simplCore/SimplUtils.lhs

% % (c) The AQUA Project, Glasgow University, 1993-1998 % \section[SimplUtils]{The simplifier utilities} \begin{code}

    module SimplUtils (
            -- Rebuilding
            mkLam, mkCase, prepareAlts, tryEtaExpandRhs,

            -- Inlining,
            preInlineUnconditionally, postInlineUnconditionally,
            activeUnfolding, activeRule,
            getUnfoldingInRuleMatch,
            simplEnvForGHCi, updModeForInlineRules,

            -- The continuation type
            SimplCont(..), DupFlag(..),
            isSimplified,
            contIsDupable, contResultType, contInputType,
            contIsTrivial, contArgs, dropArgs,
            pushSimplifiedArgs, countValArgs, countArgs,
            mkBoringStop, mkRhsStop, mkLazyArgStop, contIsRhsOrArg,
            interestingCallContext, interestingArg,

            -- ArgInfo
            ArgInfo(..), ArgSpec(..), mkArgInfo, addArgTo, addCastTo,
            argInfoExpr, argInfoValArgs,

            abstractFloats
        ) where

    #include "HsVersions.h"

    import SimplEnv
    import CoreMonad        ( SimplifierMode(..), Tick(..) )
    import MkCore           ( sortQuantVars )
    import DynFlags
    import CoreSyn
    import qualified CoreSubst
    import PprCore
    import CoreFVs
    import CoreUtils
    import CoreArity
    import CoreUnfold
    import Name
    import Id
    import Var
    import Demand
    import SimplMonad
    import Type     hiding( substTy )
    import Coercion hiding( substCo, substTy )
    import DataCon          ( dataConWorkId )
    import VarSet
    import BasicTypes
    import Util
    import MonadUtils
    import Outputable
    import FastString
    import Pair

    import Control.Monad    ( when )

\end{code} %************************************************************************ %* * The SimplCont type %* * %************************************************************************ A SimplCont allows the simplifier to traverse the expression in a zipper-like fashion. The SimplCont represents the rest of the expression, "above" the point of interest. You can also think of a SimplCont as an "evaluation context", using that term in the way it is used for operational semantics. This is the way I usually think of it, For example you'll often see a syntax for evaluation context looking like C ::= [] | C e | case C of alts | C `cast` co That's the kind of thing we are doing here, and I use that syntax in the comments. Key points: * A SimplCont describes a *strict* context (just like evaluation contexts do). E.g. Just [] is not a SimplCont * A SimplCont describes a context that *does not* bind any variables. E.g. \x. [] is not a SimplCont \begin{code}

    data SimplCont
      = Stop                -- An empty context, or <hole>
            OutType         -- Type of the <hole>
            CallCtxt        -- Tells if there is something interesting about
                            --          the context, and hence the inliner
                            --          should be a bit keener (see interestingCallContext)
                            -- Specifically:
                            --     This is an argument of a function that has RULES
                            --     Inlining the call might allow the rule to fire
                            -- Never ValAppCxt (use ApplyTo instead)
                            -- or CaseCtxt (use Select instead)

      | CoerceIt            -- <hole> `cast` co
            OutCoercion             -- The coercion simplified
                                    -- Invariant: never an identity coercion
            SimplCont

      | ApplyTo             -- <hole> arg
            DupFlag                 -- See Note [DupFlag invariants]
            InExpr StaticEnv        -- The argument and its static env
            SimplCont

      | Select              -- case <hole> of alts
            DupFlag                 -- See Note [DupFlag invariants]
            InId [InAlt] StaticEnv  -- The case binder, alts type, alts, and subst-env
            SimplCont

      -- The two strict forms have no DupFlag, because we never duplicate them
      | StrictBind                  -- (\x* \xs. e) <hole>
            InId [InBndr]           -- let x* = <hole> in e
            InExpr StaticEnv        --      is a special case
            SimplCont

      | StrictArg           -- f e1 ..en <hole>
            ArgInfo         -- Specifies f, e1..en, Whether f has rules, etc
                            --     plus strictness flags for *further* args
            CallCtxt        -- Whether *this* argument position is interesting
            SimplCont

      | TickIt
            (Tickish Id)    -- Tick tickish <hole>
            SimplCont

    data ArgInfo
      = ArgInfo {
            ai_fun   :: OutId,      -- The function
            ai_args  :: [ArgSpec],  -- ...applied to these args (which are in *reverse* order)
            ai_type  :: OutType,    -- Type of (f a1 ... an)

            ai_rules :: [CoreRule], -- Rules for this function

            ai_encl :: Bool,        -- Flag saying whether this function
                                    -- or an enclosing one has rules (recursively)
                                    --      True => be keener to inline in all args

            ai_strs :: [Bool],      -- Strictness of remaining arguments
                                    --   Usually infinite, but if it is finite it guarantees
                                    --   that the function diverges after being given
                                    --   that number of args
            ai_discs :: [Int]       -- Discounts for remaining arguments; non-zero => be keener to inline
                                    --   Always infinite
        }

    data ArgSpec = ValArg OutExpr       -- Apply to this
                 | CastBy OutCoercion   -- Cast by this

    instance Outputable ArgSpec where
      ppr (ValArg e) = ptext (sLit "ValArg") <+> ppr e
      ppr (CastBy c) = ptext (sLit "CastBy") <+> ppr c

    addArgTo :: ArgInfo -> OutExpr -> ArgInfo
    addArgTo ai arg = ai { ai_args = ValArg arg : ai_args ai
                         , ai_type = applyTypeToArg (ai_type ai) arg  }

