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Diffstat (limited to 'compiler/simplCore/FloatOut.lhs')
| -rw-r--r-- | compiler/simplCore/FloatOut.lhs | 443 |
1 files changed, 443 insertions, 0 deletions
diff --git a/compiler/simplCore/FloatOut.lhs b/compiler/simplCore/FloatOut.lhs new file mode 100644 index 0000000000..988bd53015 --- /dev/null +++ b/compiler/simplCore/FloatOut.lhs @@ -0,0 +1,443 @@ +% +% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 +% +\section[FloatOut]{Float bindings outwards (towards the top level)} + +``Long-distance'' floating of bindings towards the top level. + +\begin{code} +module FloatOut ( floatOutwards ) where + +#include "HsVersions.h" + +import CoreSyn +import CoreUtils ( mkSCC, exprIsHNF, exprIsTrivial ) + +import DynFlags ( DynFlags, DynFlag(..), FloatOutSwitches(..) ) +import ErrUtils ( dumpIfSet_dyn ) +import CostCentre ( dupifyCC, CostCentre ) +import Id ( Id, idType ) +import Type ( isUnLiftedType ) +import CoreLint ( showPass, endPass ) +import SetLevels ( Level(..), LevelledExpr, LevelledBind, + setLevels, ltMajLvl, ltLvl, isTopLvl ) +import UniqSupply ( UniqSupply ) +import List ( partition ) +import Outputable +import Util ( notNull ) +\end{code} + + ----------------- + Overall game plan + ----------------- + +The Big Main Idea is: + + To float out sub-expressions that can thereby get outside + a non-one-shot value lambda, and hence may be shared. + + +To achieve this we may need to do two thing: + + a) Let-bind the sub-expression: + + f (g x) ==> let lvl = f (g x) in lvl + + Now we can float the binding for 'lvl'. + + b) More than that, we may need to abstract wrt a type variable + + \x -> ... /\a -> let v = ...a... in .... + + Here the binding for v mentions 'a' but not 'x'. So we + abstract wrt 'a', to give this binding for 'v': + + vp = /\a -> ...a... + v = vp a + + Now the binding for vp can float out unimpeded. + I can't remember why this case seemed important enough to + deal with, but I certainly found cases where important floats + didn't happen if we did not abstract wrt tyvars. + +With this in mind we can also achieve another goal: lambda lifting. +We can make an arbitrary (function) binding float to top level by +abstracting wrt *all* local variables, not just type variables, leaving +a binding that can be floated right to top level. Whether or not this +happens is controlled by a flag. + + +Random comments +~~~~~~~~~~~~~~~ + +At the moment we never float a binding out to between two adjacent +lambdas. For example: + +@ + \x y -> let t = x+x in ... +===> + \x -> let t = x+x in \y -> ... +@ +Reason: this is less efficient in the case where the original lambda +is never partially applied. + +But there's a case I've seen where this might not be true. Consider: +@ +elEm2 x ys + = elem' x ys + where + elem' _ [] = False + elem' x (y:ys) = x==y || elem' x ys +@ +It turns out that this generates a subexpression of the form +@ + \deq x ys -> let eq = eqFromEqDict deq in ... +@ +vwhich might usefully be separated to +@ + \deq -> let eq = eqFromEqDict deq in \xy -> ... +@ +Well, maybe. We don't do this at the moment. + +\begin{code} +type FloatBind = (Level, CoreBind) -- INVARIANT: a FloatBind is always lifted +type FloatBinds = [FloatBind] +\end{code} + +%************************************************************************ +%* * +\subsection[floatOutwards]{@floatOutwards@: let-floating interface function} +%* * +%************************************************************************ + +\begin{code} +floatOutwards :: FloatOutSwitches + -> DynFlags + -> UniqSupply + -> [CoreBind] -> IO [CoreBind] + +floatOutwards float_sws dflags us pgm + = do { + showPass dflags float_msg ; + + let { annotated_w_levels = setLevels float_sws pgm us ; + (fss, binds_s') = unzip (map floatTopBind annotated_w_levels) + } ; + + dumpIfSet_dyn dflags Opt_D_verbose_core2core "Levels added:" + (vcat (map ppr annotated_w_levels)); + + let { (tlets, ntlets, lams) = get_stats (sum_stats fss) }; + + dumpIfSet_dyn dflags Opt_D_dump_simpl_stats "FloatOut stats:" + (hcat [ int tlets, ptext SLIT(" Lets floated to top level; "), + int ntlets, ptext SLIT(" Lets floated elsewhere; from "), + int lams, ptext SLIT(" Lambda groups")]); + + endPass dflags float_msg Opt_D_verbose_core2core (concat binds_s') + {- no specific flag for dumping float-out -} + } + where + float_msg = showSDoc (text "Float out" <+> parens (sws float_sws)) + sws (FloatOutSw lam const) = pp_not lam <+> text "lambdas" <> comma <+> + pp_not const <+> text "constants" + pp_not True = empty + pp_not False = text "not" + +floatTopBind bind@(NonRec _ _) + = case (floatBind bind) of { (fs, floats, bind') -> + (fs, floatsToBinds floats ++ [bind']) + } + +floatTopBind bind@(Rec _) + = case (floatBind bind) of { (fs, floats, Rec pairs') -> + WARN( notNull floats, ppr bind $$ ppr floats ) + (fs, [Rec (floatsToBindPairs floats ++ pairs')]) } +\end{code} + +%************************************************************************ +%* * +\subsection[FloatOut-Bind]{Floating in a binding (the business end)} +%* * +%************************************************************************ + + +\begin{code} +floatBind :: LevelledBind + -> (FloatStats, FloatBinds, CoreBind) + +floatBind (NonRec (TB name level) rhs) + = case (floatNonRecRhs level rhs) of { (fs, rhs_floats, rhs') -> + (fs, rhs_floats, NonRec name rhs') } + +floatBind bind@(Rec pairs) + = case (unzip3 (map do_pair pairs)) of { (fss, rhss_floats, new_pairs) -> + + if not (isTopLvl bind_dest_level) then + -- Standard case; the floated bindings can't mention the + -- binders, because they couldn't be escaping a major level + -- if so. + (sum_stats fss, concat rhss_floats, Rec new_pairs) + else + -- In a recursive binding, *destined for* the top level + -- (only), the rhs floats may contain references to the + -- bound things. For example + -- f = ...(let v = ...f... in b) ... + -- might get floated to + -- v = ...f... + -- f = ... b ... + -- and hence we must (pessimistically) make all the floats recursive + -- with the top binding. Later dependency analysis will unravel it. + -- + -- This can only happen for bindings destined for the top level, + -- because only then will partitionByMajorLevel allow through a binding + -- that only differs in its minor level + (sum_stats fss, [], + Rec (new_pairs ++ floatsToBindPairs (concat rhss_floats))) + } + where + bind_dest_level = getBindLevel bind + + do_pair (TB name level, rhs) + = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') -> + (fs, rhs_floats, (name, rhs')) + } +\end{code} + +%************************************************************************ + +\subsection[FloatOut-Expr]{Floating in expressions} +%* * +%************************************************************************ + +\begin{code} +floatExpr, floatRhs, floatNonRecRhs + :: Level + -> LevelledExpr + -> (FloatStats, FloatBinds, CoreExpr) + +floatRhs lvl arg -- Used rec rhss, and case-alternative rhss + = case (floatExpr lvl arg) of { (fsa, floats, arg') -> + case (partitionByMajorLevel lvl floats) of { (floats', heres) -> + -- Dump bindings that aren't going to escape from a lambda; + -- in particular, we must dump the ones that are bound by + -- the rec or case alternative + (fsa, floats', install heres arg') }} + +floatNonRecRhs lvl arg -- Used for nested non-rec rhss, and fn args + = case (floatExpr lvl arg) of { (fsa, floats, arg') -> + -- Dump bindings that aren't going to escape from a lambda + -- This isn't a scoping issue (the binder isn't in scope in the RHS of a non-rec binding) + -- Rather, it is to avoid floating the x binding out of + -- f (let x = e in b) + -- unnecessarily. But we first test for values or trival rhss, + -- because (in particular) we don't want to insert new bindings between + -- the "=" and the "\". E.g. + -- f = \x -> let <bind> in <body> + -- We do not want + -- f = let <bind> in \x -> <body> + -- (a) The simplifier will immediately float it further out, so we may + -- as well do so right now; in general, keeping rhss as manifest + -- values is good + -- (b) If a float-in pass follows immediately, it might add yet more + -- bindings just after the '='. And some of them might (correctly) + -- be strict even though the 'let f' is lazy, because f, being a value, + -- gets its demand-info zapped by the simplifier. + if exprIsHNF arg' || exprIsTrivial arg' then + (fsa, floats, arg') + else + case (partitionByMajorLevel lvl floats) of { (floats', heres) -> + (fsa, floats', install heres arg') }} + +floatExpr _ (Var v) = (zeroStats, [], Var v) +floatExpr _ (Type ty) = (zeroStats, [], Type ty) +floatExpr _ (Lit lit) = (zeroStats, [], Lit lit) + +floatExpr lvl (App e a) + = case (floatExpr lvl e) of { (fse, floats_e, e') -> + case (floatNonRecRhs lvl a) of { (fsa, floats_a, a') -> + (fse `add_stats` fsa, floats_e ++ floats_a, App e' a') }} + +floatExpr lvl lam@(Lam _ _) + = let + (bndrs_w_lvls, body) = collectBinders lam + bndrs = [b | TB b _ <- bndrs_w_lvls] + lvls = [l | TB b l <- bndrs_w_lvls] + + -- For the all-tyvar case we are prepared to pull + -- the lets out, to implement the float-out-of-big-lambda + -- transform; but otherwise we only float bindings that are + -- going to escape a value lambda. + -- In particular, for one-shot lambdas we don't float things + -- out; we get no saving by so doing. + partition_fn | all isTyVar bndrs = partitionByLevel + | otherwise = partitionByMajorLevel + in + case (floatExpr (last lvls) body) of { (fs, floats, body') -> + + -- Dump any bindings which absolutely cannot go any further + case (partition_fn (head lvls) floats) of { (floats', heres) -> + + (add_to_stats fs floats', floats', mkLams bndrs (install heres body')) + }} + +floatExpr lvl (Note note@(SCC cc) expr) + = case (floatExpr lvl expr) of { (fs, floating_defns, expr') -> + let + -- Annotate bindings floated outwards past an scc expression + -- with the cc. We mark that cc as "duplicated", though. + + annotated_defns = annotate (dupifyCC cc) floating_defns + in + (fs, annotated_defns, Note note expr') } + where + annotate :: CostCentre -> FloatBinds -> FloatBinds + + annotate dupd_cc defn_groups + = [ (level, ann_bind floater) | (level, floater) <- defn_groups ] + where + ann_bind (NonRec binder rhs) + = NonRec binder (mkSCC dupd_cc rhs) + + ann_bind (Rec pairs) + = Rec [(binder, mkSCC dupd_cc rhs) | (binder, rhs) <- pairs] + +floatExpr lvl (Note InlineMe expr) -- Other than SCCs + = case floatExpr InlineCtxt expr of { (fs, floating_defns, expr') -> + -- There can be some floating_defns, arising from + -- ordinary lets that were there all the time. It seems + -- more efficient to test once here than to avoid putting + -- them into floating_defns (which would mean testing for + -- inlineCtxt at every let) + (fs, [], Note InlineMe (install floating_defns expr')) } -- See notes in SetLevels + +floatExpr lvl (Note note expr) -- Other than SCCs + = case (floatExpr lvl expr) of { (fs, floating_defns, expr') -> + (fs, floating_defns, Note note expr') } + +floatExpr lvl (Let (NonRec (TB bndr bndr_lvl) rhs) body) + | isUnLiftedType (idType bndr) -- Treat unlifted lets just like a case + = case floatExpr lvl rhs of { (fs, rhs_floats, rhs') -> + case floatRhs bndr_lvl body of { (fs, body_floats, body') -> + (fs, rhs_floats ++ body_floats, Let (NonRec bndr rhs') body') }} + +floatExpr lvl (Let bind body) + = case (floatBind bind) of { (fsb, rhs_floats, bind') -> + case (floatExpr lvl body) of { (fse, body_floats, body') -> + (add_stats fsb fse, + rhs_floats ++ [(bind_lvl, bind')] ++ body_floats, + body') }} + where + bind_lvl = getBindLevel bind + +floatExpr lvl (Case scrut (TB case_bndr case_lvl) ty alts) + = case floatExpr lvl scrut of { (fse, fde, scrut') -> + case floatList float_alt alts of { (fsa, fda, alts') -> + (add_stats fse fsa, fda ++ fde, Case scrut' case_bndr ty alts') + }} + where + -- Use floatRhs for the alternatives, so that we + -- don't gratuitiously float bindings out of the RHSs + float_alt (con, bs, rhs) + = case (floatRhs case_lvl rhs) of { (fs, rhs_floats, rhs') -> + (fs, rhs_floats, (con, [b | TB b _ <- bs], rhs')) } + + +floatList :: (a -> (FloatStats, FloatBinds, b)) -> [a] -> (FloatStats, FloatBinds, [b]) +floatList f [] = (zeroStats, [], []) +floatList f (a:as) = case f a of { (fs_a, binds_a, b) -> + case floatList f as of { (fs_as, binds_as, bs) -> + (fs_a `add_stats` fs_as, binds_a ++ binds_as, b:bs) }} +\end{code} + +%************************************************************************ +%* * +\subsection{Utility bits for floating stats} +%* * +%************************************************************************ + +I didn't implement this with unboxed numbers. I don't want to be too +strict in this stuff, as it is rarely turned on. (WDP 95/09) + +\begin{code} +data FloatStats + = FlS Int -- Number of top-floats * lambda groups they've been past + Int -- Number of non-top-floats * lambda groups they've been past + Int -- Number of lambda (groups) seen + +get_stats (FlS a b c) = (a, b, c) + +zeroStats = FlS 0 0 0 + +sum_stats xs = foldr add_stats zeroStats xs + +add_stats (FlS a1 b1 c1) (FlS a2 b2 c2) + = FlS (a1 + a2) (b1 + b2) (c1 + c2) + +add_to_stats (FlS a b c) floats + = FlS (a + length top_floats) (b + length other_floats) (c + 1) + where + (top_floats, other_floats) = partition to_very_top floats + + to_very_top (my_lvl, _) = isTopLvl my_lvl +\end{code} + + +%************************************************************************ +%* * +\subsection{Utility bits for floating} +%* * +%************************************************************************ + +\begin{code} +getBindLevel (NonRec (TB _ lvl) _) = lvl +getBindLevel (Rec (((TB _ lvl), _) : _)) = lvl +\end{code} + +\begin{code} +partitionByMajorLevel, partitionByLevel + :: Level -- Partitioning level + + -> FloatBinds -- Defns to be divided into 2 piles... + + -> (FloatBinds, -- Defns with level strictly < partition level, + FloatBinds) -- The rest + + +partitionByMajorLevel ctxt_lvl defns + = partition float_further defns + where + -- Float it if we escape a value lambda, or if we get to the top level + float_further (my_lvl, bind) = my_lvl `ltMajLvl` ctxt_lvl || isTopLvl my_lvl + -- The isTopLvl part says that if we can get to the top level, say "yes" anyway + -- This means that + -- x = f e + -- transforms to + -- lvl = e + -- x = f lvl + -- which is as it should be + +partitionByLevel ctxt_lvl defns + = partition float_further defns + where + float_further (my_lvl, _) = my_lvl `ltLvl` ctxt_lvl +\end{code} + +\begin{code} +floatsToBinds :: FloatBinds -> [CoreBind] +floatsToBinds floats = map snd floats + +floatsToBindPairs :: FloatBinds -> [(Id,CoreExpr)] + +floatsToBindPairs floats = concat (map mk_pairs floats) + where + mk_pairs (_, Rec pairs) = pairs + mk_pairs (_, NonRec binder rhs) = [(binder,rhs)] + +install :: FloatBinds -> CoreExpr -> CoreExpr + +install defn_groups expr + = foldr install_group expr defn_groups + where + install_group (_, defns) body = Let defns body +\end{code} |