coreSyn/CorePrep.hs

    {-
    (c) The University of Glasgow, 1994-2006

    Core pass to saturate constructors and PrimOps
    -}

    {-# LANGUAGE BangPatterns, CPP #-}

    module CorePrep (
          corePrepPgm, corePrepExpr, cvtLitInteger,
          lookupMkIntegerName, lookupIntegerSDataConName
      ) where

    #include "HsVersions.h"

    import OccurAnal

    import HscTypes
    import PrelNames
    import MkId             ( realWorldPrimId )
    import CoreUtils
    import CoreArity
    import CoreFVs
    import CoreMonad        ( CoreToDo(..) )
    import CoreLint         ( endPassIO )
    import CoreSyn
    import CoreSubst
    import MkCore hiding( FloatBind(..) )   -- We use our own FloatBind here
    import Type
    import Literal
    import Coercion
    import TcEnv
    import TyCon
    import Demand
    import Var
    import VarSet
    import VarEnv
    import Id
    import IdInfo
    import TysWiredIn
    import DataCon
    import PrimOp
    import BasicTypes
    import Module
    import UniqSupply
    import Maybes
    import OrdList
    import ErrUtils
    import DynFlags
    import Util
    import Pair
    import Outputable
    import Platform
    import FastString
    import Config
    import Name             ( NamedThing(..), nameSrcSpan )
    import SrcLoc           ( SrcSpan(..), realSrcLocSpan, mkRealSrcLoc )
    import Data.Bits
    import MonadUtils       ( mapAccumLM )
    import Data.List        ( mapAccumL )
    import Control.Monad

    #if __GLASGOW_HASKELL__ < 710
    import Control.Applicative
    #endif

    {-
    -- ---------------------------------------------------------------------------
    -- Overview
    -- ---------------------------------------------------------------------------

    The goal of this pass is to prepare for code generation.

    1.  Saturate constructor and primop applications.

    2.  Convert to A-normal form; that is, function arguments
        are always variables.

        * Use case for strict arguments:
            f E ==> case E of x -> f x
            (where f is strict)

        * Use let for non-trivial lazy arguments
            f E ==> let x = E in f x
            (were f is lazy and x is non-trivial)

    3.  Similarly, convert any unboxed lets into cases.
        [I'm experimenting with leaving 'ok-for-speculation'
         rhss in let-form right up to this point.]

    4.  Ensure that *value* lambdas only occur as the RHS of a binding
        (The code generator can't deal with anything else.)
        Type lambdas are ok, however, because the code gen discards them.

    5.  [Not any more; nuked Jun 2002] Do the seq/par munging.

    6.  Clone all local Ids.
        This means that all such Ids are unique, rather than the
        weaker guarantee of no clashes which the simplifier provides.
        And that is what the code generator needs.

        We don't clone TyVars or CoVars. The code gen doesn't need that,
        and doing so would be tiresome because then we'd need
        to substitute in types and coercions.

    7.  Give each dynamic CCall occurrence a fresh unique; this is
        rather like the cloning step above.

    8.  Inject bindings for the "implicit" Ids:
            * Constructor wrappers
            * Constructor workers
        We want curried definitions for all of these in case they
        aren't inlined by some caller.

    9.  Replace (lazy e) by e.  See Note [lazyId magic] in MkId.hs

    10. Convert (LitInteger i t) into the core representation
        for the Integer i. Normally this uses mkInteger, but if
        we are using the integer-gmp implementation then there is a
        special case where we use the S# constructor for Integers that
        are in the range of Int.

    11. Uphold tick consistency while doing this: We move ticks out of
        (non-type) applications where we can, and make sure that we
        annotate according to scoping rules when floating.

    This is all done modulo type applications and abstractions, so that
    when type erasure is done for conversion to STG, we don't end up with
    any trivial or useless bindings.

    Invariants
    ~~~~~~~~~~
    Here is the syntax of the Core produced by CorePrep:

        Trivial expressions
           triv ::= lit |  var
                  | triv ty  |  /\a. triv
                  | truv co  |  /\c. triv  |  triv |> co

        Applications
           app ::= lit  |  var  |  app triv  |  app ty  | app co | app |> co

        Expressions
           body ::= app
                  | let(rec) x = rhs in body     -- Boxed only
                  | case body of pat -> body
                  | /\a. body | /\c. body
                  | body |> co

        Right hand sides (only place where value lambdas can occur)
           rhs ::= /\a.rhs  |  \x.rhs  |  body

    We define a synonym for each of these non-terminals.  Functions
    with the corresponding name produce a result in that syntax.
    -}

    type CpeTriv = CoreExpr    -- Non-terminal 'triv'
    type CpeApp  = CoreExpr    -- Non-terminal 'app'
    type CpeBody = CoreExpr    -- Non-terminal 'body'
    type CpeRhs  = CoreExpr    -- Non-terminal 'rhs'

    {-
    ************************************************************************
    *                                                                      *
                    Top level stuff
    *                                                                      *
    ************************************************************************
    -}

    corePrepPgm :: HscEnv -> ModLocation -> CoreProgram -> [TyCon] -> IO CoreProgram
    corePrepPgm hsc_env mod_loc binds data_tycons = do
        let dflags = hsc_dflags hsc_env
        showPass dflags "CorePrep"
        us <- mkSplitUniqSupply 's'
        initialCorePrepEnv <- mkInitialCorePrepEnv dflags hsc_env

        let implicit_binds = mkDataConWorkers dflags mod_loc data_tycons
                -- NB: we must feed mkImplicitBinds through corePrep too
                -- so that they are suitably cloned and eta-expanded

            binds_out = initUs_ us $ do
                          floats1 <- corePrepTopBinds initialCorePrepEnv binds
                          floats2 <- corePrepTopBinds initialCorePrepEnv implicit_binds
                          return (deFloatTop (floats1 `appendFloats` floats2))

        endPassIO hsc_env alwaysQualify CorePrep binds_out []
        return binds_out

    corePrepExpr :: DynFlags -> HscEnv -> CoreExpr -> IO CoreExpr
    corePrepExpr dflags hsc_env expr = do
        showPass dflags "CorePrep"
        us <- mkSplitUniqSupply 's'
        initialCorePrepEnv <- mkInitialCorePrepEnv dflags hsc_env
        let new_expr = initUs_ us (cpeBodyNF initialCorePrepEnv expr)
        dumpIfSet_dyn dflags Opt_D_dump_prep "CorePrep" (ppr new_expr)
        return new_expr

    corePrepTopBinds :: CorePrepEnv -> [CoreBind] -> UniqSM Floats
    -- Note [Floating out of top level bindings]
    corePrepTopBinds initialCorePrepEnv binds
      = go initialCorePrepEnv binds
      where
        go _   []             = return emptyFloats
        go env (bind : binds) = do (env', bind') <- cpeBind TopLevel env bind
                                   binds' <- go env' binds
                                   return (bind' `appendFloats` binds')