    addCastTo :: ArgInfo -> OutCoercion -> ArgInfo
    addCastTo ai co = ai { ai_args = CastBy co : ai_args ai
                         , ai_type = pSnd (coercionKind co) }

    argInfoValArgs :: SimplEnv -> [ArgSpec] -> SimplCont -> ([OutExpr], SimplCont)
    argInfoValArgs env args cont
      = go args [] cont
      where
        go :: [ArgSpec] -> [OutExpr] -> SimplCont -> ([OutExpr], SimplCont)
        go (ValArg e  : as) acc cont = go as (e:acc) cont
        go (CastBy co : as) acc cont = go as [] (CoerceIt co (pushSimplifiedArgs env acc cont))
        go []               acc cont = (acc, cont)

    argInfoExpr :: OutId -> [ArgSpec] -> OutExpr
    argInfoExpr fun args
      = go args
      where
        go []               = Var fun
        go (ValArg a : as)  = go as `App` a
        go (CastBy co : as) = mkCast (go as) co

    instance Outputable SimplCont where
      ppr (Stop ty interesting)          = ptext (sLit "Stop") <> brackets (ppr interesting) <+> ppr ty
      ppr (ApplyTo dup arg _ cont)       = ((ptext (sLit "ApplyTo") <+> ppr dup <+> pprParendExpr arg)
                                              {-  $$ nest 2 (pprSimplEnv se) -}) $$ ppr cont
      ppr (StrictBind b _ _ _ cont)      = (ptext (sLit "StrictBind") <+> ppr b) $$ ppr cont
      ppr (StrictArg ai _ cont)          = (ptext (sLit "StrictArg") <+> ppr (ai_fun ai)) $$ ppr cont
      ppr (Select dup bndr alts se cont) = (ptext (sLit "Select") <+> ppr dup <+> ppr bndr) $$
                                             (nest 2 $ vcat [ppr (seTvSubst se), ppr alts]) $$ ppr cont
      ppr (CoerceIt co cont)             = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont
      ppr (TickIt t cont)                = (ptext (sLit "TickIt") <+> ppr t) $$ ppr cont

    data DupFlag = NoDup       -- Unsimplified, might be big
                 | Simplified  -- Simplified
                 | OkToDup     -- Simplified and small

    isSimplified :: DupFlag -> Bool
    isSimplified NoDup = False
    isSimplified _     = True       -- Invariant: the subst-env is empty

    instance Outputable DupFlag where
      ppr OkToDup    = ptext (sLit "ok")
      ppr NoDup      = ptext (sLit "nodup")
      ppr Simplified = ptext (sLit "simpl")

\end{code} Note [DupFlag invariants] ~~~~~~~~~~~~~~~~~~~~~~~~~ In both (ApplyTo dup _ env k) and (Select dup _ _ env k) the following invariants hold (a) if dup = OkToDup, then continuation k is also ok-to-dup (b) if dup = OkToDup or Simplified, the subst-env is empty (and and hence no need to re-simplify) \begin{code}

    -------------------
    mkBoringStop :: OutType -> SimplCont
    mkBoringStop ty = Stop ty BoringCtxt

    mkRhsStop :: OutType -> SimplCont       -- See Note [RHS of lets] in CoreUnfold
    mkRhsStop ty = Stop ty RhsCtxt

    mkLazyArgStop :: OutType -> CallCtxt -> SimplCont
    mkLazyArgStop ty cci = Stop ty cci

    -------------------
    contIsRhsOrArg :: SimplCont -> Bool
    contIsRhsOrArg (Stop {})       = True
    contIsRhsOrArg (StrictBind {}) = True
    contIsRhsOrArg (StrictArg {})  = True
    contIsRhsOrArg _               = False

    contIsRhs :: SimplCont -> Bool
    contIsRhs (Stop _ RhsCtxt) = True
    contIsRhs _                = False

    -------------------
    contIsDupable :: SimplCont -> Bool
    contIsDupable (Stop {})                  = True
    contIsDupable (ApplyTo  OkToDup _ _ _)   = True -- See Note [DupFlag invariants]
    contIsDupable (Select   OkToDup _ _ _ _) = True -- ...ditto...
    contIsDupable (CoerceIt _ cont)          = contIsDupable cont
    contIsDupable _                          = False

    -------------------
    contIsTrivial :: SimplCont -> Bool
    contIsTrivial (Stop {})                   = True
    contIsTrivial (ApplyTo _ (Type _) _ cont) = contIsTrivial cont
    contIsTrivial (ApplyTo _ (Coercion _) _ cont) = contIsTrivial cont
    contIsTrivial (CoerceIt _ cont)           = contIsTrivial cont
    contIsTrivial _                           = False

    -------------------
    contResultType :: SimplCont -> OutType
    contResultType (Stop ty _)            = ty
    contResultType (CoerceIt _ k)         = contResultType k
    contResultType (StrictBind _ _ _ _ k) = contResultType k
    contResultType (StrictArg _ _ k)      = contResultType k
    contResultType (Select _ _ _ _ k)     = contResultType k
    contResultType (ApplyTo _ _ _ k)      = contResultType k
    contResultType (TickIt _ k)           = contResultType k

    contInputType :: SimplCont -> OutType
    contInputType (Stop ty _)             = ty
    contInputType (CoerceIt co _)         = pFst (coercionKind co)
    contInputType (Select d b _ se _)     = perhapsSubstTy d se (idType b)
    contInputType (StrictBind b _ _ se _) = substTy se (idType b)
    contInputType (StrictArg ai _ _)      = funArgTy (ai_type ai)
    contInputType (ApplyTo d e se k)      = mkFunTy (perhapsSubstTy d se (exprType e)) (contInputType k)
    contInputType (TickIt _ k)            = contInputType k

    perhapsSubstTy :: DupFlag -> SimplEnv -> InType -> OutType
    perhapsSubstTy dup_flag se ty
      | isSimplified dup_flag = ty
      | otherwise             = substTy se ty

