, but just inline it un-simplified. + +This is much better than first simplifying a perhaps-huge RHS and then +inlining and re-simplifying it. Indeed, it can be at least quadratically +better. Consider + + x1 = e1 + x2 = e2[x1] + x3 = e3[x2] + ...etc... + xN = eN[xN-1] + +We may end up simplifying e1 N times, e2 N-1 times, e3 N-3 times etc. +This can happen with cascades of functions too: + + f1 = \x1.e1 + f2 = \xs.e2[f1] + f3 = \xs.e3[f3] + ...etc... + +THE MAIN INVARIANT is this: + + ---- preInlineUnconditionally invariant ----- + IF preInlineUnconditionally chooses to inline x = <rhs> + THEN doing the inlining should not change the occurrence + info for the free vars of <rhs> + ---------------------------------------------- + +For example, it's tempting to look at trivial binding like + x = y +and inline it unconditionally. But suppose x is used many times, +but this is the unique occurrence of y. Then inlining x would change +y's occurrence info, which breaks the invariant. It matters: y +might have a BIG rhs, which will now be dup'd at every occurrenc of x. + + +Evne RHSs labelled InlineMe aren't caught here, because there might be +no benefit from inlining at the call site. + +[Sept 01] Don't unconditionally inline a top-level thing, because that +can simply make a static thing into something built dynamically. E.g. + x = (a,b) + main = \s -> h x + +[Remember that we treat \s as a one-shot lambda.] No point in +inlining x unless there is something interesting about the call site. + +But watch out: if you aren't careful, some useful foldr/build fusion +can be lost (most notably in spectral/hartel/parstof) because the +foldr didn't see the build. Doing the dynamic allocation isn't a big +deal, in fact, but losing the fusion can be. But the right thing here +seems to be to do a callSiteInline based on the fact that there is +something interesting about the call site (it's strict). Hmm. That +seems a bit fragile. + +Conclusion: inline top level things gaily until Phase 0 (the last +phase), at which point don't. + +\begin{code} +preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool +preInlineUnconditionally env top_lvl bndr rhs + | not active = False + | opt_SimplNoPreInlining = False + | otherwise = case idOccInfo bndr of + IAmDead -> True -- Happens in ((\x.1) v) + OneOcc in_lam True int_cxt -> try_once in_lam int_cxt + other -> False + where + phase = getMode env + active = case phase of + SimplGently -> isAlwaysActive prag + SimplPhase n -> isActive n prag + prag = idInlinePragma bndr + + try_once in_lam int_cxt -- There's one textual occurrence + | not in_lam = isNotTopLevel top_lvl || early_phase + | otherwise = int_cxt && canInlineInLam rhs + +-- Be very careful before inlining inside a lambda, becuase (a) we must not +-- invalidate occurrence information, and (b) we want to avoid pushing a +-- single allocation (here) into multiple allocations (inside lambda). +-- Inlining a *function* with a single *saturated* call would be ok, mind you. +-- || (if is_cheap && not (canInlineInLam rhs) then pprTrace "preinline" (ppr bndr <+> ppr rhs) ok else ok) +-- where +-- is_cheap = exprIsCheap rhs +-- ok = is_cheap && int_cxt + + -- int_cxt The context isn't totally boring + -- E.g. let f = \ab.BIG in \y. map f xs + -- Don't want to substitute for f, because then we allocate + -- its closure every time the \y is called + -- But: let f = \ab.BIG in \y. map (f y) xs + -- Now we do want to substitute for f, even though it's not + -- saturated, because we're going to allocate a closure for + -- (f y) every time round the loop anyhow. + + -- canInlineInLam => free vars of rhs are (Once in_lam) or Many, + -- so substituting rhs inside a lambda doesn't change the occ info. + -- Sadly, not quite the same as exprIsHNF. + canInlineInLam (Lit l) = True + canInlineInLam (Lam b e) = isRuntimeVar b || canInlineInLam e + canInlineInLam (Note _ e) = canInlineInLam e + canInlineInLam _ = False + + early_phase = case phase of + SimplPhase 0 -> False + other -> 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 +-- top level, and preInlineUnconditionally floats them all back in. +-- Result is (a) static allocation replaced by dynamic allocation +-- (b) many simplifier iterations because this tickles +-- a related problem; only one inlining per pass +-- +-- On the other hand, I have seen cases where top-level fusion is +-- lost if we don't inline top level thing (e.g. string constants) +-- Hence the test for phase zero (which is the phase for all the