    mkDataConWorkers :: DynFlags -> ModLocation -> [TyCon] -> [CoreBind]
    -- See Note [Data constructor workers]
    -- c.f. Note [Injecting implicit bindings] in TidyPgm
    mkDataConWorkers dflags mod_loc data_tycons
      = [ NonRec id (tick_it (getName data_con) (Var id))
                                    -- The ice is thin here, but it works
        | tycon <- data_tycons,     -- CorePrep will eta-expand it
          data_con <- tyConDataCons tycon,
          let id = dataConWorkId data_con
        ]
     where
       -- If we want to generate debug info, we put a source note on the
       -- worker. This is useful, especially for heap profiling.
       tick_it name
         | debugLevel dflags == 0                = id
         | RealSrcSpan span <- nameSrcSpan name  = tick span
         | Just file <- ml_hs_file mod_loc       = tick (span1 file)
         | otherwise                             = tick (span1 "???")
         where tick span  = Tick (SourceNote span $ showSDoc dflags (ppr name))
               span1 file = realSrcLocSpan $ mkRealSrcLoc (mkFastString file) 1 1

    {-
    Note [Floating out of top level bindings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NB: we do need to float out of top-level bindings
    Consider        x = length [True,False]
    We want to get
                    s1 = False : []
                    s2 = True  : s1
                    x  = length s2

    We return a *list* of bindings, because we may start with
            x* = f (g y)
    where x is demanded, in which case we want to finish with
            a = g y
            x* = f a
    And then x will actually end up case-bound

    Note [CafInfo and floating]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    What happens when we try to float bindings to the top level?  At this
    point all the CafInfo is supposed to be correct, and we must make certain
    that is true of the new top-level bindings.  There are two cases
    to consider

    a) The top-level binding is marked asCafRefs.  In that case we are
       basically fine.  The floated bindings had better all be lazy lets,
       so they can float to top level, but they'll all have HasCafRefs
       (the default) which is safe.

    b) The top-level binding is marked NoCafRefs.  This really happens
       Example.  CoreTidy produces
          $fApplicativeSTM [NoCafRefs] = D:Alternative retry# ...blah...
       Now CorePrep has to eta-expand to
          $fApplicativeSTM = let sat = \xy. retry x y
                             in D:Alternative sat ...blah...
       So what we *want* is
          sat [NoCafRefs] = \xy. retry x y
          $fApplicativeSTM [NoCafRefs] = D:Alternative sat ...blah...

       So, gruesomely, we must set the NoCafRefs flag on the sat bindings,
       *and* substutite the modified 'sat' into the old RHS.

       It should be the case that 'sat' is itself [NoCafRefs] (a value, no
       cafs) else the original top-level binding would not itself have been
       marked [NoCafRefs].  The DEBUG check in CoreToStg for
       consistentCafInfo will find this.

    This is all very gruesome and horrible. It would be better to figure
    out CafInfo later, after CorePrep.  We'll do that in due course.
    Meanwhile this horrible hack works.

    Note [Data constructor workers]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Create any necessary "implicit" bindings for data con workers.  We
    create the rather strange (non-recursive!) binding

            $wC = \x y -> $wC x y

    i.e. a curried constructor that allocates.  This means that we can
    treat the worker for a constructor like any other function in the rest
    of the compiler.  The point here is that CoreToStg will generate a
    StgConApp for the RHS, rather than a call to the worker (which would
    give a loop).  As Lennart says: the ice is thin here, but it works.

    Hmm.  Should we create bindings for dictionary constructors?  They are
    always fully applied, and the bindings are just there to support
    partial applications. But it's easier to let them through.

    Note [Dead code in CorePrep]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Imagine that we got an input program like this (see Trac #4962):

      f :: Show b => Int -> (Int, b -> Maybe Int -> Int)
      f x = (g True (Just x) + g () (Just x), g)
        where
          g :: Show a => a -> Maybe Int -> Int
          g _ Nothing = x
          g y (Just z) = if z > 100 then g y (Just (z + length (show y))) else g y unknown

    After specialisation and SpecConstr, we would get something like this:

      f :: Show b => Int -> (Int, b -> Maybe Int -> Int)
      f x = (g$Bool_True_Just x + g$Unit_Unit_Just x, g)
        where
          {-# RULES g $dBool = g$Bool
                    g $dUnit = g$Unit #-}
          g = ...
          {-# RULES forall x. g$Bool True (Just x) = g$Bool_True_Just x #-}
          g$Bool = ...
          {-# RULES forall x. g$Unit () (Just x) = g$Unit_Unit_Just x #-}
          g$Unit = ...
          g$Bool_True_Just = ...
          g$Unit_Unit_Just = ...

    Note that the g$Bool and g$Unit functions are actually dead code: they
    are only kept alive by the occurrence analyser because they are
    referred to by the rules of g, which is being kept alive by the fact
    that it is used (unspecialised) in the returned pair.

    However, at the CorePrep stage there is no way that the rules for g
    will ever fire, and it really seems like a shame to produce an output
    program that goes to the trouble of allocating a closure for the
    unreachable g$Bool and g$Unit functions.

    The way we fix this is to:
     * In cloneBndr, drop all unfoldings/rules

     * In deFloatTop, run a simple dead code analyser on each top-level
       RHS to drop the dead local bindings. For that call to OccAnal, we
       disable the binder swap, else the occurrence analyser sometimes
       introduces new let bindings for cased binders, which lead to the bug
       in #5433.

    The reason we don't just OccAnal the whole output of CorePrep is that
    the tidier ensures that all top-level binders are GlobalIds, so they
    don't show up in the free variables any longer. So if you run the
    occurrence analyser on the output of CoreTidy (or later) you e.g. turn
    this program:

      Rec {
      f = ... f ...
      }

    Into this one:

      f = ... f ...

    (Since f is not considered to be free in its own RHS.)