    -------------------
    countValArgs :: SimplCont -> Int
    countValArgs (ApplyTo _ (Type _) _ cont) = countValArgs cont
    countValArgs (ApplyTo _ (Coercion _) _ cont) = countValArgs cont
    countValArgs (ApplyTo _ _        _ cont) = 1 + countValArgs cont
    countValArgs _                           = 0

    countArgs :: SimplCont -> Int
    countArgs (ApplyTo _ _ _ cont) = 1 + countArgs cont
    countArgs _                    = 0

    contArgs :: SimplCont -> (Bool, [ArgSummary], SimplCont)
    -- Summarises value args, discards type args and coercions
    -- The returned continuation of the call is only used to
    -- answer questions like "are you interesting?"
    contArgs cont
      | lone cont = (True, [], cont)
      | otherwise = go [] cont
      where
        lone (ApplyTo {})  = False  -- See Note [Lone variables] in CoreUnfold
        lone (CoerceIt {}) = False
        lone _             = True

        go args (ApplyTo _ arg se cont)
          | isTypeArg arg         = go args                           cont
          | otherwise             = go (is_interesting arg se : args) cont
        go args (CoerceIt _ cont) = go args cont
        go args cont              = (False, reverse args, cont)

        is_interesting arg se = interestingArg (substExpr (text "contArgs") se arg)
                       -- Do *not* use short-cutting substitution here
                       -- because we want to get as much IdInfo as possible

    pushSimplifiedArgs :: SimplEnv -> [CoreExpr] -> SimplCont -> SimplCont
    pushSimplifiedArgs _env []         cont = cont
    pushSimplifiedArgs env  (arg:args) cont = ApplyTo Simplified arg env (pushSimplifiedArgs env args cont)
                       -- The env has an empty SubstEnv

    dropArgs :: Int -> SimplCont -> SimplCont
    dropArgs 0 cont = cont
    dropArgs n (ApplyTo _ _ _ cont) = dropArgs (n-1) cont
    dropArgs n other                = pprPanic "dropArgs" (ppr n <+> ppr other)

\end{code} Note [Interesting call context] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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 :: SimplCont -> CallCtxt
    -- See Note [Interesting call context]
    interestingCallContext cont
      = interesting cont
      where
        interesting (Select _ _bndr _ _ _) = CaseCtxt

        interesting (ApplyTo _ arg _ cont)
            | isTypeArg arg = interesting cont
            | otherwise     = ValAppCtxt    -- Can happen if we have (f Int |> co) y
                                            -- If f has an INLINE prag we need to give it some
                                            -- motivation to inline. See Note [Cast then apply]
                                            -- in CoreUnfold

        interesting (StrictArg _ cci _) = cci
        interesting (StrictBind {})     = BoringCtxt
        interesting (Stop _ cci)        = cci
        interesting (TickIt _ cci)      = interesting cci
        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.

    -------------------
    mkArgInfo :: Id
              -> [CoreRule] -- Rules for function
              -> Int        -- Number of value args
              -> SimplCont  -- Context of the call
              -> ArgInfo

    mkArgInfo fun rules n_val_args call_cont
      | n_val_args < idArity fun            -- Note [Unsaturated functions]
      = ArgInfo { ai_fun = fun, ai_args = [], ai_type = fun_ty
                , ai_rules = rules, ai_encl = False
                , ai_strs = vanilla_stricts
                , ai_discs = vanilla_discounts }
      | otherwise
      = ArgInfo { ai_fun = fun, ai_args = [], ai_type = fun_ty
                , ai_rules = rules
                , ai_encl = interestingArgContext rules call_cont
                , ai_strs  = add_type_str fun_ty arg_stricts
                , ai_discs = arg_discounts }
      where
        fun_ty = idType fun

        vanilla_discounts, arg_discounts :: [Int]
        vanilla_discounts = repeat 0
        arg_discounts = case idUnfolding fun of
                            CoreUnfolding {uf_guidance = UnfIfGoodArgs {ug_args = discounts}}
                                  -> discounts ++ vanilla_discounts
                            _     -> vanilla_discounts

        vanilla_stricts, arg_stricts :: [Bool]
        vanilla_stricts  = repeat False

        arg_stricts
          = case splitStrictSig (idStrictness fun) of
              (demands, result_info)
                    | not (demands `lengthExceeds` n_val_args)
                    ->      -- 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
                   | otherwise
                   -> WARN( True, text "More demands than arity" <+> ppr fun <+> ppr (idArity fun)
                                    <+> ppr n_val_args <+> ppr demands )
                       vanilla_stricts      -- Not enough args, or no strictness

        add_type_str :: Type -> [Bool] -> [Bool]
        -- If the function arg types are strict, record that in the 'strictness bits'
        -- No need to instantiate because unboxed types (which dominate the strict
        -- types) can't instantiate type variables.
        -- add_type_str is done repeatedly (for each call); might be better
        -- once-for-all in the function
        -- But beware primops/datacons with no strictness
        add_type_str _ [] = []
        add_type_str fun_ty strs            -- Look through foralls
            | Just (_, fun_ty') <- splitForAllTy_maybe fun_ty       -- Includes coercions
            = add_type_str fun_ty' strs
        add_type_str fun_ty (str:strs)      -- Add strict-type info
            | Just (arg_ty, fun_ty') <- splitFunTy_maybe fun_ty
            = (str || isStrictType arg_ty) : add_type_str fun_ty' strs
        add_type_str _ strs
            = strs