final +-- simplifications). Until phase zero we take no special notice of +-- top level things, but then we become more leery about inlining +-- them. + +\end{code} + +postInlineUnconditionally +~~~~~~~~~~~~~~~~~~~~~~~~~ +@postInlineUnconditionally@ decides whether to unconditionally inline +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 + +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 +ocurrence(s) + +Note that we do this unconditional inlining only for trival RHSs. +Don't inline even WHNFs inside lambdas; doing so may simply increase +allocation when the function is called. This isn't the last chance; see +NOTE above. + +NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here Why? +Because we don't even want to inline them into the RHS of constructor +arguments. See NOTE above + +NB: At one time even NOINLINE was ignored here: if the rhs is trivial +it's best to inline it anyway. We often get a=E; b=a from desugaring, +with both a and b marked NOINLINE. But that seems incompatible with +our new view that inlining is like a RULE, so I'm sticking to the 'active' +story for now. + +\begin{code} +postInlineUnconditionally :: SimplEnv -> TopLevelFlag -> OutId -> OccInfo -> OutExpr -> Unfolding -> Bool +postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding + | not active = False + | isLoopBreaker occ_info = False + | isExportedId bndr = False + | exprIsTrivial rhs = True + | otherwise + = case occ_info of + OneOcc in_lam one_br int_cxt + -> (one_br || smallEnoughToInline unfolding) -- Small enough to dup + -- ToDo: consider discount on smallEnoughToInline if int_cxt is true + -- + -- NB: Do we want to inline arbitrarily big things becuase + -- one_br is True? that can lead to inline cascades. But + -- preInlineUnconditionlly has dealt with all the common cases + -- so perhaps it's worth the risk. Here's an example + -- let f = if b then Left (\x.BIG) else Right (\y.BIG) + -- in \y. ....f.... + -- We can't preInlineUnconditionally because that woud invalidate + -- the occ info for b. Yet f is used just once, and duplicating + -- the case work is fine (exprIsCheap). + + && ((isNotTopLevel top_lvl && not in_lam) || + -- But outside a lambda, we want to be reasonably aggressive + -- about inlining into multiple branches of case + -- e.g. let x = <non-value> + -- in case y of { C1 -> ..x..; C2 -> ..x..; C3 -> ... } + -- Inlining can be a big win if C3 is the hot-spot, even if + -- the uses in C1, C2 are not 'interesting' + -- An example that gets worse if you add int_cxt here is 'clausify' + + (isCheapUnfolding unfolding && int_cxt)) + -- isCheap => acceptable work duplication; in_lam may be true + -- int_cxt to prevent us inlining inside a lambda without some + -- good reason. See the notes on int_cxt in preInlineUnconditionally + + other -> False + -- The point here is that for *non-values* that occur + -- outside a lambda, the call-site inliner won't have + -- a chance (becuase it doesn't know that the thing + -- only occurs once). The pre-inliner won't have gotten + -- it either, if the thing occurs in more than one branch + -- So the main target is things like + -- let x = f y in + -- case v of + -- True -> case x of ... + -- False -> case x of ... + -- I'm not sure how important this is in practice + where + active = case getMode env of + SimplGently -> isAlwaysActive prag + SimplPhase n -> isActive n prag + prag = idInlinePragma bndr + +activeInline :: SimplEnv -> OutId -> OccInfo -> Bool +activeInline env id occ + = case getMode env of + SimplGently -> isOneOcc occ && isAlwaysActive prag + -- No inlining at all when doing gentle stuff, + -- except for local things that occur once + -- The reason is that too little clean-up happens if you + -- don't inline use-once things. Also a bit of inlining is *good* for + -- full laziness; it can expose constant sub-expressions. + -- Example in spectral/mandel/Mandel.hs, where the mandelset + -- function gets a useful let-float if you inline windowToViewport + + -- NB: we used to have a second exception, for data con wrappers. + -- On the grounds that we use gentle mode for rule LHSs, and + -- they match better when data con wrappers are inlined. + -- But that only really applies to the trivial wrappers (like (:)), + -- and they are now constructed as Compulsory unfoldings (in MkId) + -- so they'll happen anyway. + + SimplPhase n -> isActive n prag + where + prag = idInlinePragma id + +activeRule :: SimplEnv -> Maybe (Activation -> Bool) +-- Nothing => No rules at all +activeRule