    {-
    (c) The GRASP/AQUA Project, Glasgow University, 1993-1998

    \section[WorkWrap]{Worker/wrapper-generating back-end of strictness analyser}
    -}

    module GHC.Core.Opt.WorkWrap
     ( WwOpts (..)
     , wwTopBinds
     )
    where

    import GHC.Prelude

    import GHC.Core
    import GHC.Core.Unfold.Make
    import GHC.Core.Utils  ( exprType, exprIsHNF )
    import GHC.Core.Type
    import GHC.Core.Opt.WorkWrap.Utils
    import GHC.Core.SimpleOpt

    import GHC.Data.FastString

    import GHC.Types.Var
    import GHC.Types.Id
    import GHC.Types.Id.Info
    import GHC.Types.Unique.Supply
    import GHC.Types.Basic
    import GHC.Types.Demand
    import GHC.Types.Cpr
    import GHC.Types.SourceText
    import GHC.Types.Unique

    import GHC.Utils.Misc
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Utils.Monad
    import GHC.Core.DataCon

    {-
    We take Core bindings whose binders have:

    \begin{enumerate}

    \item Strictness attached (by the front-end of the strictness
    analyser), and / or

    \item Constructed Product Result information attached by the CPR
    analysis pass.

    \end{enumerate}

    and we return some ``plain'' bindings which have been
    worker/wrapper-ified, meaning:

    \begin{enumerate}

    \item Functions have been split into workers and wrappers where
    appropriate.  If a function has both strictness and CPR properties
    then only one worker/wrapper doing both transformations is produced;

    \item Binders' @IdInfos@ have been updated to reflect the existence of
    these workers/wrappers (this is where we get STRICTNESS and CPR pragma
    info for exported values).
    \end{enumerate}
    -}

    wwTopBinds :: WwOpts -> UniqSupply -> CoreProgram -> CoreProgram

    wwTopBinds :: WwOpts -> UniqSupply -> CoreProgram -> CoreProgram
    wwTopBinds WwOpts
    ww_opts UniqSupply
    us CoreProgram
    top_binds
      = UniqSupply -> UniqSM CoreProgram -> CoreProgram
    forall a. UniqSupply -> UniqSM a -> a
    initUs_ UniqSupply
    us (UniqSM CoreProgram -> CoreProgram)
    -> UniqSM CoreProgram -> CoreProgram
    forall a b. (a -> b) -> a -> b
    $ (CoreBind -> UniqSM CoreProgram)
    -> CoreProgram -> UniqSM CoreProgram
    forall (m :: * -> *) (f :: * -> *) a b.
    (Monad m, Traversable f) =>
    (a -> m [b]) -> f a -> m [b]
    concatMapM (WwOpts -> CoreBind -> UniqSM CoreProgram
    wwBind WwOpts
    ww_opts) CoreProgram
    top_binds

    {-
    ************************************************************************
    *                                                                      *
    \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
    *                                                                      *
    ************************************************************************

    @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
    turn.  Non-recursive case first, then recursive...
    -}

    wwBind  :: WwOpts
            -> CoreBind
            -> UniqSM [CoreBind]    -- returns a WwBinding intermediate form;
                                    -- the caller will convert to Expr/Binding,
                                    -- as appropriate.

    wwBind :: WwOpts -> CoreBind -> UniqSM CoreProgram
    wwBind WwOpts
    ww_opts (NonRec Var
    binder CoreExpr
    rhs) = do
        new_rhs   <- WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    rhs
        new_pairs <- tryWW ww_opts NonRecursive binder new_rhs
        return [NonRec b e | (b,e) <- new_pairs]
          -- Generated bindings must be non-recursive
          -- because the original binding was.

    wwBind WwOpts
    ww_opts (Rec [(Var, CoreExpr)]
    pairs)
      = CoreBind -> CoreProgram
    forall a. a -> [a]
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CoreBind -> CoreProgram)
    -> ([(Var, CoreExpr)] -> CoreBind)
    -> [(Var, CoreExpr)]
    -> CoreProgram
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . [(Var, CoreExpr)] -> CoreBind
    forall b. [(b, Expr b)] -> Bind b
    Rec ([(Var, CoreExpr)] -> CoreProgram)
    -> UniqSM [(Var, CoreExpr)] -> UniqSM CoreProgram
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> ((Var, CoreExpr) -> UniqSM [(Var, CoreExpr)])
    -> [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall (m :: * -> *) (f :: * -> *) a b.
    (Monad m, Traversable f) =>
    (a -> m [b]) -> f a -> m [b]
    concatMapM (Var, CoreExpr) -> UniqSM [(Var, CoreExpr)]
    do_one [(Var, CoreExpr)]
    pairs
      where
        do_one :: (Var, CoreExpr) -> UniqSM [(Var, CoreExpr)]
    do_one (Var
    binder, CoreExpr
    rhs) = do new_rhs <- WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    rhs
                                  tryWW ww_opts Recursive binder new_rhs

