simplCore/FloatIn.hs

    {-
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    ************************************************************************
    *                                                                      *
    \section[FloatIn]{Floating Inwards pass}
    *                                                                      *
    ************************************************************************

    The main purpose of @floatInwards@ is floating into branches of a
    case, so that we don't allocate things, save them on the stack, and
    then discover that they aren't needed in the chosen branch.
    -}

    {-# LANGUAGE CPP #-}
    {-# OPTIONS_GHC -fprof-auto #-}

    module FloatIn ( floatInwards ) where

    #include "HsVersions.h"

    import GhcPrelude

    import CoreSyn
    import MkCore
    import HscTypes         ( ModGuts(..) )
    import CoreUtils
    import CoreFVs
    import CoreMonad        ( CoreM )
    import Id               ( isOneShotBndr, idType, isJoinId, isJoinId_maybe )
    import Var
    import Type
    import VarSet
    import Util
    import DynFlags
    import Outputable
    -- import Data.List        ( mapAccumL )
    import BasicTypes       ( RecFlag(..), isRec )

    {-
    Top-level interface function, @floatInwards@.  Note that we do not
    actually float any bindings downwards from the top-level.
    -}

    floatInwards :: ModGuts -> CoreM ModGuts
    floatInwards pgm@(ModGuts { mg_binds = binds })
      = do { dflags <- getDynFlags
           ; return (pgm { mg_binds = map (fi_top_bind dflags) binds }) }
      where
        fi_top_bind dflags (NonRec binder rhs)
          = NonRec binder (fiExpr dflags [] (freeVars rhs))
        fi_top_bind dflags (Rec pairs)
          = Rec [ (b, fiExpr dflags [] (freeVars rhs)) | (b, rhs) <- pairs ]

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Mail from Andr\'e [edited]}
    *                                                                      *
    ************************************************************************

    {\em Will wrote: What??? I thought the idea was to float as far
    inwards as possible, no matter what.  This is dropping all bindings
    every time it sees a lambda of any kind.  Help! }

    You are assuming we DO DO full laziness AFTER floating inwards!  We
    have to [not float inside lambdas] if we don't.

    If we indeed do full laziness after the floating inwards (we could
    check the compilation flags for that) then I agree we could be more
    aggressive and do float inwards past lambdas.

    Actually we are not doing a proper full laziness (see below), which
    was another reason for not floating inwards past a lambda.

    This can easily be fixed.  The problem is that we float lets outwards,
    but there are a few expressions which are not let bound, like case
    scrutinees and case alternatives.  After floating inwards the
    simplifier could decide to inline the let and the laziness would be
    lost, e.g.

    \begin{verbatim}
    let a = expensive             ==> \b -> case expensive of ...
    in \ b -> case a of ...
    \end{verbatim}
    The fix is
    \begin{enumerate}
    \item
    to let bind the algebraic case scrutinees (done, I think) and
    the case alternatives (except the ones with an
    unboxed type)(not done, I think). This is best done in the
    SetLevels.hs module, which tags things with their level numbers.
    \item
    do the full laziness pass (floating lets outwards).
    \item
    simplify. The simplifier inlines the (trivial) lets that were
     created but were not floated outwards.
    \end{enumerate}

    With the fix I think Will's suggestion that we can gain even more from
    strictness by floating inwards past lambdas makes sense.

    We still gain even without going past lambdas, as things may be
    strict in the (new) context of a branch (where it was floated to) or
    of a let rhs, e.g.
    \begin{verbatim}
    let a = something            case x of
    in case x of                   alt1 -> case something of a -> a + a
         alt1 -> a + a      ==>    alt2 -> b
         alt2 -> b

    let a = something           let b = case something of a -> a + a
    in let b = a + a        ==> in (b,b)
    in (b,b)
    \end{verbatim}
    Also, even if a is not found to be strict in the new context and is
    still left as a let, if the branch is not taken (or b is not entered)
    the closure for a is not built.