env + | opt_RulesOff = Nothing + | otherwise + = case getMode env of + SimplGently -> Just isAlwaysActive + -- Used to be Nothing (no rules in gentle mode) + -- Main motivation for changing is that I wanted + -- lift String ===> ... + -- to work in Template Haskell when simplifying + -- splices, so we get simpler code for literal strings + SimplPhase n -> Just (isActive n) +\end{code} + + +%************************************************************************ +%* * +\subsection{Rebuilding a lambda} +%* * +%************************************************************************ + +\begin{code} +mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr +\end{code} + +Try three things + a) eta reduction, if that gives a trivial expression + b) eta expansion [only if there are some value lambdas] + c) floating lets out through big lambdas + [only if all tyvar lambdas, and only if this lambda + is the RHS of a let] + +\begin{code} +mkLam env bndrs body cont + = getDOptsSmpl `thenSmpl` \dflags -> + mkLam' dflags env bndrs body cont + where + mkLam' dflags env bndrs body cont + | dopt Opt_DoEtaReduction dflags, + Just etad_lam <- tryEtaReduce bndrs body + = tick (EtaReduction (head bndrs)) `thenSmpl_` + returnSmpl (emptyFloats env, etad_lam) + + | dopt Opt_DoLambdaEtaExpansion dflags, + any isRuntimeVar bndrs + = tryEtaExpansion body `thenSmpl` \ body' -> + returnSmpl (emptyFloats env, mkLams bndrs body') + +{- Sept 01: I'm experimenting with getting the + full laziness pass to float out past big lambdsa + | all isTyVar 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! + = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') -> + returnSmpl (floats, mkLams bndrs body') +-} + + | otherwise + = returnSmpl (emptyFloats env, mkLams bndrs body) +\end{code} + + +%************************************************************************ +%* * +\subsection{Eta expansion and reduction} +%* * +%************************************************************************ + +We try for eta reduction here, but *only* if we get all the +way to an exprIsTrivial expression. +We don't want to remove extra lambdas unless we are going +to avoid allocating this thing altogether + +\begin{code} +tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr +tryEtaReduce bndrs body + -- We don't use CoreUtils.etaReduce, because we can be more + -- efficient here: + -- (a) we already have the binders + -- (b) we can do the triviality test before computing the free vars + = go (reverse bndrs) body + where + go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round + go [] fun | ok_fun fun = Just fun -- Success! + go _ _ = Nothing -- Failure! + + ok_fun fun = exprIsTrivial fun + && not (any (`elemVarSet` (exprFreeVars fun)) bndrs) + && (exprIsHNF fun || all ok_lam bndrs) + ok_lam v = isTyVar v || isDictId v + -- The exprIsHNF is because eta reduction is not + -- valid in general: \x. bot /= bot + -- So we need to be sure that the "fun" is a value. + -- + -- However, we always want to reduce (/\a -> f a) to f + -- This came up in a RULE: foldr (build (/\a -> g a)) + -- did not match foldr (build (/\b -> ...something complex...)) + -- The type checker can insert these eta-expanded versions, + -- with both type and dictionary lambdas; hence the slightly + -- ad-hoc isDictTy + + ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg +\end{code} + + + Try eta expansion for RHSs + +We go for: + f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym + (n >= 0) + +where (in both cases) + + * The xi can include type variables + + * The yi are all value variables + + * N is a NORMAL FORM (i.e. no redexes anywhere) + wanting a suitable number of extra args. + +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. + +\begin{code} +tryEtaExpansion :: OutExpr -> SimplM OutExpr +-- There is at least one runtime binder in the binders +tryEtaExpansion body + = getUniquesSmpl `thenSmpl` \ us -> + returnSmpl (etaExpand fun_arity us body (exprType body)) + where + fun_arity = exprEtaExpandArity body +\end{code} + + +%************************************************************************ +%* * +\subsection{Floating lets out of big lambdas} +%* * +%************************************************************************ + +tryRhsTyLam 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. + + +\begin{code} +{- Trying to do this in full laziness + +tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr +-- Call ensures that all the binders are type variables + +tryRhsTyLam env