    {- Note [Unsaturated functions]
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (test eyeball/inline4)
            x = a:as
            y = f x
    where f has arity 2.  Then we do not want to inline 'x', because
    it'll just be floated out again.  Even if f has lots of discounts
    on its first argument -- it must be saturated for these to kick in
    -}

    interestingArgContext :: [CoreRule] -> SimplCont -> Bool
    -- If the argument has form (f x y), where x,y are boring,
    -- and f is marked INLINE, then we don't want to inline f.
    -- But if the context of the argument is
    --      g (f x y)
    -- where g has rules, then we *do* want to inline f, in case it
    -- exposes a rule that might fire.  Similarly, if the context is
    --      h (g (f x x))
    -- where h has rules, then we do want to inline f; hence the
    -- call_cont argument to interestingArgContext
    --
    -- The ai-rules flag makes this happen; if it's
    -- set, the inliner gets just enough keener to inline f
    -- regardless of how boring f's arguments are, if it's marked INLINE
    --
    -- The alternative would be to *always* inline an INLINE function,
    -- regardless of how boring its context is; but that seems overkill
    -- For example, it'd mean that wrapper functions were always inlined
    interestingArgContext rules call_cont
      = notNull rules || enclosing_fn_has_rules
      where
        enclosing_fn_has_rules = go call_cont

        go (Select {})         = False
        go (ApplyTo {})        = False
        go (StrictArg _ cci _) = interesting cci
        go (StrictBind {})     = False      -- ??
        go (CoerceIt _ c)      = go c
        go (Stop _ cci)        = interesting cci
        go (TickIt _ c)        = go c

        interesting RuleArgCtxt = True
        interesting _           = False

\end{code} %************************************************************************ %* * SimplifierMode %* * %************************************************************************ 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      = gopt Opt_EnableRewriteRules   dflags
        eta_expand_on = gopt 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"
                     -- because 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 = ... {-# INLINE f #-} f x = g (g x) If we were to inline g into f's inlining, then an importing module would never be able to do f e --> g (g e) ---> RULE fires because the InlineRule for f has had g inlined into it. On the other hand, it is bad not to do ANY inlining into an InlineRule, because then recursive knots in instance declarations don't get unravelled. However, *sometimes* SimplGently must do no call-site inlining at all (hence sm_inline = False). 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-inlining 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. Note [Simplifying inside InlineRules] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We must take care with simplification inside InlineRules (which come from INLINE pragmas). First, 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 might be inlined inside g, and thence copied multiple times when g is inlined. HENCE we treat any occurrence in an InlineRule as a multiple occurrence, not a single 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. However, suppose we have {-# INLINE f #-} f = meaning "inline f in phases p where activation (p) holds". Then what inlinings/rules can we apply to the copy of captured in f's InlineRule? Our model is that literally is substituted for f when it is inlined. So our conservative plan (implemented by updModeForInlineRules) is this: ------------------------------------------------------------- When simplifying the RHS of an InlineRule, set the phase to the phase in which the InlineRule first becomes active ------------------------------------------------------------- That ensures that a) Rules/inlinings that *cease* being active before p will not apply to the InlineRule rhs, consistent with it being inlined in its *original* form in phase p. b) Rules/inlinings that only become active *after* p will not apply to the InlineRule rhs, again to be consistent with inlining the *original* rhs in phase p. For example, {-# INLINE f #-} f x = ...g... {-# NOINLINE [1] g #-} g y = ... {-# 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. Similarly, consider {-# INLINE f #-} f x = ...g... g y = ... and suppose that there are auto-generated specialisations and a strictness wrapper for g. The specialisations get activation AlwaysActive, and the strictness wrapper get activation (ActiveAfter 0). So the strictness wrepper fails the test and won't be inlined into f's InlineRule. That means f can inline, expose the specialised call to g, so the specialisation rules can fire. A note about wrappers ~~~~~~~~~~~~~~~~~~~~~ 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. \begin{code}

    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

    getUnfoldingInRuleMatch :: SimplEnv -> InScopeEnv
    -- 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
      = (in_scope, id_unf)
      where
        in_scope = seInScope env
        mode = getMode env
        id_unf id | unf_is_active id = idUnfolding id
                  | otherwise        = NoUnfolding
        unf_is_active id
         | not (sm_rules mode) = active_unfolding_minimal id
         | otherwise           = isActive (sm_phase mode) (idInlineActivation id)

    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

    ----------------------
    activeRule :: SimplEnv -> Activation -> Bool
    -- Nothing => No rules at all
    activeRule env
      | not (sm_rules mode) = \_ -> False     -- Rewriting is off
      | otherwise           = isActive (sm_phase mode)
      where
        mode = getMode env