    ************************************************************************
    *                                                                      *
                    The main code
    *                                                                      *
    ************************************************************************
    -}

    cpeBind :: TopLevelFlag -> CorePrepEnv -> CoreBind
            -> UniqSM (CorePrepEnv, Floats)
    cpeBind top_lvl env (NonRec bndr rhs)
      = do { (_, bndr1) <- cpCloneBndr env bndr
           ; let dmd         = idDemandInfo bndr
                 is_unlifted = isUnLiftedType (idType bndr)
           ; (floats, bndr2, rhs2) <- cpePair top_lvl NonRecursive
                                              dmd
                                              is_unlifted
                                              env bndr1 rhs
           ; let new_float = mkFloat dmd is_unlifted bndr2 rhs2

            -- We want bndr'' in the envt, because it records
            -- the evaluated-ness of the binder
           ; return (extendCorePrepEnv env bndr bndr2,
                     addFloat floats new_float) }

    cpeBind top_lvl env (Rec pairs)
      = do { let (bndrs,rhss) = unzip pairs
           ; (env', bndrs1) <- cpCloneBndrs env (map fst pairs)
           ; stuff <- zipWithM (cpePair top_lvl Recursive topDmd False env') bndrs1 rhss

           ; let (floats_s, bndrs2, rhss2) = unzip3 stuff
                 all_pairs = foldrOL add_float (bndrs2 `zip` rhss2)
                                               (concatFloats floats_s)
           ; return (extendCorePrepEnvList env (bndrs `zip` bndrs2),
                     unitFloat (FloatLet (Rec all_pairs))) }
      where
            -- Flatten all the floats, and the currrent
            -- group into a single giant Rec
        add_float (FloatLet (NonRec b r)) prs2 = (b,r) : prs2
        add_float (FloatLet (Rec prs1))   prs2 = prs1 ++ prs2
        add_float b                       _    = pprPanic "cpeBind" (ppr b)

    ---------------
    cpePair :: TopLevelFlag -> RecFlag -> Demand -> Bool
            -> CorePrepEnv -> Id -> CoreExpr
            -> UniqSM (Floats, Id, CpeRhs)
    -- Used for all bindings
    cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
      = do { (floats1, rhs1) <- cpeRhsE env rhs

           -- See if we are allowed to float this stuff out of the RHS
           ; (floats2, rhs2) <- float_from_rhs floats1 rhs1

           -- Make the arity match up
           ; (floats3, rhs3)
                <- if manifestArity rhs1 <= arity
                   then return (floats2, cpeEtaExpand arity rhs2)
                   else WARN(True, text "CorePrep: silly extra arguments:" <+> ppr bndr)
                                   -- Note [Silly extra arguments]
                        (do { v <- newVar (idType bndr)
                            ; let float = mkFloat topDmd False v rhs2
                            ; return ( addFloat floats2 float
                                     , cpeEtaExpand arity (Var v)) })

            -- Wrap floating ticks
           ; let (floats4, rhs4) = wrapTicks floats3 rhs3

            -- Record if the binder is evaluated
            -- and otherwise trim off the unfolding altogether
            -- It's not used by the code generator; getting rid of it reduces
            -- heap usage and, since we may be changing uniques, we'd have
            -- to substitute to keep it right
           ; let bndr' | exprIsHNF rhs3 = bndr `setIdUnfolding` evaldUnfolding
                       | otherwise      = bndr `setIdUnfolding` noUnfolding

           ; return (floats4, bndr', rhs4) }
      where
        is_strict_or_unlifted = (isStrictDmd dmd) || is_unlifted

        platform = targetPlatform (cpe_dynFlags env)

        arity = idArity bndr        -- We must match this arity

        ---------------------
        float_from_rhs floats rhs
          | isEmptyFloats floats = return (emptyFloats, rhs)
          | isTopLevel top_lvl    = float_top    floats rhs
          | otherwise             = float_nested floats rhs

        ---------------------
        float_nested floats rhs
          | wantFloatNested is_rec is_strict_or_unlifted floats rhs
                      = return (floats, rhs)
          | otherwise = dont_float floats rhs

        ---------------------
        float_top floats rhs        -- Urhgh!  See Note [CafInfo and floating]
          | mayHaveCafRefs (idCafInfo bndr)
          , allLazyTop floats
          = return (floats, rhs)

          -- So the top-level binding is marked NoCafRefs
          | Just (floats', rhs') <- canFloatFromNoCaf platform floats rhs
          = return (floats', rhs')

          | otherwise
          = dont_float floats rhs

        ---------------------
        dont_float floats rhs
          -- Non-empty floats, but do not want to float from rhs
          -- So wrap the rhs in the floats
          -- But: rhs1 might have lambdas, and we can't
          --      put them inside a wrapBinds
          = do { body <- rhsToBodyNF rhs
               ; return (emptyFloats, wrapBinds floats body) }

    {- Note [Silly extra arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we had this
            f{arity=1} = \x\y. e
    We *must* match the arity on the Id, so we have to generate
            f' = \x\y. e
            f  = \x. f' x

    It's a bizarre case: why is the arity on the Id wrong?  Reason
    (in the days of __inline_me__):
            f{arity=0} = __inline_me__ (let v = expensive in \xy. e)
    When InlineMe notes go away this won't happen any more.  But
    it seems good for CorePrep to be robust.
    -}

    -- ---------------------------------------------------------------------------
    --              CpeRhs: produces a result satisfying CpeRhs
    -- ---------------------------------------------------------------------------

    cpeRhsE :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeRhs)
    -- If
    --      e  ===>  (bs, e')
    -- then
    --      e = let bs in e'        (semantically, that is!)
    --
    -- For example
    --      f (g x)   ===>   ([v = g x], f v)

    cpeRhsE _env expr@(Type {})      = return (emptyFloats, expr)
    cpeRhsE _env expr@(Coercion {})  = return (emptyFloats, expr)
    cpeRhsE env (Lit (LitInteger i _))
        = cpeRhsE env (cvtLitInteger (cpe_dynFlags env) (getMkIntegerId env)
                       (cpe_integerSDataCon env) i)
    cpeRhsE _env expr@(Lit {}) = return (emptyFloats, expr)
    cpeRhsE env expr@(Var {})  = cpeApp env expr

    cpeRhsE env (Var f `App` _{-type-} `App` arg)
      | f `hasKey` lazyIdKey          -- Replace (lazy a) by a
      = cpeRhsE env arg               -- See Note [lazyId magic] in MkId

    cpeRhsE env (Var f `App` _levity `App` _type `App` arg)
        -- See Note [runRW magic] in MkId
      | f `hasKey` runRWKey           -- Replace (runRW# f) by (f realWorld#),
      = case arg of                   -- beta reducing if possible
          Lam s body -> cpeRhsE (extendCorePrepEnv env s realWorldPrimId) body
          _          -> cpeRhsE env (arg `App` Var realWorldPrimId)
                        -- See Note [runRW arg]