    ************************************************************************
    *                                                                      *
    \subsection{Main floating-inwards code}
    *                                                                      *
    ************************************************************************
    -}

    type FreeVarSet  = DIdSet
    type BoundVarSet = DIdSet

    data FloatInBind = FB BoundVarSet FreeVarSet FloatBind
            -- The FreeVarSet is the free variables of the binding.  In the case
            -- of recursive bindings, the set doesn't include the bound
            -- variables.

    type FloatInBinds = [FloatInBind]
            -- In reverse dependency order (innermost binder first)

    fiExpr :: DynFlags
           -> FloatInBinds      -- Binds we're trying to drop
                                -- as far "inwards" as possible
           -> CoreExprWithFVs   -- Input expr
           -> CoreExpr          -- Result

    fiExpr _ to_drop (_, AnnLit lit)     = ASSERT( null to_drop ) Lit lit
    fiExpr _ to_drop (_, AnnType ty)     = ASSERT( null to_drop ) Type ty
    fiExpr _ to_drop (_, AnnVar v)       = wrapFloats to_drop (Var v)
    fiExpr _ to_drop (_, AnnCoercion co) = wrapFloats to_drop (Coercion co)
    fiExpr dflags to_drop (_, AnnCast expr (co_ann, co))
      = wrapFloats (drop_here ++ co_drop) $
        Cast (fiExpr dflags e_drop expr) co
      where
        [drop_here, e_drop, co_drop]
          = sepBindsByDropPoint dflags False
              [freeVarsOf expr, freeVarsOfAnn co_ann]
              to_drop

    {-
    Applications: we do float inside applications, mainly because we
    need to get at all the arguments.  The next simplifier run will
    pull out any silly ones.
    -}

    fiExpr dflags to_drop ann_expr@(_,AnnApp {})
      = wrapFloats drop_here $ wrapFloats extra_drop $
        mkTicks ticks $
        mkApps (fiExpr dflags fun_drop ann_fun)
               (zipWith (fiExpr dflags) arg_drops ann_args)
      where
        (ann_fun, ann_args, ticks) = collectAnnArgsTicks tickishFloatable ann_expr
        fun_ty  = exprType (deAnnotate ann_fun)
        fun_fvs = freeVarsOf ann_fun
        arg_fvs = map freeVarsOf ann_args

        (drop_here : extra_drop : fun_drop : arg_drops)
           = sepBindsByDropPoint dflags False
                                 (extra_fvs : fun_fvs : arg_fvs)
                                 to_drop
             -- Shortcut behaviour: if to_drop is empty,
             -- sepBindsByDropPoint returns a suitable bunch of empty
             -- lists without evaluating extra_fvs, and hence without
             -- peering into each argument

        (_, extra_fvs) = foldl add_arg (fun_ty, extra_fvs0) ann_args
        extra_fvs0 = case ann_fun of
                       (_, AnnVar _) -> fun_fvs
                       _             -> emptyDVarSet
              -- Don't float the binding for f into f x y z; see Note [Join points]
              -- for why we *can't* do it when f is a join point. (If f isn't a
              -- join point, floating it in isn't especially harmful but it's
              -- useless since the simplifier will immediately float it back out.)

        add_arg :: (Type,FreeVarSet) -> CoreExprWithFVs -> (Type,FreeVarSet)
        add_arg (fun_ty, extra_fvs) (_, AnnType ty)
          = (piResultTy fun_ty ty, extra_fvs)

        add_arg (fun_ty, extra_fvs) (arg_fvs, arg)
          | noFloatIntoArg arg arg_ty
          = (res_ty, extra_fvs `unionDVarSet` arg_fvs)
          | otherwise
          = (res_ty, extra_fvs)
          where
           (arg_ty, res_ty) = splitFunTy fun_ty

    {-
    Note [Do not destroy the let/app invariant]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Watch out for
       f (x +# y)
    We don't want to float bindings into here
       f (case ... of { x -> x +# y })
    because that might destroy the let/app invariant, which requires
    unlifted function arguments to be ok-for-speculation.

    Note [Join points]
    ~~~~~~~~~~~~~~~~~~
    Generally, we don't need to worry about join points - there are places we're
    not allowed to float them, but since they can't have occurrences in those
    places, we're not tempted.

    We do need to be careful about jumps, however:

      joinrec j x y z = ... in
      jump j a b c

    Previous versions often floated the definition of a recursive function into its
    only non-recursive occurrence. But for a join point, this is a disaster:

      (joinrec j x y z = ... in
      jump j) a b c -- wrong!

    Every jump must be exact, so the jump to j must have three arguments. Hence
    we're careful not to float into the target of a jump (though we can float into
    the arguments just fine).