\end{code} %************************************************************************ %* * preInlineUnconditionally %* * %************************************************************************ 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 = THEN doing the inlining should not change the occurrence info for the free vars of ---------------------------------------------- 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. Even 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. Note [pre/postInlineUnconditionally in gentle mode] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Even in gentle mode we want to do preInlineUnconditionally. 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 However, as usual for Gentle mode, do not inline things that are inactive in the intial stages. See Note [Gentle mode]. Note [InlineRule and preInlineUnconditionally] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Surprisingly, do not pre-inline-unconditionally Ids with INLINE pragmas! Example {-# INLINE f #-} f :: Eq a => a -> a f x = ... fInt :: Int -> Int fInt = f Int dEqInt ...fInt...fInt...fInt... Here f occurs just once, in the RHS of f1. But if we inline it there we'll lose the opportunity to inline at each of fInt's call sites. The INLINE pragma will only inline when the application is saturated for exactly this reason; and we don't want PreInlineUnconditionally to second-guess it. A live example is Trac #3736. c.f. Note [InlineRule and postInlineUnconditionally] Note [Top-level botomming Ids] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Don't inline top-level Ids that are bottoming, even if they are used just once, because FloatOut has gone to some trouble to extract them out. Inlining them won't make the program run faster! Note [Do not inline CoVars unconditionally] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Coercion variables appear inside coercions, and the RHS of a let-binding is a term (not a coercion) so we can't necessarily inline the latter in the former. \begin{code}

    preInlineUnconditionally :: DynFlags -> SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
    preInlineUnconditionally dflags env top_lvl bndr rhs
      | not active                               = False
      | isStableUnfolding (idUnfolding bndr)     = False -- Note [InlineRule and preInlineUnconditionally]
      | isTopLevel top_lvl && isBottomingId bndr = False -- Note [Top-level bottoming Ids]
      | not (gopt Opt_SimplPreInlining dflags)   = False
      | isCoVar bndr                             = False -- Note [Do not inline CoVars unconditionally]
      | otherwise = case idOccInfo bndr of
                      IAmDead                    -> True -- Happens in ((\x.1) v)
                      OneOcc in_lam True int_cxt -> try_once in_lam int_cxt
                      _                          -> False
      where
        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
            | otherwise  = int_cxt && canInlineInLam rhs

    -- Be very careful before inlining inside a lambda, because (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 _)              = True
        canInlineInLam (Lam b e)            = isRuntimeVar b || canInlineInLam e
        canInlineInLam _                    = False
          -- not ticks.  Counting ticks cannot be duplicated, and non-counting
          -- ticks around a Lam will disappear anyway.

        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
    -- 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 ~~~~~~~~~~~~~~~~~~~~~~~~~ @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
        :: DynFlags -> SimplEnv -> TopLevelFlag
        -> OutId            -- The binder (an InId would be fine too)
                            --            (*not* a CoVar)
        -> OccInfo          -- From the InId
        -> OutExpr
        -> Unfolding
        -> Bool
    postInlineUnconditionally dflags env top_lvl bndr occ_info rhs unfolding
      | not active                  = False
      | isWeakLoopBreaker occ_info  = False -- If it's a loop-breaker of any kind, don't inline
                                            -- because it might be referred to "earlier"
      | isExportedId bndr           = False
      | isStableUnfolding unfolding = False -- Note [InlineRule and postInlineUnconditionally]
      | 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*
            -- that occur outside a lambda, the call-site inliner won't have
            -- a chance (because 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 ...
            -- This is very important in practice; e.g. wheel-seive1 doubles
            -- in allocation if you miss this out
          OneOcc in_lam _one_br int_cxt     -- OneOcc => no code-duplication issue
            ->     smallEnoughToInline dflags unfolding     -- Small enough to dup
                            -- ToDo: consider discount on smallEnoughToInline if int_cxt is true
                            --
                            -- NB: Do NOT inline arbitrarily big things, even if one_br is True
                            -- Reason: doing so risks exponential behaviour.  We simplify a big
                            --         expression, inline it, and simplify it again.  But if the
                            --         very same thing happens in the big expression, we get
                            --         exponential cost!
                            -- PRINCIPLE: when we've already simplified an expression once,
                            -- make sure that we only inline it if it's reasonably small.

               && (not in_lam ||
                            -- 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

          IAmDead -> True   -- This happens; for example, the case_bndr during case of
                            -- known constructor:  case (a,b) of x { (p,q) -> ... }
                            -- Here x isn't mentioned in the RHS, so we don't want to
                            -- create the (dead) let-binding  let x = (a,b) in ...

          _ -> False

    -- Here's an example that we don't handle well:
    --      let f = if b then Left (\x.BIG) else Right (\y.BIG)
    --      in \y. ....case f of {...} ....
    -- Here f is used just once, and duplicating the case work is fine (exprIsCheap).
    -- But
    --  - We can't preInlineUnconditionally because that woud invalidate
    --    the occ info for b.
    --  - We can't postInlineUnconditionally because the RHS is big, and
    --    that risks exponential behaviour
    --  - We can't call-site inline, because the rhs is big
    -- Alas!

      where
        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 (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. * The inliner should inline trivial things at call sites anyway. Note [InlineRule and postInlineUnconditionally] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Do not do postInlineUnconditionally if the Id has an InlineRule, otherwise we lose the unfolding. Example -- f has InlineRule with rhs (e |> co) -- where 'e' is big f = e |> co Then there's a danger we'll optimise to f' = e f = f' |> co and now postInlineUnconditionally, losing the InlineRule on f. Now f' won't inline because 'e' is too big. c.f. Note [InlineRule and preInlineUnconditionally] %************************************************************************ %* * Rebuilding a lambda %* * %************************************************************************ \begin{code}

    mkLam :: [OutBndr] -> OutExpr -> SimplCont -> SimplM OutExpr
    -- mkLam tries three things
    --      a) eta reduction, if that gives a trivial expression
    --      b) eta expansion [only if there are some value lambdas]