    {- Note [runRW arg]
    ~~~~~~~~~~~~~~~~~~~
    If we got, say
       runRW# (case bot of {})
    which happened in Trac #11291, we do /not/ want to turn it into
       (case bot of {}) realWorldPrimId#
    because that gives a panic in CoreToStg.myCollectArgs, which expects
    only variables in function position.  But if we are sure to make
    runRW# strict (which we do in MkId), this can't happen
    -}

    cpeRhsE env expr@(App {}) = cpeApp env expr

    cpeRhsE env (Let bind expr)
      = do { (env', new_binds) <- cpeBind NotTopLevel env bind
           ; (floats, body) <- cpeRhsE env' expr
           ; return (new_binds `appendFloats` floats, body) }

    cpeRhsE env (Tick tickish expr)
      | tickishPlace tickish == PlaceNonLam && tickish `tickishScopesLike` SoftScope
      = do { (floats, body) <- cpeRhsE env expr
             -- See [Floating Ticks in CorePrep]
           ; return (unitFloat (FloatTick tickish) `appendFloats` floats, body) }
      | otherwise
      = do { body <- cpeBodyNF env expr
           ; return (emptyFloats, mkTick tickish' body) }
      where
        tickish' | Breakpoint n fvs <- tickish
                 = Breakpoint n (map (lookupCorePrepEnv env) fvs)
                 | otherwise
                 = tickish

    cpeRhsE env (Cast expr co)
       = do { (floats, expr') <- cpeRhsE env expr
            ; return (floats, Cast expr' co) }

    cpeRhsE env expr@(Lam {})
       = do { let (bndrs,body) = collectBinders expr
            ; (env', bndrs') <- cpCloneBndrs env bndrs
            ; body' <- cpeBodyNF env' body
            ; return (emptyFloats, mkLams bndrs' body') }

    cpeRhsE env (Case scrut bndr ty alts)
      = do { (floats, scrut') <- cpeBody env scrut
           ; let bndr1 = bndr `setIdUnfolding` evaldUnfolding
                -- Record that the case binder is evaluated in the alternatives
           ; (env', bndr2) <- cpCloneBndr env bndr1
           ; alts' <- mapM (sat_alt env') alts
           ; return (floats, Case scrut' bndr2 ty alts') }
      where
        sat_alt env (con, bs, rhs)
           = do { (env2, bs') <- cpCloneBndrs env bs
                ; rhs' <- cpeBodyNF env2 rhs
                ; return (con, bs', rhs') }

    cvtLitInteger :: DynFlags -> Id -> Maybe DataCon -> Integer -> CoreExpr
    -- Here we convert a literal Integer to the low-level
    -- represenation. Exactly how we do this depends on the
    -- library that implements Integer.  If it's GMP we
    -- use the S# data constructor for small literals.
    -- See Note [Integer literals] in Literal
    cvtLitInteger dflags _ (Just sdatacon) i
      | inIntRange dflags i -- Special case for small integers
        = mkConApp sdatacon [Lit (mkMachInt dflags i)]

    cvtLitInteger dflags mk_integer _ i
        = mkApps (Var mk_integer) [isNonNegative, ints]
      where isNonNegative = if i < 0 then mkConApp falseDataCon []
                                     else mkConApp trueDataCon  []
            ints = mkListExpr intTy (f (abs i))
            f 0 = []
            f x = let low  = x .&. mask
                      high = x `shiftR` bits
                  in mkConApp intDataCon [Lit (mkMachInt dflags low)] : f high
            bits = 31
            mask = 2 ^ bits - 1

    -- ---------------------------------------------------------------------------
    --              CpeBody: produces a result satisfying CpeBody
    -- ---------------------------------------------------------------------------

    cpeBodyNF :: CorePrepEnv -> CoreExpr -> UniqSM CpeBody
    cpeBodyNF env expr
      = do { (floats, body) <- cpeBody env expr
           ; return (wrapBinds floats body) }

    --------
    cpeBody :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeBody)
    cpeBody env expr
      = do { (floats1, rhs) <- cpeRhsE env expr
           ; (floats2, body) <- rhsToBody rhs
           ; return (floats1 `appendFloats` floats2, body) }

    --------
    rhsToBodyNF :: CpeRhs -> UniqSM CpeBody
    rhsToBodyNF rhs = do { (floats,body) <- rhsToBody rhs
                         ; return (wrapBinds floats body) }

    --------
    rhsToBody :: CpeRhs -> UniqSM (Floats, CpeBody)
    -- Remove top level lambdas by let-binding

    rhsToBody (Tick t expr)
      | tickishScoped t == NoScope  -- only float out of non-scoped annotations
      = do { (floats, expr') <- rhsToBody expr
           ; return (floats, mkTick t expr') }

    rhsToBody (Cast e co)
            -- You can get things like
            --      case e of { p -> coerce t (\s -> ...) }
      = do { (floats, e') <- rhsToBody e
           ; return (floats, Cast e' co) }

    rhsToBody expr@(Lam {})
      | Just no_lam_result <- tryEtaReducePrep bndrs body
      = return (emptyFloats, no_lam_result)
      | all isTyVar bndrs           -- Type lambdas are ok
      = return (emptyFloats, expr)
      | otherwise                   -- Some value lambdas
      = do { fn <- newVar (exprType expr)
           ; let rhs   = cpeEtaExpand (exprArity expr) expr
                 float = FloatLet (NonRec fn rhs)
           ; return (unitFloat float, Var fn) }
      where
        (bndrs,body) = collectBinders expr

    rhsToBody expr = return (emptyFloats, expr)

    -- ---------------------------------------------------------------------------
    --              CpeApp: produces a result satisfying CpeApp
    -- ---------------------------------------------------------------------------

    cpeApp :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeRhs)
    -- May return a CpeRhs because of saturating primops
    cpeApp env expr
      = do { (app, (head,depth), _, floats, ss) <- collect_args expr 0
           ; MASSERT(null ss)       -- make sure we used all the strictness info

            -- Now deal with the function
           ; case head of
               Var fn_id -> do { sat_app <- maybeSaturate fn_id app depth
                               ; return (floats, sat_app) }
               _other    -> return (floats, app) }

      where
        -- Deconstruct and rebuild the application, floating any non-atomic
        -- arguments to the outside.  We collect the type of the expression,
        -- the head of the application, and the number of actual value arguments,
        -- all of which are used to possibly saturate this application if it
        -- has a constructor or primop at the head.

        collect_args
            :: CoreExpr
            -> Int                     -- Current app depth
            -> UniqSM (CpeApp,         -- The rebuilt expression
                       (CoreExpr,Int), -- The head of the application,
                                       -- and no. of args it was applied to
                       Type,           -- Type of the whole expr
                       Floats,         -- Any floats we pulled out
                       [Demand])       -- Remaining argument demands

        collect_args (App fun arg@(Type arg_ty)) depth
          = do { (fun',hd,fun_ty,floats,ss) <- collect_args fun depth
               ; return (App fun' arg, hd, piResultTy fun_ty arg_ty, floats, ss) }

        collect_args (App fun arg@(Coercion {})) depth
          = do { (fun',hd,fun_ty,floats,ss) <- collect_args fun depth
               ; return (App fun' arg, hd, funResultTy fun_ty, floats, ss) }

        collect_args (App fun arg) depth
          = do { (fun',hd,fun_ty,floats,ss) <- collect_args fun (depth+1)
               ; let
                  (ss1, ss_rest)   = case ss of
                                       (ss1:ss_rest)             -> (ss1,     ss_rest)
                                       []                        -> (topDmd, [])
                  (arg_ty, res_ty) = expectJust "cpeBody:collect_args" $
                                     splitFunTy_maybe fun_ty