tyvars body -- Only does something if there's a let + | not (all isTyVar tyvars) + || not (worth_it body) -- inside a type lambda, + = returnSmpl (emptyFloats env, body) -- and a WHNF inside that + + | otherwise + = go env (\x -> x) body + + where + 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 + + main_tyvar_set = mkVarSet tyvars + + go env fn (Let bind@(NonRec var rhs) body) + | exprIsTrivial rhs + = go env (fn . Let bind) body + + go env fn (Let (NonRec var rhs) body) + = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') -> + addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env -> + go env (fn . Let (mk_silly_bind var rhs')) body + + where + + tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs) + -- Abstract only over the type variables free in the rhs + -- 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 env 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')) + pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss] + in + addAuxiliaryBind env (Rec pairs) $ \ env -> + go env gn body + where + (vars,rhss) = unzip prs + tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs)) + -- See notes with tyvars_here above + + go env fn body = returnSmpl (emptyFloats env, 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} + +%************************************************************************ +%* * +\subsection{Case alternative filtering +%* * +%************************************************************************ + +prepareAlts does two things: + +1. Eliminate alternatives that cannot match, including the + DEFAULT alternative. + +2. If the DEFAULT alternative can match only one possible constructor, + then make that constructor explicit. + e.g. + case e of x { DEFAULT -> rhs } + ===> + case e of x { (a,b) -> rhs } + where the type is a single constructor type. This gives better code + when rhs also scrutinises x or e. + +It's a good idea do do this stuff before simplifying the alternatives, to +avoid simplifying alternatives we know can't happen, and to come up with +the list of constructors that are handled, to put into the IdInfo of the +case binder, for use when simplifying the alternatives. + +Eliminating the default alternative in (1) isn't so obvious, but it can +happen: + +data Colour = Red | Green | Blue + +f x = case x of + Red -> .. + Green -> .. + DEFAULT -> h x + +h y = case y of + Blue -> .. + DEFAULT -> [ case y of ... ] + +If we inline h into f, the default case of the inlined h can't happen. +If we don't notice this, we may end up filtering out *all* the cases +of the inner case y, which give us nowhere to go! + + +\begin{code} +prepareAlts :: OutExpr -- Scrutinee + -> InId -- Case binder (passed only to use in statistics) + -> [InAlt] -- Increasing order + -> SimplM ([InAlt], -- Better alternatives, still incresaing order + [AltCon]) -- These cases are handled + +prepareAlts scrut case_bndr alts + = let + (alts_wo_default, maybe_deflt) = findDefault alts + + impossible_cons = case scrut of + Var v -> otherCons (idUnfolding v) + other -> [] + + -- Filter out alternatives that can't possibly match + better_alts | null impossible_cons = alts_wo_default + | otherwise = [alt | alt@(con,_,_) <- alts_wo_default, + not (con `elem` impossible_cons)] + + -- "handled_cons" are handled either by the context, + -- or by a branch in this case expression + -- (Don't add DEFAULT to the handled_cons!!) + handled_cons = impossible_cons ++ [con | (con,_,_) <- better_alts] + in + -- Filter out the default, if it can't happen, + -- or replace it with "proper" alternative if there + -- is only one constructor left + prepareDefault scrut case_bndr handled_cons maybe_deflt `thenSmpl` \ deflt_alt -> + + returnSmpl (mergeAlts better_alts deflt_alt, handled_cons) + -- We need the mergeAlts in case the new default_alt + -- has turned into a constructor alternative. + +prepareDefault scrut case_bndr handled_cons (Just rhs) + | Just (tycon, inst_tys) <- splitTyConApp_maybe (exprType scrut), + -- Use exprType scrut here, rather than idType case_bndr, because + -- case_bndr is an InId, so exprType scrut may have more information + -- Test simpl013 is an example + isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples. + not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval: + -- case x of { DEFAULT -> e } + -- and we don't want to fill in a default for them! + Just all_cons <- tyConDataCons_maybe tycon, + not (null all_cons), -- This is a tricky corner case. If the data type has no constructors, + -- which