    mkLam [] body _cont
      = return body
    mkLam bndrs body cont
      = do  { dflags <- getDynFlags
            ; mkLam' dflags bndrs body }
      where
        mkLam' :: DynFlags -> [OutBndr] -> OutExpr -> SimplM OutExpr
        mkLam' dflags bndrs (Cast body co)
          | not (any bad bndrs)
            -- Note [Casts and lambdas]
          = do { lam <- mkLam' dflags bndrs body
               ; return (mkCast lam (mkPiCos Representational bndrs co)) }
          where
            co_vars  = tyCoVarsOfCo co
            bad bndr = isCoVar bndr && bndr `elemVarSet` co_vars

        mkLam' dflags bndrs body@(Lam {})
          = mkLam' dflags (bndrs ++ bndrs1) body1
          where
            (bndrs1, body1) = collectBinders body

        mkLam' dflags bndrs body
          | gopt Opt_DoEtaReduction dflags
          , Just etad_lam <- tryEtaReduce bndrs body
          = do { tick (EtaReduction (head bndrs))
               ; return etad_lam }

          | not (contIsRhs cont)   -- See Note [Eta-expanding lambdas]
          , gopt Opt_DoLambdaEtaExpansion dflags
          , any isRuntimeVar bndrs
          , let body_arity = exprEtaExpandArity dflags body
          , body_arity > 0
          = do { tick (EtaExpansion (head bndrs))
               ; return (mkLams bndrs (etaExpand body_arity body)) }

          | otherwise
          = return (mkLams bndrs body)

\end{code} Note [Eta expanding lambdas] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general we *do* want to eta-expand lambdas. Consider f (\x -> case x of (a,b) -> \s -> blah) where 's' is a state token, and hence can be eta expanded. This showed up in the code for GHc.IO.Handle.Text.hPutChar, a rather important function! The eta-expansion will never happen unless we do it now. (Well, it's possible that CorePrep will do it, but CorePrep only has a half-baked eta-expander that can't deal with casts. So it's much better to do it here.) However, when the lambda is let-bound, as the RHS of a let, we have a better eta-expander (in the form of tryEtaExpandRhs), so we don't bother to try expansion in mkLam in that case; hence the contIsRhs guard. Note [Casts and lambdas] ~~~~~~~~~~~~~~~~~~~~~~~~ Consider (\x. (\y. e) `cast` g1) `cast` g2 There is a danger here that the two lambdas look separated, and the full laziness pass might float an expression to between the two. So this equation in mkLam' floats the g1 out, thus: (\x. e `cast` g1) --> (\x.e) `cast` (tx -> g1) where x:tx. In general, this floats casts outside lambdas, where (I hope) they might meet and cancel with some other cast: \x. e `cast` co ===> (\x. e) `cast` (tx -> co) /\a. e `cast` co ===> (/\a. e) `cast` (/\a. co) /\g. e `cast` co ===> (/\g. e) `cast` (/\g. co) (if not (g `in` co)) Notice that it works regardless of 'e'. Originally it worked only if 'e' was itself a lambda, but in some cases that resulted in fruitless iteration in the simplifier. A good example was when compiling Text.ParserCombinators.ReadPrec, where we had a definition like (\x. Get `cast` g) where Get is a constructor with nonzero arity. Then mkLam eta-expanded the Get, and the next iteration eta-reduced it, and then eta-expanded it again. Note also the side condition for the case of coercion binders. It does not make sense to transform /\g. e `cast` g ==> (/\g.e) `cast` (/\g.g) because the latter is not well-kinded. %************************************************************************ %* * Eta expansion %* * %************************************************************************ \begin{code}

    tryEtaExpandRhs :: SimplEnv -> OutId -> OutExpr -> SimplM (Arity, OutExpr)
    -- See Note [Eta-expanding at let bindings]
    -- and Note [Eta expansion to manifest arity]
    tryEtaExpandRhs env bndr rhs
      = do { dflags <- getDynFlags
           ; (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
          | exprIsTrivial rhs
          = return (exprArity rhs, rhs)

          | sm_eta_expand (getMode env)      -- Provided eta-expansion is on
          , let new_arity = findRhsArity dflags bndr rhs old_arity
          , new_arity > manifest_arity      -- And the curent manifest arity isn't enough
          = do { tick (EtaExpansion bndr)
               ; return (new_arity, etaExpand new_arity rhs) }
          | otherwise
          = return (manifest_arity, rhs)

        manifest_arity = manifestArity rhs
        old_arity  = idArity bndr
        _dmd_arity = length $ fst $ splitStrictSig $ idStrictness bndr

\end{code} Note [Eta-expanding at let bindings] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We now eta expand at let-bindings, which is where the payoff comes. The most significant thing is that we can do a simple arity analysis (in CoreArity.findRhsArity), which we can't do for free-floating lambdas 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! %************************************************************************ %* * \subsection{Floating lets out of big lambdas} %* * %************************************************************************ Note [Floating and type abstraction] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this: x = /\a. C e1 e2 We'd like to float this to y1 = /\a. e1 y2 = /\a. e2 x = /\a. C (y1 a) (y2 a) for the usual reasons: we want to inline x rather vigorously. You may think that this kind of thing is rare. But in some programs it is common. For example, if you do closure conversion you might get: data a :-> b = forall e. (e -> a -> b) :$ e f_cc :: forall a. a :-> a f_cc = /\a. (\e. id a) :$ () Now we really want to inline that f_cc thing so that the construction of the closure goes away. So I have elaborated simplLazyBind to understand right-hand sides that look like /\ a1..an. body and treat them specially. The real work is done in SimplUtils.abstractFloats, but there is quite a bit of plumbing in simplLazyBind as well. The same transformation is good when there are lets in the body: /\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! [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. That's why the whole transformation is part of the same process that floats let-bindings and constructor arguments out of RHSs. In particular, it is guarded by the doFloatFromRhs call in simplLazyBind. \begin{code}