               ; (fs, arg') <- cpeArg env ss1 arg arg_ty
               ; return (App fun' arg', hd, res_ty, fs `appendFloats` floats, ss_rest) }

        collect_args (Var v) depth
          = do { v1 <- fiddleCCall v
               ; let v2 = lookupCorePrepEnv env v1
               ; return (Var v2, (Var v2, depth), idType v2, emptyFloats, stricts) }
            where
              stricts = case idStrictness v of
                                StrictSig (DmdType _ demands _)
                                  | listLengthCmp demands depth /= GT -> demands
                                        -- length demands <= depth
                                  | otherwise                         -> []
                    -- If depth < length demands, then we have too few args to
                    -- satisfy strictness  info so we have to  ignore all the
                    -- strictness info, e.g. + (error "urk")
                    -- Here, we can't evaluate the arg strictly, because this
                    -- partial application might be seq'd

        collect_args (Cast fun co) depth
          = do { let Pair _ty1 ty2 = coercionKind co
               ; (fun', hd, _, floats, ss) <- collect_args fun depth
               ; return (Cast fun' co, hd, ty2, floats, ss) }

        collect_args (Tick tickish fun) depth
          | tickishPlace tickish == PlaceNonLam
            && tickish `tickishScopesLike` SoftScope
          = do { (fun',hd,fun_ty,floats,ss) <- collect_args fun depth
                 -- See [Floating Ticks in CorePrep]
               ; return (fun',hd,fun_ty,addFloat floats (FloatTick tickish),ss) }

            -- N-variable fun, better let-bind it
        collect_args fun depth
          = do { (fun_floats, fun') <- cpeArg env evalDmd fun ty
                              -- The evalDmd says that it's sure to be evaluated,
                              -- so we'll end up case-binding it
               ; return (fun', (fun', depth), ty, fun_floats, []) }
            where
              ty = exprType fun

    -- ---------------------------------------------------------------------------
    --      CpeArg: produces a result satisfying CpeArg
    -- ---------------------------------------------------------------------------

    -- This is where we arrange that a non-trivial argument is let-bound
    cpeArg :: CorePrepEnv -> Demand
           -> CoreArg -> Type -> UniqSM (Floats, CpeTriv)
    cpeArg env dmd arg arg_ty
      = do { (floats1, arg1) <- cpeRhsE env arg     -- arg1 can be a lambda
           ; (floats2, arg2) <- if want_float floats1 arg1
                                then return (floats1, arg1)
                                else do { body1 <- rhsToBodyNF arg1
                                        ; return (emptyFloats, wrapBinds floats1 body1) }
                    -- Else case: arg1 might have lambdas, and we can't
                    --            put them inside a wrapBinds

           ; if cpe_ExprIsTrivial arg2    -- Do not eta expand a trivial argument
             then return (floats2, arg2)
             else do
           { v <- newVar arg_ty
           ; let arg3      = cpeEtaExpand (exprArity arg2) arg2
                 arg_float = mkFloat dmd is_unlifted v arg3
           ; return (addFloat floats2 arg_float, varToCoreExpr v) } }
      where
        is_unlifted = isUnLiftedType arg_ty
        is_strict   = isStrictDmd dmd
        want_float  = wantFloatNested NonRecursive (is_strict || is_unlifted)

    {-
    Note [Floating unlifted arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider    C (let v* = expensive in v)

    where the "*" indicates "will be demanded".  Usually v will have been
    inlined by now, but let's suppose it hasn't (see Trac #2756).  Then we
    do *not* want to get

         let v* = expensive in C v

    because that has different strictness.  Hence the use of 'allLazy'.
    (NB: the let v* turns into a FloatCase, in mkLocalNonRec.)

    ------------------------------------------------------------------------------
    -- Building the saturated syntax
    -- ---------------------------------------------------------------------------

    maybeSaturate deals with saturating primops and constructors
    The type is the type of the entire application
    -}

    maybeSaturate :: Id -> CpeApp -> Int -> UniqSM CpeRhs
    maybeSaturate fn expr n_args
      | Just DataToTagOp <- isPrimOpId_maybe fn     -- DataToTag must have an evaluated arg
                                                    -- A gruesome special case
      = saturateDataToTag sat_expr

      | hasNoBinding fn        -- There's no binding
      = return sat_expr

      | otherwise
      = return expr
      where
        fn_arity     = idArity fn
        excess_arity = fn_arity - n_args
        sat_expr     = cpeEtaExpand excess_arity expr

    -------------
    saturateDataToTag :: CpeApp -> UniqSM CpeApp
    -- See Note [dataToTag magic]
    saturateDataToTag sat_expr
      = do { let (eta_bndrs, eta_body) = collectBinders sat_expr
           ; eta_body' <- eval_data2tag_arg eta_body
           ; return (mkLams eta_bndrs eta_body') }
      where
        eval_data2tag_arg :: CpeApp -> UniqSM CpeBody
        eval_data2tag_arg app@(fun `App` arg)
            | exprIsHNF arg         -- Includes nullary constructors
            = return app            -- The arg is evaluated
            | otherwise                     -- Arg not evaluated, so evaluate it
            = do { arg_id <- newVar (exprType arg)
                 ; let arg_id1 = setIdUnfolding arg_id evaldUnfolding
                 ; return (Case arg arg_id1 (exprType app)
                                [(DEFAULT, [], fun `App` Var arg_id1)]) }

        eval_data2tag_arg (Tick t app)    -- Scc notes can appear
            = do { app' <- eval_data2tag_arg app
                 ; return (Tick t app') }

        eval_data2tag_arg other     -- Should not happen
            = pprPanic "eval_data2tag" (ppr other)

    {-
    Note [dataToTag magic]
    ~~~~~~~~~~~~~~~~~~~~~~
    Horrid: we must ensure that the arg of data2TagOp is evaluated
      (data2tag x) -->  (case x of y -> data2tag y)
    (yuk yuk) take into account the lambdas we've now introduced

    How might it not be evaluated?  Well, we might have floated it out
    of the scope of a `seq`, or dropped the `seq` altogether.