    Note [Floating in past a lambda group]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    * We must be careful about floating inside a value lambda.
      That risks losing laziness.
      The float-out pass might rescue us, but then again it might not.

    * We must be careful about type lambdas too.  At one time we did, and
      there is no risk of duplicating work thereby, but we do need to be
      careful.  In particular, here is a bad case (it happened in the
      cichelli benchmark:
            let v = ...
            in let f = /\t -> \a -> ...
               ==>
            let f = /\t -> let v = ... in \a -> ...
      This is bad as now f is an updatable closure (update PAP)
      and has arity 0.

    * Hack alert!  We only float in through one-shot lambdas,
      not (as you might guess) through lone big lambdas.
      Reason: we float *out* past big lambdas (see the test in the Lam
      case of FloatOut.floatExpr) and we don't want to float straight
      back in again.

      It *is* important to float into one-shot lambdas, however;
      see the remarks with noFloatIntoRhs.

    So we treat lambda in groups, using the following rule:

     Float in if (a) there is at least one Id,
             and (b) there are no non-one-shot Ids

     Otherwise drop all the bindings outside the group.

    This is what the 'go' function in the AnnLam case is doing.

    (Join points are handled similarly: a join point is considered one-shot iff
    it's non-recursive, so we float only into non-recursive join points.)

    Urk! if all are tyvars, and we don't float in, we may miss an
          opportunity to float inside a nested case branch

    Note [Floating coercions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    We could, in principle, have a coercion binding like
       case f x of co { DEFAULT -> e1 e2 }
    It's not common to have a function that returns a coercion, but nothing
    in Core prohibits it.  If so, 'co' might be mentioned in e1 or e2
    /only in a type/.  E.g. suppose e1 was
      let (x :: Int |> co) = blah in blah2

    But, with coercions appearing in types, there is a complication: we
    might be floating in a "strict let" -- that is, a case. Case expressions
    mention their return type. We absolutely can't float a coercion binding
    inward to the point that the type of the expression it's about to wrap
    mentions the coercion. So we include the union of the sets of free variables
    of the types of all the drop points involved. If any of the floaters
    bind a coercion variable mentioned in any of the types, that binder must
    be dropped right away.

    -}

    fiExpr dflags to_drop lam@(_, AnnLam _ _)
      | noFloatIntoLam bndrs       -- Dump it all here
         -- NB: Must line up with noFloatIntoRhs (AnnLam...); see Trac #7088
      = wrapFloats to_drop (mkLams bndrs (fiExpr dflags [] body))

      | otherwise           -- Float inside
      = mkLams bndrs (fiExpr dflags to_drop body)

      where
        (bndrs, body) = collectAnnBndrs lam

    {-
    We don't float lets inwards past an SCC.
            ToDo: keep info on current cc, and when passing
            one, if it is not the same, annotate all lets in binds with current
            cc, change current cc to the new one and float binds into expr.
    -}

    fiExpr dflags to_drop (_, AnnTick tickish expr)
      | tickish `tickishScopesLike` SoftScope
      = Tick tickish (fiExpr dflags to_drop expr)

      | otherwise -- Wimp out for now - we could push values in
      = wrapFloats to_drop (Tick tickish (fiExpr dflags [] expr))

    {-
    For @Lets@, the possible ``drop points'' for the \tr{to_drop}
    bindings are: (a)~in the body, (b1)~in the RHS of a NonRec binding,
    or~(b2), in each of the RHSs of the pairs of a @Rec@.

    Note that we do {\em weird things} with this let's binding.  Consider:
    \begin{verbatim}
    let
        w = ...
    in {
        let v = ... w ...
        in ... v .. w ...
    }
    \end{verbatim}
    Look at the inner \tr{let}.  As \tr{w} is used in both the bind and
    body of the inner let, we could panic and leave \tr{w}'s binding where
    it is.  But \tr{v} is floatable further into the body of the inner let, and
    {\em then} \tr{w} will also be only in the body of that inner let.

    So: rather than drop \tr{w}'s binding here, we add it onto the list of
    things to drop in the outer let's body, and let nature take its
    course.

    Note [extra_fvs (1): avoid floating into RHS]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider let x=\y....t... in body.  We do not necessarily want to float
    a binding for t into the RHS, because it'll immediately be floated out
    again.  (It won't go inside the lambda else we risk losing work.)
    In letrec, we need to be more careful still. We don't want to transform
            let x# = y# +# 1#
            in
            letrec f = \z. ...x#...f...
            in ...
    into
            letrec f = let x# = y# +# 1# in \z. ...x#...f... in ...
    because now we can't float the let out again, because a letrec
    can't have unboxed bindings.