    abstractFloats :: [OutTyVar] -> SimplEnv -> OutExpr -> SimplM ([OutBind], OutExpr)
    abstractFloats main_tvs body_env body
      = ASSERT( notNull body_floats )
        do  { (subst, float_binds) <- mapAccumLM abstract empty_subst body_floats
            ; return (float_binds, CoreSubst.substExpr (text "abstract_floats1") subst body) }
      where
        main_tv_set = mkVarSet main_tvs
        body_floats = getFloatBinds body_env
        empty_subst = CoreSubst.mkEmptySubst (seInScope body_env)

        abstract :: CoreSubst.Subst -> OutBind -> SimplM (CoreSubst.Subst, OutBind)
        abstract subst (NonRec id rhs)
          = do { (poly_id, poly_app) <- mk_poly tvs_here id
               ; let poly_rhs = mkLams tvs_here rhs'
                     subst'   = CoreSubst.extendIdSubst subst id poly_app
               ; return (subst', (NonRec poly_id poly_rhs)) }
          where
            rhs' = CoreSubst.substExpr (text "abstract_floats2") subst rhs
            tvs_here = varSetElemsKvsFirst (main_tv_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

        abstract subst (Rec prs)
           = do { (poly_ids, poly_apps) <- mapAndUnzipM (mk_poly tvs_here) ids
                ; let subst' = CoreSubst.extendSubstList subst (ids `zip` poly_apps)
                      poly_rhss = [mkLams tvs_here (CoreSubst.substExpr (text "abstract_floats3") subst' rhs)
                                  | rhs <- rhss]
                ; return (subst', Rec (poly_ids `zip` poly_rhss)) }
           where
             (ids,rhss) = unzip prs
                    -- For a recursive group, it's a bit of a pain to work out the minimal
                    -- set of tyvars over which to abstract:
                    --      /\ a b c.  let x = ...a... in
                    --                 letrec { p = ...x...q...
                    --                          q = .....p...b... } in
                    --                 ...
                    -- Since 'x' is abstracted over 'a', the {p,q} group must be abstracted
                    -- over 'a' (because x is replaced by (poly_x a)) as well as 'b'.
                    -- Since it's a pain, we just use the whole set, which is always safe
                    --
                    -- If you ever want to be more selective, remember this bizarre case too:
                    --      x::a = x
                    -- Here, we must abstract 'x' over 'a'.
             tvs_here = sortQuantVars main_tvs

        mk_poly tvs_here var
          = do { uniq <- getUniqueM
               ; let  poly_name = setNameUnique (idName var) uniq           -- Keep same name
                      poly_ty   = mkForAllTys tvs_here (idType var) -- But new type of course
                      poly_id   = transferPolyIdInfo var tvs_here $ -- Note [transferPolyIdInfo] in Id.lhs
                                  mkLocalId poly_name poly_ty
               ; return (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tvs_here)) }
                    -- 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.

\end{code} Note [Abstract over coercions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If a coercion variable (g :: a ~ Int) is free in the RHS, then so is the type variable a. Rather than sort this mess out, we simply bale out and abstract wrt all the type variables if any of them are coercion variables. Historical note: if you use let-bindings instead of a substitution, beware of this: -- 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.) %************************************************************************ %* * prepareAlts %* * %************************************************************************ prepareAlts tries these 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. 3. Returns a list of the constructors that cannot holds in the DEFAULT alternative (if there is one) Here "cannot match" includes knowledge from GADTs It's a good idea to 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 -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
    -- The returned alternatives can be empty, none are possible
    prepareAlts scrut case_bndr' alts
               -- Case binder is needed just for its type. Note that as an
               --   OutId, it has maximum information; this is important.
               --   Test simpl013 is an example
      = do { us <- getUniquesM
           ; let (imposs_deflt_cons, refined_deflt, alts')
                    = filterAlts us (varType case_bndr') imposs_cons alts
           ; when refined_deflt $ tick (FillInCaseDefault case_bndr')

           ; alts'' <- combineIdenticalAlts case_bndr' alts'
           ; return (imposs_deflt_cons, alts'') }
      where
        imposs_cons = case scrut of
                        Var v -> otherCons (idUnfolding v)
                        _     -> []

\end{code} Note [Combine identical alternatives] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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) The case where Combine Identical Alternatives transformation showed up was like this (base/Foreign/C/Err/Error.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! NB: it's important that all this is done in [InAlt], *before* we work on the alternatives themselves, because Simpify.simplAlt may zap the occurrence info on the binders in the alternatives, which in turn defeats combineIdenticalAlts (see Trac #7360). \begin{code}

    combineIdenticalAlts :: OutId -> [InAlt] -> SimplM [InAlt]
    -- See Note [Combine identical alternatives]
    combineIdenticalAlts case_bndr ((_con1,bndrs1,rhs1) : con_alts)
      | all isDeadBinder bndrs1                     -- Remember the default
      , length filtered_alts < length con_alts      -- alternative comes first
      = do  { tick (AltMerge case_bndr)
            ; return ((DEFAULT, [], rhs1) : filtered_alts) }
      where
        filtered_alts = filterOut identical_to_alt1 con_alts
        identical_to_alt1 (_con,bndrs,rhs) = all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1

    combineIdenticalAlts _ alts = return alts

\end{code} %************************************************************************ %* * mkCase %* * %************************************************************************ mkCase tries these things 1. Merge Nested Cases 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. 2. Eliminate Identity Case case e of ===> e True -> True; False -> False and similar friends. \begin{code}

    mkCase, mkCase1, mkCase2
       :: DynFlags
       -> OutExpr -> OutId
       -> OutType -> [OutAlt]               -- Alternatives in standard (increasing) order
       -> SimplM OutExpr