    ************************************************************************
    *                                                                      *
                    Simple CoreSyn operations
    *                                                                      *
    ************************************************************************
    -}

    cpe_ExprIsTrivial :: CoreExpr -> Bool
    -- Version that doesn't consider an scc annotation to be trivial.
    cpe_ExprIsTrivial (Var _)         = True
    cpe_ExprIsTrivial (Type _)        = True
    cpe_ExprIsTrivial (Coercion _)    = True
    cpe_ExprIsTrivial (Lit _)         = True
    cpe_ExprIsTrivial (App e arg)     = not (isRuntimeArg arg) && cpe_ExprIsTrivial e
    cpe_ExprIsTrivial (Lam b e)       = not (isRuntimeVar b) && cpe_ExprIsTrivial e
    cpe_ExprIsTrivial (Tick t e)      = not (tickishIsCode t) && cpe_ExprIsTrivial e
    cpe_ExprIsTrivial (Cast e _)      = cpe_ExprIsTrivial e
    cpe_ExprIsTrivial (Case e _ _ []) = cpe_ExprIsTrivial e
                                        -- See Note [Empty case is trivial] in CoreUtils
    cpe_ExprIsTrivial _               = False

    {-
    -- -----------------------------------------------------------------------------
    --      Eta reduction
    -- -----------------------------------------------------------------------------

    Note [Eta expansion]
    ~~~~~~~~~~~~~~~~~~~~~
    Eta expand to match the arity claimed by the binder Remember,
    CorePrep must not change arity

    Eta expansion might not have happened already, because it is done by
    the simplifier only when there at least one lambda already.

    NB1:we could refrain when the RHS is trivial (which can happen
        for exported things).  This would reduce the amount of code
        generated (a little) and make things a little words for
        code compiled without -O.  The case in point is data constructor
        wrappers.

    NB2: we have to be careful that the result of etaExpand doesn't
       invalidate any of the assumptions that CorePrep is attempting
       to establish.  One possible cause is eta expanding inside of
       an SCC note - we're now careful in etaExpand to make sure the
       SCC is pushed inside any new lambdas that are generated.

    Note [Eta expansion and the CorePrep invariants]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It turns out to be much much easier to do eta expansion
    *after* the main CorePrep stuff.  But that places constraints
    on the eta expander: given a CpeRhs, it must return a CpeRhs.

    For example here is what we do not want:
                    f = /\a -> g (h 3)      -- h has arity 2
    After ANFing we get
                    f = /\a -> let s = h 3 in g s
    and now we do NOT want eta expansion to give
                    f = /\a -> \ y -> (let s = h 3 in g s) y

    Instead CoreArity.etaExpand gives
                    f = /\a -> \y -> let s = h 3 in g s y
    -}

    cpeEtaExpand :: Arity -> CpeRhs -> CpeRhs
    cpeEtaExpand arity expr
      | arity == 0 = expr
      | otherwise  = etaExpand arity expr

    {-
    -- -----------------------------------------------------------------------------
    --      Eta reduction
    -- -----------------------------------------------------------------------------

    Why try eta reduction?  Hasn't the simplifier already done eta?
    But the simplifier only eta reduces if that leaves something
    trivial (like f, or f Int).  But for deLam it would be enough to
    get to a partial application:
            case x of { p -> \xs. map f xs }
        ==> case x of { p -> map f }
    -}

    tryEtaReducePrep :: [CoreBndr] -> CoreExpr -> Maybe CoreExpr
    tryEtaReducePrep bndrs expr@(App _ _)
      | ok_to_eta_reduce f
      , n_remaining >= 0
      , and (zipWith ok bndrs last_args)
      , not (any (`elemVarSet` fvs_remaining) bndrs)
      , exprIsHNF remaining_expr   -- Don't turn value into a non-value
                                   -- else the behaviour with 'seq' changes
      = Just remaining_expr
      where
        (f, args) = collectArgs expr
        remaining_expr = mkApps f remaining_args
        fvs_remaining = exprFreeVars remaining_expr
        (remaining_args, last_args) = splitAt n_remaining args
        n_remaining = length args - length bndrs

        ok bndr (Var arg) = bndr == arg
        ok _    _         = False

              -- We can't eta reduce something which must be saturated.
        ok_to_eta_reduce (Var f) = not (hasNoBinding f)
        ok_to_eta_reduce _       = False -- Safe. ToDo: generalise

    tryEtaReducePrep bndrs (Let bind@(NonRec _ r) body)
      | not (any (`elemVarSet` fvs) bndrs)
      = case tryEtaReducePrep bndrs body of
            Just e -> Just (Let bind e)
            Nothing -> Nothing
      where
        fvs = exprFreeVars r

    tryEtaReducePrep bndrs (Tick tickish e)
      = fmap (mkTick tickish) $ tryEtaReducePrep bndrs e

    tryEtaReducePrep _ _ = Nothing

    {-
    ************************************************************************
    *                                                                      *
                    Floats
    *                                                                      *
    ************************************************************************

    Note [Pin demand info on floats]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We pin demand info on floated lets so that we can see the one-shot thunks.
    -}

    data FloatingBind
      = FloatLet CoreBind    -- Rhs of bindings are CpeRhss
                             -- They are always of lifted type;
                             -- unlifted ones are done with FloatCase

     | FloatCase
          Id CpeBody
          Bool              -- The bool indicates "ok-for-speculation"

     -- | See Note [Floating Ticks in CorePrep]
     | FloatTick (Tickish Id)

    data Floats = Floats OkToSpec (OrdList FloatingBind)

    instance Outputable FloatingBind where
      ppr (FloatLet b) = ppr b
      ppr (FloatCase b r ok) = brackets (ppr ok) <+> ppr b <+> equals <+> ppr r
      ppr (FloatTick t) = ppr t

    instance Outputable Floats where
      ppr (Floats flag fs) = text "Floats" <> brackets (ppr flag) <+>
                             braces (vcat (map ppr (fromOL fs)))

    instance Outputable OkToSpec where
      ppr OkToSpec    = text "OkToSpec"
      ppr IfUnboxedOk = text "IfUnboxedOk"
      ppr NotOkToSpec = text "NotOkToSpec"

    -- Can we float these binds out of the rhs of a let?  We cache this decision
    -- to avoid having to recompute it in a non-linear way when there are
    -- deeply nested lets.
    data OkToSpec
       = OkToSpec           -- Lazy bindings of lifted type
       | IfUnboxedOk        -- A mixture of lazy lifted bindings and n
                            -- ok-to-speculate unlifted bindings
       | NotOkToSpec        -- Some not-ok-to-speculate unlifted bindings

    mkFloat :: Demand -> Bool -> Id -> CpeRhs -> FloatingBind
    mkFloat dmd is_unlifted bndr rhs
      | use_case  = FloatCase bndr rhs (exprOkForSpeculation rhs)
      | is_hnf    = FloatLet (NonRec bndr                       rhs)
      | otherwise = FloatLet (NonRec (setIdDemandInfo bndr dmd) rhs)
                       -- See Note [Pin demand info on floats]
      where
        is_hnf    = exprIsHNF rhs
        is_strict = isStrictDmd dmd
        use_case  = is_unlifted || is_strict && not is_hnf
                    -- Don't make a case for a value binding,
                    -- even if it's strict.  Otherwise we get
                    --      case (\x -> e) of ...!