    So we make "extra_fvs" which is the rhs_fvs of such bindings, and
    arrange to dump bindings that bind extra_fvs before the entire let.

    Note [extra_fvs (2): free variables of rules]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      let x{rule mentioning y} = rhs in body
    Here y is not free in rhs or body; but we still want to dump bindings
    that bind y outside the let.  So we augment extra_fvs with the
    idRuleAndUnfoldingVars of x.  No need for type variables, hence not using
    idFreeVars.
    -}

    fiExpr dflags to_drop (_,AnnLet bind body)
      = fiExpr dflags (after ++ new_float : before) body
               -- to_drop is in reverse dependency order
      where
        (before, new_float, after) = fiBind dflags to_drop bind body_fvs
        body_fvs    = freeVarsOf body

    {- Note [Floating primops]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    We try to float-in a case expression over an unlifted type.  The
    motivating example was Trac #5658: in particular, this change allows
    array indexing operations, which have a single DEFAULT alternative
    without any binders, to be floated inward.

    SIMD primops for unpacking SIMD vectors into an unboxed tuple of unboxed
    scalars also need to be floated inward, but unpacks have a single non-DEFAULT
    alternative that binds the elements of the tuple. We now therefore also support
    floating in cases with a single alternative that may bind values.

    But there are wrinkles

    * Which unlifted cases do we float? See PrimOp.hs
      Note [PrimOp can_fail and has_side_effects] which explains:
       - We can float-in can_fail primops, but we can't float them out.
       - But we can float a has_side_effects primop, but NOT inside a lambda,
         so for now we don't float them at all.
      Hence exprOkForSideEffects

    * Because we can float can-fail primops (array indexing, division) inwards
      but not outwards, we must be careful not to transform
         case a /# b of r -> f (F# r)
      ===>
        f (case a /# b of r -> F# r)
      because that creates a new thunk that wasn't there before.  And
      because it can't be floated out (can_fail), the thunk will stay
      there.  Disaster!  (This happened in nofib 'simple' and 'scs'.)

      Solution: only float cases into the branches of other cases, and
      not into the arguments of an application, or the RHS of a let. This
      is somewhat conservative, but it's simple.  And it still hits the
      cases like Trac #5658.   This is implemented in sepBindsByJoinPoint;
      if is_case is False we dump all floating cases right here.

    * Trac #14511 is another example of why we want to restrict float-in
      of case-expressions.  Consider
         case indexArray# a n of (# r #) -> writeArray# ma i (f r)
      Now, floating that indexing operation into the (f r) thunk will
      not create any new thunks, but it will keep the array 'a' alive
      for much longer than the programmer expected.

      So again, not floating a case into a let or argument seems like
      the Right Thing

    For @Case@, the possible drop points for the 'to_drop'
    bindings are:
      (a) inside the scrutinee
      (b) inside one of the alternatives/default (default FVs always /first/!).

    -}

    fiExpr dflags to_drop (_, AnnCase scrut case_bndr _ [(con,alt_bndrs,rhs)])
      | isUnliftedType (idType case_bndr)
      , exprOkForSideEffects (deAnnotate scrut)
          -- See Note [Floating primops]
      = wrapFloats shared_binds $
        fiExpr dflags (case_float : rhs_binds) rhs
      where
        case_float = FB (mkDVarSet (case_bndr : alt_bndrs)) scrut_fvs
                        (FloatCase scrut' case_bndr con alt_bndrs)
        scrut'     = fiExpr dflags scrut_binds scrut
        rhs_fvs    = freeVarsOf rhs `delDVarSetList` (case_bndr : alt_bndrs)
        scrut_fvs  = freeVarsOf scrut

        [shared_binds, scrut_binds, rhs_binds]
           = sepBindsByDropPoint dflags False
               [scrut_fvs, rhs_fvs]
               to_drop

    fiExpr dflags to_drop (_, AnnCase scrut case_bndr ty alts)
      = wrapFloats drop_here1 $
        wrapFloats drop_here2 $
        Case (fiExpr dflags scrut_drops scrut) case_bndr ty
             (zipWith fi_alt alts_drops_s alts)
      where
            -- Float into the scrut and alts-considered-together just like App
        [drop_here1, scrut_drops, alts_drops]
           = sepBindsByDropPoint dflags False
               [scrut_fvs, all_alts_fvs]
               to_drop