GHC allows, then the case expression will have at most a default + -- alternative. We don't want to eliminate that alternative, because the + -- invariant is that there's always one alternative. It's more convenient + -- to leave + -- case x of { DEFAULT -> e } + -- as it is, rather than transform it to + -- error "case cant match" + -- which would be quite legitmate. But it's a really obscure corner, and + -- not worth wasting code on. + let handled_data_cons = [data_con | DataAlt data_con <- handled_cons], + let missing_cons = [con | con <- all_cons, + not (con `elem` handled_data_cons)] + = case missing_cons of + [] -> returnSmpl [] -- Eliminate the default alternative + -- if it can't match + + [con] -> -- It matches exactly one constructor, so fill it in + tick (FillInCaseDefault case_bndr) `thenSmpl_` + mk_args con inst_tys `thenSmpl` \ args -> + returnSmpl [(DataAlt con, args, rhs)] + + two_or_more -> returnSmpl [(DEFAULT, [], rhs)] + + | otherwise + = returnSmpl [(DEFAULT, [], rhs)] + +prepareDefault scrut case_bndr handled_cons Nothing + = returnSmpl [] + +mk_args missing_con inst_tys + = mk_tv_bndrs missing_con inst_tys `thenSmpl` \ (tv_bndrs, inst_tys') -> + getUniquesSmpl `thenSmpl` \ id_uniqs -> + let arg_tys = dataConInstArgTys missing_con inst_tys' + arg_ids = zipWith (mkSysLocal FSLIT("a")) id_uniqs arg_tys + in + returnSmpl (tv_bndrs ++ arg_ids) + +mk_tv_bndrs missing_con inst_tys + | isVanillaDataCon missing_con + = returnSmpl ([], inst_tys) + | otherwise + = getUniquesSmpl `thenSmpl` \ tv_uniqs -> + let new_tvs = zipWith mk tv_uniqs (dataConTyVars missing_con) + mk uniq tv = mkTyVar (mkSysTvName uniq FSLIT("t")) (tyVarKind tv) + in + returnSmpl (new_tvs, mkTyVarTys new_tvs) +\end{code} + + +%************************************************************************ +%* * +\subsection{Case absorption and identity-case elimination} +%* * +%************************************************************************ + +mkCase puts a case expression back together, trying various transformations first. + +\begin{code} +mkCase :: OutExpr -> OutId -> OutType + -> [OutAlt] -- Increasing order + -> SimplM OutExpr + +mkCase scrut case_bndr ty alts + = getDOptsSmpl `thenSmpl` \dflags -> + mkAlts dflags scrut case_bndr alts `thenSmpl` \ better_alts -> + mkCase1 scrut case_bndr ty better_alts +\end{code} + + +mkAlts tries these things: + +1. If several alternatives are identical, merge them into + a single DEFAULT alternative. I've occasionally seen this + making a big difference: + + case e of =====> case e of + C _ -> f x D v -> ....v.... + D v -> ....v.... DEFAULT -> f x + DEFAULT -> f x + + The point is that we merge common RHSs, at least for the DEFAULT case. + [One could do something more elaborate but I've never seen it needed.] + To avoid an expensive test, we just merge branches equal to the *first* + alternative; this picks up the common cases + a) all branches equal + b) some branches equal to the DEFAULT (which occurs first) + +2. Case merging: + case e of b { ==> case e of b { + p1 -> rhs1 p1 -> rhs1 + ... ... + pm -> rhsm pm -> rhsm + _ -> case b of b' { pn -> let b'=b in rhsn + pn -> rhsn ... + ... po -> let b'=b in rhso + po -> rhso _ -> let b'=b in rhsd + _ -> rhsd + } + + which merges two cases in one case when -- the default alternative of + the outer case scrutises the same variable as the outer case This + transformation is called Case Merging. It avoids that the same + variable is scrutinised multiple times. + + +The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs): + + x | p `is` 1 -> e1 + | p `is` 2 -> e2 + ...etc... + +where @is@ was something like + + p `is` n = p /= (-1) && p == n + +This gave rise to a horrible sequence of cases + + case p of + (-1) -> $j p + 1 -> e1 + DEFAULT -> $j p + +and similarly in cascade for all the join points! + + + +\begin{code} +-------------------------------------------------- +-- 1. Merge identical branches +-------------------------------------------------- +mkAlts dflags scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts) + | all isDeadBinder bndrs1, -- Remember the default + length filtered_alts < length con_alts -- alternative comes first + = tick (AltMerge case_bndr) `thenSmpl_` + returnSmpl better_alts + where + filtered_alts = filter keep con_alts + keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1) + better_alts = (DEFAULT, [], rhs1) : filtered_alts + + +-------------------------------------------------- +-- 2. Merge nested cases +-------------------------------------------------- + +mkAlts dflags scrut outer_bndr outer_alts + | dopt Opt_CaseMerge dflags, + (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts, + Just (Case (Var scrut_var) inner_bndr _ inner_alts) <- maybe_outer_deflt, + scruting_same_var scrut_var + = let + munged_inner_alts = [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts] + munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs + + new_alts = mergeAlts outer_alts_without_deflt munged_inner_alts + -- The merge keeps the inner DEFAULT at the front, if there is one + -- and eliminates any inner_alts that are shadowed by the outer_alts + in + tick (CaseMerge outer_bndr) `thenSmpl_` + returnSmpl new_alts + -- Warning: don't call mkAlts recursively! + -- Firstly, there's no point, because inner alts have already had + -- mkCase applied to them, so they won't have a case in their default + -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr + -- in munge_rhs may put a case into the DEFAULT branch! + where + -- We are scrutinising the same variable if it's + -- the outer case-binder, or if the outer case scrutinises a variable + -- (and it's the same). Testing both allows us not to replace the + -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder). + scruting_same_var = case scrut of + Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut + other -> \ v -> v == outer_bndr + +------------------------------------------------ +-- Catch-all +------------------------------------------------ + +mkAlts dflags scrut case_bndr other_alts = returnSmpl other_alts + + +--------------------------------- +mergeAlts :: [OutAlt] -> [OutAlt] -> [OutAlt] +-- Merge preserving order; alternatives in the first arg +-- shadow ones in the second +mergeAlts [] as2 = as2 +mergeAlts as1 [] = as1 +mergeAlts (a1:as1) (a2:as2) + = case a1 `cmpAlt` a2 of + LT -> a1 : mergeAlts as1 (a2:as2) + EQ -> a1 : mergeAlts as1 as2 -- Discard a2 + GT -> a2 : mergeAlts (a1:as1) as2 +\end{code} + + + +================================================================================= + +mkCase1 tries these things + +1. Eliminate the case altogether if possible + +2. Case-identity: + + case e of ===> e + True -> True; + False -> False + + and similar friends. + + +Start with a simple situation: + + case x# of ===> e[x#/y#] + y# -> e + +(when x#, y# are of primitive type, of course). We can't (in general) +do this for algebraic cases, because we might turn bottom into +non-bottom! + +Actually, we generalise this idea to look for a case where we're +scrutinising a variable, and we know that only the default case can +match. For example: +\begin{verbatim} + case x of + 0# -> ... + other -> ...(case x of + 0# -> ... + other -> ...) ... +\end{code} +Here the inner case can be eliminated. This really only shows up in +eliminating error-checking code. + +We also make sure that we deal with this very common case: + + case e of + x -> ...x... + +Here we are using the case as a strict let; if x is used only once +then we want to inline it. We have to be careful that this doesn't +make the program terminate when it would have diverged before, so we +check that + - x is used strictly, or + - e is already evaluated (it may so if e is a variable) + +Lastly, we generalise the transformation to handle this: + + case e of ===> r + True -> r + False -> r + +We only do this for very cheaply compared r's (constructors, literals +and variables). If pedantic bottoms is on, we only do it when the +scrutinee is a PrimOp which can't fail. + +We do it *here*, looking at un-simplified alternatives, because we +have to check that r doesn't mention the variables bound by the +pattern in each alternative, so the binder-info is rather useful. + +So the case-elimination algorithm is: + + 1. Eliminate alternatives which can't match + + 2. Check whether all the remaining alternatives + (a) do not mention in their rhs any of the variables bound in their pattern + and (b) have equal rhss + + 3. Check we can safely ditch the case: + * PedanticBottoms is off, + or * the scrutinee is an already-evaluated variable + or * the scrutinee is a primop which is ok for speculation + -- ie we want to preserve divide-by-zero errors, and + -- calls to error itself! + + or * [Prim cases] the scrutinee is a primitive variable + + or * [Alg cases] the scrutinee is a variable and + either * the rhs is the same variable + (eg case x of C a b -> x ===> x) + or * there is only one alternative, the default alternative, + and the binder is used