    emptyFloats :: Floats
    emptyFloats = Floats OkToSpec nilOL

    isEmptyFloats :: Floats -> Bool
    isEmptyFloats (Floats _ bs) = isNilOL bs

    wrapBinds :: Floats -> CpeBody -> CpeBody
    wrapBinds (Floats _ binds) body
      = foldrOL mk_bind body binds
      where
        mk_bind (FloatCase bndr rhs _) body = Case rhs bndr (exprType body) [(DEFAULT, [], body)]
        mk_bind (FloatLet bind)        body = Let bind body
        mk_bind (FloatTick tickish)    body = mkTick tickish body

    addFloat :: Floats -> FloatingBind -> Floats
    addFloat (Floats ok_to_spec floats) new_float
      = Floats (combine ok_to_spec (check new_float)) (floats `snocOL` new_float)
      where
        check (FloatLet _) = OkToSpec
        check (FloatCase _ _ ok_for_spec)
            | ok_for_spec  =  IfUnboxedOk
            | otherwise    =  NotOkToSpec
        check FloatTick{}  = OkToSpec
            -- The ok-for-speculation flag says that it's safe to
            -- float this Case out of a let, and thereby do it more eagerly
            -- We need the top-level flag because it's never ok to float
            -- an unboxed binding to the top level

    unitFloat :: FloatingBind -> Floats
    unitFloat = addFloat emptyFloats

    appendFloats :: Floats -> Floats -> Floats
    appendFloats (Floats spec1 floats1) (Floats spec2 floats2)
      = Floats (combine spec1 spec2) (floats1 `appOL` floats2)

    concatFloats :: [Floats] -> OrdList FloatingBind
    concatFloats = foldr (\ (Floats _ bs1) bs2 -> appOL bs1 bs2) nilOL

    combine :: OkToSpec -> OkToSpec -> OkToSpec
    combine NotOkToSpec _ = NotOkToSpec
    combine _ NotOkToSpec = NotOkToSpec
    combine IfUnboxedOk _ = IfUnboxedOk
    combine _ IfUnboxedOk = IfUnboxedOk
    combine _ _           = OkToSpec

    deFloatTop :: Floats -> [CoreBind]
    -- For top level only; we don't expect any FloatCases
    deFloatTop (Floats _ floats)
      = foldrOL get [] floats
      where
        get (FloatLet b) bs = occurAnalyseRHSs b : bs
        get b            _  = pprPanic "corePrepPgm" (ppr b)

        -- See Note [Dead code in CorePrep]
        occurAnalyseRHSs (NonRec x e) = NonRec x (occurAnalyseExpr_NoBinderSwap e)
        occurAnalyseRHSs (Rec xes)    = Rec [(x, occurAnalyseExpr_NoBinderSwap e) | (x, e) <- xes]

    ---------------------------------------------------------------------------

    canFloatFromNoCaf :: Platform -> Floats -> CpeRhs -> Maybe (Floats, CpeRhs)
           -- Note [CafInfo and floating]
    canFloatFromNoCaf platform (Floats ok_to_spec fs) rhs
      | OkToSpec <- ok_to_spec           -- Worth trying
      , Just (subst, fs') <- go (emptySubst, nilOL) (fromOL fs)
      = Just (Floats OkToSpec fs', subst_expr subst rhs)
      | otherwise
      = Nothing
      where
        subst_expr = substExpr (text "CorePrep")

        go :: (Subst, OrdList FloatingBind) -> [FloatingBind]
           -> Maybe (Subst, OrdList FloatingBind)

        go (subst, fbs_out) [] = Just (subst, fbs_out)

        go (subst, fbs_out) (FloatLet (NonRec b r) : fbs_in)
          | rhs_ok r
          = go (subst', fbs_out `snocOL` new_fb) fbs_in
          where
            (subst', b') = set_nocaf_bndr subst b
            new_fb = FloatLet (NonRec b' (subst_expr subst r))

        go (subst, fbs_out) (FloatLet (Rec prs) : fbs_in)
          | all rhs_ok rs
          = go (subst', fbs_out `snocOL` new_fb) fbs_in
          where
            (bs,rs) = unzip prs
            (subst', bs') = mapAccumL set_nocaf_bndr subst bs
            rs' = map (subst_expr subst') rs
            new_fb = FloatLet (Rec (bs' `zip` rs'))

        go (subst, fbs_out) (ft@FloatTick{} : fbs_in)
          = go (subst, fbs_out `snocOL` ft) fbs_in

        go _ _ = Nothing      -- Encountered a caffy binding

        ------------
        set_nocaf_bndr subst bndr
          = (extendIdSubst subst bndr (Var bndr'), bndr')
          where
            bndr' = bndr `setIdCafInfo` NoCafRefs

        ------------
        rhs_ok :: CoreExpr -> Bool
        -- We can only float to top level from a NoCaf thing if
        -- the new binding is static. However it can't mention
        -- any non-static things or it would *already* be Caffy
        rhs_ok = rhsIsStatic platform (\_ -> False)
                             (\i -> pprPanic "rhsIsStatic" (integer i))
                             -- Integer literals should not show up

    wantFloatNested :: RecFlag -> Bool -> Floats -> CpeRhs -> Bool
    wantFloatNested is_rec strict_or_unlifted floats rhs
      =  isEmptyFloats floats
      || strict_or_unlifted
      || (allLazyNested is_rec floats && exprIsHNF rhs)
            -- Why the test for allLazyNested?
            --      v = f (x `divInt#` y)
            -- we don't want to float the case, even if f has arity 2,
            -- because floating the case would make it evaluated too early

    allLazyTop :: Floats -> Bool
    allLazyTop (Floats OkToSpec _) = True
    allLazyTop _                   = False

    allLazyNested :: RecFlag -> Floats -> Bool
    allLazyNested _      (Floats OkToSpec    _) = True
    allLazyNested _      (Floats NotOkToSpec _) = False
    allLazyNested is_rec (Floats IfUnboxedOk _) = isNonRec is_rec

    {-
    ************************************************************************
    *                                                                      *
                    Cloning
    *                                                                      *
    ************************************************************************
    -}