            -- Float into the alts with the is_case flag set
        (drop_here2 : alts_drops_s)
          | [ _ ] <- alts = [] : [alts_drops]
          | otherwise     = sepBindsByDropPoint dflags True alts_fvs alts_drops

        scrut_fvs    = freeVarsOf scrut
        alts_fvs     = map alt_fvs alts
        all_alts_fvs = unionDVarSets alts_fvs
        alt_fvs (_con, args, rhs)
          = foldl delDVarSet (freeVarsOf rhs) (case_bndr:args)
               -- Delete case_bndr and args from free vars of rhs
               -- to get free vars of alt

        fi_alt to_drop (con, args, rhs) = (con, args, fiExpr dflags to_drop rhs)

    ------------------
    fiBind :: DynFlags
           -> FloatInBinds      -- Binds we're trying to drop
                                -- as far "inwards" as possible
           -> CoreBindWithFVs   -- Input binding
           -> DVarSet           -- Free in scope of binding
           -> ( FloatInBinds    -- Land these before
              , FloatInBind     -- The binding itself
              , FloatInBinds)   -- Land these after

    fiBind dflags to_drop (AnnNonRec id ann_rhs@(rhs_fvs, rhs)) body_fvs
      = ( extra_binds ++ shared_binds          -- Land these before
                                               -- See Note [extra_fvs (1,2)]
        , FB (unitDVarSet id) rhs_fvs'         -- The new binding itself
              (FloatLet (NonRec id rhs'))
        , body_binds )                         -- Land these after

      where
        body_fvs2 = body_fvs `delDVarSet` id

        rule_fvs = bndrRuleAndUnfoldingVarsDSet id        -- See Note [extra_fvs (2): free variables of rules]
        extra_fvs | noFloatIntoRhs NonRecursive id rhs
                  = rule_fvs `unionDVarSet` rhs_fvs
                  | otherwise
                  = rule_fvs
            -- See Note [extra_fvs (1): avoid floating into RHS]
            -- No point in floating in only to float straight out again
            -- We *can't* float into ok-for-speculation unlifted RHSs
            -- But do float into join points

        [shared_binds, extra_binds, rhs_binds, body_binds]
            = sepBindsByDropPoint dflags False
                [extra_fvs, rhs_fvs, body_fvs2]
                to_drop

            -- Push rhs_binds into the right hand side of the binding
        rhs'     = fiRhs dflags rhs_binds id ann_rhs
        rhs_fvs' = rhs_fvs `unionDVarSet` floatedBindsFVs rhs_binds `unionDVarSet` rule_fvs
                            -- Don't forget the rule_fvs; the binding mentions them!

    fiBind dflags to_drop (AnnRec bindings) body_fvs
      = ( extra_binds ++ shared_binds
        , FB (mkDVarSet ids) rhs_fvs'
             (FloatLet (Rec (fi_bind rhss_binds bindings)))
        , body_binds )
      where
        (ids, rhss) = unzip bindings
        rhss_fvs = map freeVarsOf rhss

            -- See Note [extra_fvs (1,2)]
        rule_fvs = mapUnionDVarSet bndrRuleAndUnfoldingVarsDSet ids
        extra_fvs = rule_fvs `unionDVarSet`
                    unionDVarSets [ rhs_fvs | (bndr, (rhs_fvs, rhs)) <- bindings
                                  , noFloatIntoRhs Recursive bndr rhs ]

        (shared_binds:extra_binds:body_binds:rhss_binds)
            = sepBindsByDropPoint dflags False
                (extra_fvs:body_fvs:rhss_fvs)
                to_drop

        rhs_fvs' = unionDVarSets rhss_fvs `unionDVarSet`
                   unionDVarSets (map floatedBindsFVs rhss_binds) `unionDVarSet`
                   rule_fvs         -- Don't forget the rule variables!