    --------------------------------------------------
    --      1. Merge Nested Cases
    --------------------------------------------------

    mkCase dflags scrut outer_bndr alts_ty ((DEFAULT, _, deflt_rhs) : outer_alts)
      | gopt Opt_CaseMerge dflags
      , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs
      , inner_scrut_var == outer_bndr
      = do  { tick (CaseMerge outer_bndr)

            ; let wrap_alt (con, args, rhs) = ASSERT( outer_bndr `notElem` args )
                                              (con, args, wrap_rhs rhs)
                    -- Simplifier's no-shadowing invariant should ensure
                    -- that outer_bndr is not shadowed by the inner patterns
                  wrap_rhs rhs = Let (NonRec inner_bndr (Var outer_bndr)) rhs
                    -- The let is OK even for unboxed binders,

                  wrapped_alts | isDeadBinder inner_bndr = inner_alts
                               | otherwise               = map wrap_alt inner_alts

                  merged_alts = mergeAlts outer_alts wrapped_alts
                    -- NB: mergeAlts gives priority to the left
                    --      case x of
                    --        A -> e1
                    --        DEFAULT -> case x of
                    --                      A -> e2
                    --                      B -> e3
                    -- When we merge, we must ensure that e1 takes
                    -- precedence over e2 as the value for A!

            ; mkCase1 dflags scrut outer_bndr alts_ty merged_alts
            }
            -- Warning: don't call mkCase recursively!
            -- Firstly, there's no point, because inner alts have already had
            -- mkCase applied to them, so they won't have a case in their default
            -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
            -- in munge_rhs may put a case into the DEFAULT branch!

    mkCase dflags scrut bndr alts_ty alts = mkCase1 dflags scrut bndr alts_ty alts

    --------------------------------------------------
    --      2. Eliminate Identity Case
    --------------------------------------------------

    mkCase1 _dflags scrut case_bndr _ alts@((_,_,rhs1) : _)      -- Identity case
      | all identity_alt alts
      = do { tick (CaseIdentity case_bndr)
           ; return (re_cast scrut rhs1) }
      where
        identity_alt (con, args, rhs) = check_eq rhs con args

        check_eq (Cast rhs co) con args         = not (any (`elemVarSet` tyCoVarsOfCo co) args)
            {- See Note [RHS casts] -}            && check_eq rhs con args
        check_eq (Lit lit) (LitAlt lit') _      = lit == lit'
        check_eq (Var v)   _ _ | v == case_bndr = True
        check_eq (Var v)   (DataAlt con) []     = v == dataConWorkId con   -- Optimisation only
        check_eq rhs       (DataAlt con) args   = rhs `cheapEqExpr` mkConApp con (arg_tys ++ varsToCoreExprs args)
        check_eq _ _ _ = False

        arg_tys = map Type (tyConAppArgs (idType case_bndr))

            -- Note [RHS casts]
            -- ~~~~~~~~~~~~~~~~
            -- We've seen this:
            --      case e of x { _ -> x `cast` c }
            -- And we definitely want to eliminate this case, to give
            --      e `cast` c
            -- So we throw away the cast from the RHS, and reconstruct
            -- it at the other end.  All the RHS casts must be the same
            -- if (all identity_alt alts) holds.
            --
            -- Don't worry about nested casts, because the simplifier combines them

        re_cast scrut (Cast rhs co) = Cast (re_cast scrut rhs) co
        re_cast scrut _             = scrut

    mkCase1 dflags scrut bndr alts_ty alts = mkCase2 dflags scrut bndr alts_ty alts

    --------------------------------------------------
    --      Catch-all
    --------------------------------------------------
    mkCase2 _dflags scrut bndr alts_ty alts
      = return (Case scrut bndr alts_ty alts)

\end{code} Note [Dead binders] ~~~~~~~~~~~~~~~~~~~~ Note that dead-ness is maintained by the simplifier, so that it is accurate after simplification as well as before. Note [Cascading case merge] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Case merging should cascade in one sweep, because it happens bottom-up case e of a { DEFAULT -> case a of b DEFAULT -> case b of c { DEFAULT -> e A -> ea B -> eb C -> ec ==> case e of a { DEFAULT -> case a of b DEFAULT -> let c = b in e A -> let c = b in ea B -> eb C -> ec ==> case e of a { DEFAULT -> let b = a in let c = b in e A -> let b = a in let c = b in ea B -> let b = a in eb C -> ec However here's a tricky case that we still don't catch, and I don't see how to catch it in one pass: case x of c1 { I# a1 -> case a1 of c2 -> 0 -> ... DEFAULT -> case x of c3 { I# a2 -> case a2 of ... After occurrence analysis (and its binder-swap) we get this case x of c1 { I# a1 -> let x = c1 in -- Binder-swap addition case a1 of c2 -> 0 -> ... DEFAULT -> case x of c3 { I# a2 -> case a2 of ... When we simplify the inner case x, we'll see that x=c1=I# a1. So we'll bind a2 to a1, and get case x of c1 { I# a1 -> case a1 of c2 -> 0 -> ... DEFAULT -> case a1 of ... This is corect, but we can't do a case merge in this sweep because c2 /= a1. Reason: the binding c1=I# a1 went inwards without getting changed to c1=I# c2. I don't think this is worth fixing, even if I knew how. It'll all come out in the next pass anyway.