strictly in its scope. + [NB this is helped by the "use default binder where + possible" transformation; see below.] + + +If so, then we can replace the case with one of the rhss. + +Further notes about case elimination +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider: test :: Integer -> IO () + test = print + +Turns out that this compiles to: + Print.test + = \ eta :: Integer + eta1 :: State# RealWorld -> + case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT -> + case hPutStr stdout + (PrelNum.jtos eta ($w[] @ Char)) + eta1 + of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }} + +Notice the strange '<' which has no effect at all. This is a funny one. +It started like this: + +f x y = if x < 0 then jtos x + else if y==0 then "" else jtos x + +At a particular call site we have (f v 1). So we inline to get + + if v < 0 then jtos x + else if 1==0 then "" else jtos x + +Now simplify the 1==0 conditional: + + if v<0 then jtos v else jtos v + +Now common-up the two branches of the case: + + case (v<0) of DEFAULT -> jtos v + +Why don't we drop the case? Because it's strict in v. It's technically +wrong to drop even unnecessary evaluations, and in practice they +may be a result of 'seq' so we *definitely* don't want to drop those. +I don't really know how to improve this situation. + + +\begin{code} +-------------------------------------------------- +-- 0. Check for empty alternatives +-------------------------------------------------- + +-- This isn't strictly an error. It's possible that the simplifer might "see" +-- that an inner case has no accessible alternatives before it "sees" that the +-- entire branch of an outer case is inaccessible. So we simply +-- put an error case here insteadd +mkCase1 scrut case_bndr ty [] + = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $ + return (mkApps (Var eRROR_ID) + [Type ty, Lit (mkStringLit "Impossible alternative")]) + +-------------------------------------------------- +-- 1. Eliminate the case altogether if poss +-------------------------------------------------- + +mkCase1 scrut case_bndr ty [(con,bndrs,rhs)] + -- See if we can get rid of the case altogether + -- See the extensive notes on case-elimination above + -- mkCase made sure that if all the alternatives are equal, + -- then there is now only one (DEFAULT) rhs + | all isDeadBinder bndrs, + + -- Check that the scrutinee can be let-bound instead of case-bound + exprOkForSpeculation scrut + -- OK not to evaluate it + -- This includes things like (==# a# b#)::Bool + -- so that we simplify + -- case ==# a# b# of { True -> x; False -> x } + -- to just + -- x + -- This particular example shows up in default methods for + -- comparision operations (e.g. in (>=) for Int.Int32) + || exprIsHNF scrut -- It's already evaluated + || var_demanded_later scrut -- It'll be demanded later + +-- || not opt_SimplPedanticBottoms) -- Or we don't care! +-- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on, +-- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate +-- its argument: case x of { y -> dataToTag# y } +-- Here we must *not* discard the case, because dataToTag# just fetches the tag from +-- the info pointer. So we'll be pedantic all the time, and see if that gives any +-- other problems +-- Also we don't want to discard 'seq's + = tick (CaseElim case_bndr) `thenSmpl_` + returnSmpl (bindCaseBndr case_bndr scrut rhs) + + where + -- The case binder is going to be evaluated later, + -- and the scrutinee is a simple variable + var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr) + var_demanded_later other = False + + +-------------------------------------------------- +-- 2. Identity case +-------------------------------------------------- + +mkCase1 scrut case_bndr ty alts -- Identity case + | all identity_alt alts + = tick (CaseIdentity case_bndr) `thenSmpl_` + returnSmpl (re_note scrut) + where + 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 _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut + other -> scrut + + +-------------------------------------------------- +-- Catch-all +-------------------------------------------------- +mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts) +\end{code} + + +When adding auxiliary bindings for the case binder, it's worth checking if +its dead, because it often is, and occasionally these mkCase transformations +cascade rather nicely. + +\begin{code} +bindCaseBndr bndr rhs body + | isDeadBinder bndr = body + | otherwise = bindNonRec bndr rhs body +\end{code} |