        -- Push rhs_binds into the right hand side of the binding
        fi_bind :: [FloatInBinds]       -- one per "drop pt" conjured w/ fvs_of_rhss
                -> [(Id, CoreExprWithFVs)]
                -> [(Id, CoreExpr)]

        fi_bind to_drops pairs
          = [ (binder, fiRhs dflags to_drop binder rhs)
            | ((binder, rhs), to_drop) <- zipEqual "fi_bind" pairs to_drops ]

    ------------------
    fiRhs :: DynFlags -> FloatInBinds -> CoreBndr -> CoreExprWithFVs -> CoreExpr
    fiRhs dflags to_drop bndr rhs
      | Just join_arity <- isJoinId_maybe bndr
      , let (bndrs, body) = collectNAnnBndrs join_arity rhs
      = mkLams bndrs (fiExpr dflags to_drop body)
      | otherwise
      = fiExpr dflags to_drop rhs

    ------------------
    noFloatIntoLam :: [Var] -> Bool
    noFloatIntoLam bndrs = any bad bndrs
      where
        bad b = isId b && not (isOneShotBndr b)
        -- Don't float inside a non-one-shot lambda

    noFloatIntoRhs :: RecFlag -> Id -> CoreExprWithFVs' -> Bool
    -- ^ True if it's a bad idea to float bindings into this RHS
    noFloatIntoRhs is_rec bndr rhs
      | isJoinId bndr
      = isRec is_rec -- Joins are one-shot iff non-recursive

      | otherwise
      = noFloatIntoArg rhs (idType bndr)

    noFloatIntoArg :: CoreExprWithFVs' -> Type -> Bool
    noFloatIntoArg expr expr_ty
      | isUnliftedType expr_ty
      = True  -- See Note [Do not destroy the let/app invariant]

       | AnnLam bndr e <- expr
       , (bndrs, _) <- collectAnnBndrs e
       =  noFloatIntoLam (bndr:bndrs)  -- Wrinkle 1 (a)
       || all isTyVar (bndr:bndrs)     -- Wrinkle 1 (b)
          -- See Note [noFloatInto considerations] wrinkle 2

      | otherwise  -- Note [noFloatInto considerations] wrinkle 2
      = exprIsTrivial deann_expr || exprIsHNF deann_expr
      where
        deann_expr = deAnnotate' expr

    {- Note [noFloatInto considerations]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When do we want to float bindings into
       - noFloatIntoRHs: the RHS of a let-binding
       - noFloatIntoArg: the argument of a function application

    Definitely don't float in if it has unlifted type; that
    would destroy the let/app invariant.

    * Wrinkle 1: do not float in if
         (a) any non-one-shot value lambdas
      or (b) all type lambdas
      In both cases we'll float straight back out again
      NB: Must line up with fiExpr (AnnLam...); see Trac #7088

      (a) is important: we /must/ float into a one-shot lambda group
      (which includes join points). This makes a big difference
      for things like
         f x# = let x = I# x#
                in let j = \() -> ...x...
                   in if <condition> then normal-path else j ()
      If x is used only in the error case join point, j, we must float the
      boxing constructor into it, else we box it every time which is very
      bad news indeed.

    * Wrinkle 2: for RHSs, do not float into a HNF; we'll just float right
      back out again... not tragic, but a waste of time.

      For function arguments we will still end up with this
      in-then-out stuff; consider
        letrec x = e in f x
      Here x is not a HNF, so we'll produce
        f (letrec x = e in x)
      which is OK... it's not that common, and we'll end up
      floating out again, in CorePrep if not earlier.
      Still, we use exprIsTrivial to catch this case (sigh)

    ************************************************************************
    *                                                                      *
    \subsection{@sepBindsByDropPoint@}
    *                                                                      *
    ************************************************************************

    This is the crucial function.  The idea is: We have a wad of bindings
    that we'd like to distribute inside a collection of {\em drop points};
    insides the alternatives of a \tr{case} would be one example of some
    drop points; the RHS and body of a non-recursive \tr{let} binding
    would be another (2-element) collection.

    So: We're given a list of sets-of-free-variables, one per drop point,
    and a list of floating-inwards bindings.  If a binding can go into
    only one drop point (without suddenly making something out-of-scope),
    in it goes.  If a binding is used inside {\em multiple} drop points,
    then it has to go in a you-must-drop-it-above-all-these-drop-points
    point.