    -- ---------------------------------------------------------------------------
    --                      The environment
    -- ---------------------------------------------------------------------------

    data CorePrepEnv
      = CPE { cpe_dynFlags        :: DynFlags
            , cpe_env             :: IdEnv Id   -- Clone local Ids
            , cpe_mkIntegerId     :: Id
            , cpe_integerSDataCon :: Maybe DataCon
        }

    lookupMkIntegerName :: DynFlags -> HscEnv -> IO Id
    lookupMkIntegerName dflags hsc_env
        = guardIntegerUse dflags $ liftM tyThingId $
          lookupGlobal hsc_env mkIntegerName

    lookupIntegerSDataConName :: DynFlags -> HscEnv -> IO (Maybe DataCon)
    lookupIntegerSDataConName dflags hsc_env = case cIntegerLibraryType of
        IntegerGMP -> guardIntegerUse dflags $ liftM (Just . tyThingDataCon) $
                      lookupGlobal hsc_env integerSDataConName
        IntegerSimple -> return Nothing

    -- | Helper for 'lookupMkIntegerName' and 'lookupIntegerSDataConName'
    guardIntegerUse :: DynFlags -> IO a -> IO a
    guardIntegerUse dflags act
      | thisPackage dflags == primUnitId
      = return $ panic "Can't use Integer in ghc-prim"
      | thisPackage dflags == integerUnitId
      = return $ panic "Can't use Integer in integer-*"
      | otherwise = act

    mkInitialCorePrepEnv :: DynFlags -> HscEnv -> IO CorePrepEnv
    mkInitialCorePrepEnv dflags hsc_env
        = do mkIntegerId <- lookupMkIntegerName dflags hsc_env
             integerSDataCon <- lookupIntegerSDataConName dflags hsc_env
             return $ CPE {
                          cpe_dynFlags = dflags,
                          cpe_env = emptyVarEnv,
                          cpe_mkIntegerId = mkIntegerId,
                          cpe_integerSDataCon = integerSDataCon
                      }

    extendCorePrepEnv :: CorePrepEnv -> Id -> Id -> CorePrepEnv
    extendCorePrepEnv cpe id id'
        = cpe { cpe_env = extendVarEnv (cpe_env cpe) id id' }

    extendCorePrepEnvList :: CorePrepEnv -> [(Id,Id)] -> CorePrepEnv
    extendCorePrepEnvList cpe prs
        = cpe { cpe_env = extendVarEnvList (cpe_env cpe) prs }

    lookupCorePrepEnv :: CorePrepEnv -> Id -> Id
    lookupCorePrepEnv cpe id
      = case lookupVarEnv (cpe_env cpe) id of
            Nothing  -> id
            Just id' -> id'

    getMkIntegerId :: CorePrepEnv -> Id
    getMkIntegerId = cpe_mkIntegerId

    ------------------------------------------------------------------------------
    -- Cloning binders
    -- ---------------------------------------------------------------------------

    cpCloneBndrs :: CorePrepEnv -> [Var] -> UniqSM (CorePrepEnv, [Var])
    cpCloneBndrs env bs = mapAccumLM cpCloneBndr env bs

    cpCloneBndr  :: CorePrepEnv -> Var -> UniqSM (CorePrepEnv, Var)
    cpCloneBndr env bndr
      | isLocalId bndr, not (isCoVar bndr)
      = do bndr' <- setVarUnique bndr <$> getUniqueM

           -- We are going to OccAnal soon, so drop (now-useless) rules/unfoldings
           -- so that we can drop more stuff as dead code.
           -- See also Note [Dead code in CorePrep]
           let bndr'' = bndr' `setIdUnfolding` noUnfolding
                              `setIdSpecialisation` emptyRuleInfo
           return (extendCorePrepEnv env bndr bndr'', bndr'')

      | otherwise   -- Top level things, which we don't want
                    -- to clone, have become GlobalIds by now
                    -- And we don't clone tyvars, or coercion variables
      = return (env, bndr)

    ------------------------------------------------------------------------------
    -- Cloning ccall Ids; each must have a unique name,
    -- to give the code generator a handle to hang it on
    -- ---------------------------------------------------------------------------

    fiddleCCall :: Id -> UniqSM Id
    fiddleCCall id
      | isFCallId id = (id `setVarUnique`) <$> getUniqueM
      | otherwise    = return id

    ------------------------------------------------------------------------------
    -- Generating new binders
    -- ---------------------------------------------------------------------------

    newVar :: Type -> UniqSM Id
    newVar ty
     = seqType ty `seq` do
         uniq <- getUniqueM
         return (mkSysLocalOrCoVar (fsLit "sat") uniq ty)

    ------------------------------------------------------------------------------
    -- Floating ticks
    -- ---------------------------------------------------------------------------
    --
    -- Note [Floating Ticks in CorePrep]
    --
    -- It might seem counter-intuitive to float ticks by default, given
    -- that we don't actually want to move them if we can help it. On the
    -- other hand, nothing gets very far in CorePrep anyway, and we want
    -- to preserve the order of let bindings and tick annotations in
    -- relation to each other. For example, if we just wrapped let floats
    -- when they pass through ticks, we might end up performing the
    -- following transformation:
    --
    --   src<...> let foo = bar in baz
    --   ==>  let foo = src<...> bar in src<...> baz
    --
    -- Because the let-binding would float through the tick, and then
    -- immediately materialize, achieving nothing but decreasing tick
    -- accuracy. The only special case is the following scenario:
    --
    --   let foo = src<...> (let a = b in bar) in baz
    --   ==>  let foo = src<...> bar; a = src<...> b in baz
    --
    -- Here we would not want the source tick to end up covering "baz" and
    -- therefore refrain from pushing ticks outside. Instead, we copy them
    -- into the floating binds (here "a") in cpePair. Note that where "b"
    -- or "bar" are (value) lambdas we have to push the annotations
    -- further inside in order to uphold our rules.
    --
    -- All of this is implemented below in @wrapTicks@.

    -- | Like wrapFloats, but only wraps tick floats
    wrapTicks :: Floats -> CoreExpr -> (Floats, CoreExpr)
    wrapTicks (Floats flag floats0) expr = (Floats flag floats1, expr')
      where (floats1, expr') = foldrOL go (nilOL, expr) floats0
            go (FloatTick t) (fs, e) = ASSERT(tickishPlace t == PlaceNonLam)
                                       (mapOL (wrap t) fs, mkTick t e)
            go other         (fs, e) = (other `consOL` fs, e)
            wrap t (FloatLet bind)    = FloatLet (wrapBind t bind)
            wrap t (FloatCase b r ok) = FloatCase b (mkTick t r) ok
            wrap _ other              = pprPanic "wrapTicks: unexpected float!"
                                                 (ppr other)
            wrapBind t (NonRec binder rhs) = NonRec binder (mkTick t rhs)
            wrapBind t (Rec pairs)         = Rec (mapSnd (mkTick t) pairs)