    We have to maintain the order on these drop-point-related lists.
    -}

    -- pprFIB :: FloatInBinds -> SDoc
    -- pprFIB fibs = text "FIB:" <+> ppr [b | FB _ _ b <- fibs]

    sepBindsByDropPoint
        :: DynFlags
        -> Bool                -- True <=> is case expression
        -> [FreeVarSet]        -- One set of FVs per drop point
                               -- Always at least two long!
        -> FloatInBinds        -- Candidate floaters
        -> [FloatInBinds]      -- FIRST one is bindings which must not be floated
                               -- inside any drop point; the rest correspond
                               -- one-to-one with the input list of FV sets

    -- Every input floater is returned somewhere in the result;
    -- none are dropped, not even ones which don't seem to be
    -- free in *any* of the drop-point fvs.  Why?  Because, for example,
    -- a binding (let x = E in B) might have a specialised version of
    -- x (say x') stored inside x, but x' isn't free in E or B.

    type DropBox = (FreeVarSet, FloatInBinds)

    sepBindsByDropPoint dflags is_case drop_pts floaters
      | null floaters  -- Shortcut common case
      = [] : [[] | _ <- drop_pts]

      | otherwise
      = ASSERT( drop_pts `lengthAtLeast` 2 )
        go floaters (map (\fvs -> (fvs, [])) (emptyDVarSet : drop_pts))
      where
        n_alts = length drop_pts

        go :: FloatInBinds -> [DropBox] -> [FloatInBinds]
            -- The *first* one in the argument list is the drop_here set
            -- The FloatInBinds in the lists are in the reverse of
            -- the normal FloatInBinds order; that is, they are the right way round!

        go [] drop_boxes = map (reverse . snd) drop_boxes

        go (bind_w_fvs@(FB bndrs bind_fvs bind) : binds) drop_boxes@(here_box : fork_boxes)
            = go binds new_boxes
            where
              -- "here" means the group of bindings dropped at the top of the fork

              (used_here : used_in_flags) = [ fvs `intersectsDVarSet` bndrs
                                            | (fvs, _) <- drop_boxes]

              drop_here = used_here || cant_push

              n_used_alts = count id used_in_flags -- returns number of Trues in list.

              cant_push
                | is_case   = n_used_alts == n_alts   -- Used in all, don't push
                                                      -- Remember n_alts > 1
                              || (n_used_alts > 1 && not (floatIsDupable dflags bind))
                                 -- floatIsDupable: see Note [Duplicating floats]

                | otherwise = floatIsCase bind || n_used_alts > 1
                                 -- floatIsCase: see Note [Floating primops]

              new_boxes | drop_here = (insert here_box : fork_boxes)
                        | otherwise = (here_box : new_fork_boxes)

              new_fork_boxes = zipWithEqual "FloatIn.sepBinds" insert_maybe
                                            fork_boxes used_in_flags

              insert :: DropBox -> DropBox
              insert (fvs,drops) = (fvs `unionDVarSet` bind_fvs, bind_w_fvs:drops)

              insert_maybe box True  = insert box
              insert_maybe box False = box

        go _ _ = panic "sepBindsByDropPoint/go"

    {- Note [Duplicating floats]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    For case expressions we duplicate the binding if it is reasonably
    small, and if it is not used in all the RHSs This is good for
    situations like
         let x = I# y in
         case e of
           C -> error x
           D -> error x
           E -> ...not mentioning x...

    If the thing is used in all RHSs there is nothing gained,
    so we don't duplicate then.
    -}

    floatedBindsFVs :: FloatInBinds -> FreeVarSet
    floatedBindsFVs binds = mapUnionDVarSet fbFVs binds

    fbFVs :: FloatInBind -> DVarSet
    fbFVs (FB _ fvs _) = fvs

    wrapFloats :: FloatInBinds -> CoreExpr -> CoreExpr
    -- Remember FloatInBinds is in *reverse* dependency order
    wrapFloats []               e = e
    wrapFloats (FB _ _ fl : bs) e = wrapFloats bs (wrapFloat fl e)

    floatIsDupable :: DynFlags -> FloatBind -> Bool
    floatIsDupable dflags (FloatCase scrut _ _ _) = exprIsDupable dflags scrut
    floatIsDupable dflags (FloatLet (Rec prs))    = all (exprIsDupable dflags . snd) prs
    floatIsDupable dflags (FloatLet (NonRec _ r)) = exprIsDupable dflags r

    floatIsCase :: FloatBind -> Bool
    floatIsCase (FloatCase {}) = True
    floatIsCase (FloatLet {})  = False