    {-
    @wwExpr@ basically just walks the tree, looking for appropriate
    annotations that can be used. Remember it is @wwBind@ that does the
    matching by looking for strict arguments of the correct type.
    @wwExpr@ is a version that just returns the ``Plain'' Tree.
    -}

    wwExpr :: WwOpts -> CoreExpr -> UniqSM CoreExpr

    wwExpr :: WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    _ e :: CoreExpr
    e@(Type {}) = CoreExpr -> UniqSM CoreExpr
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return CoreExpr
    e
    wwExpr WwOpts
    _ e :: CoreExpr
    e@(Coercion {}) = CoreExpr -> UniqSM CoreExpr
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return CoreExpr
    e
    wwExpr WwOpts
    _ e :: CoreExpr
    e@(Lit  {}) = CoreExpr -> UniqSM CoreExpr
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return CoreExpr
    e
    wwExpr WwOpts
    _ e :: CoreExpr
    e@(Var  {}) = CoreExpr -> UniqSM CoreExpr
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return CoreExpr
    e

    wwExpr WwOpts
    ww_opts (Lam Var
    binder CoreExpr
    expr)
      = Var -> CoreExpr -> CoreExpr
    forall b. b -> Expr b -> Expr b
    Lam Var
    new_binder (CoreExpr -> CoreExpr) -> UniqSM CoreExpr -> UniqSM CoreExpr
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    expr
      where new_binder :: Var
    new_binder | Var -> Bool
    isId Var
    binder = Var -> Var
    zapIdUsedOnceInfo Var
    binder
                       | Bool
    otherwise   = Var
    binder
      -- See Note [Zapping Used Once info in WorkWrap]

    wwExpr WwOpts
    ww_opts (App CoreExpr
    f CoreExpr
    a)
      = CoreExpr -> CoreExpr -> CoreExpr
    forall b. Expr b -> Expr b -> Expr b
    App (CoreExpr -> CoreExpr -> CoreExpr)
    -> UniqSM CoreExpr -> UniqSM (CoreExpr -> CoreExpr)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    f UniqSM (CoreExpr -> CoreExpr) -> UniqSM CoreExpr -> UniqSM CoreExpr
    forall a b. UniqSM (a -> b) -> UniqSM a -> UniqSM b
    forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
    <*> WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    a

    wwExpr WwOpts
    ww_opts (Tick CoreTickish
    note CoreExpr
    expr)
      = CoreTickish -> CoreExpr -> CoreExpr
    forall b. CoreTickish -> Expr b -> Expr b
    Tick CoreTickish
    note (CoreExpr -> CoreExpr) -> UniqSM CoreExpr -> UniqSM CoreExpr
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    expr

    wwExpr WwOpts
    ww_opts (Cast CoreExpr
    expr CoercionR
    co) = do
        new_expr <- WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    expr
        return (Cast new_expr co)

    wwExpr WwOpts
    ww_opts (Let CoreBind
    bind CoreExpr
    expr)
      = CoreProgram -> CoreExpr -> CoreExpr
    forall b. [Bind b] -> Expr b -> Expr b
    mkLets (CoreProgram -> CoreExpr -> CoreExpr)
    -> UniqSM CoreProgram -> UniqSM (CoreExpr -> CoreExpr)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> WwOpts -> CoreBind -> UniqSM CoreProgram
    wwBind WwOpts
    ww_opts CoreBind
    bind UniqSM (CoreExpr -> CoreExpr) -> UniqSM CoreExpr -> UniqSM CoreExpr
    forall a b. UniqSM (a -> b) -> UniqSM a -> UniqSM b
    forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
    <*> WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    expr

    wwExpr WwOpts
    ww_opts (Case CoreExpr
    expr Var
    binder Type
    ty [Alt Var]
    alts) = do
        new_expr <- WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    expr
        new_alts <- mapM ww_alt alts
        let new_binder = Var -> Var
    zapIdUsedOnceInfo Var
    binder
          -- See Note [Zapping Used Once info in WorkWrap]
        return (Case new_expr new_binder ty new_alts)
      where
        ww_alt :: Alt Var -> UniqSM (Alt Var)
    ww_alt (Alt AltCon
    con [Var]
    binders CoreExpr
    rhs) = do
            new_rhs <- WwOpts -> CoreExpr -> UniqSM CoreExpr
    wwExpr WwOpts
    ww_opts CoreExpr
    rhs
            let new_binders = [ if Var -> Bool
    isId Var
    b then Var -> Var
    zapIdUsedOnceInfo Var
    b else Var
    b
                              | Var
    b <- [Var]
    binders ]
               -- See Note [Zapping Used Once info in WorkWrap]
            return (Alt con new_binders new_rhs)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
    *                                                                      *
    ************************************************************************

    @tryWW@ just accumulates arguments, converts strictness info from the
    front-end into the proper form, then calls @mkWwBodies@ to do
    the business.

    The only reason this is monadised is for the unique supply.

    Note [Don't w/w INLINE things]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's very important to refrain from w/w-ing an INLINE function (ie one
    with a stable unfolding) because the wrapper will then overwrite the
    old stable unfolding with the wrapper code.

    Furthermore, if the programmer has marked something as INLINE,
    we may lose by w/w'ing it.

    If the strictness analyser is run twice, this test also prevents
    wrappers (which are INLINEd) from being re-done.  (You can end up with
    several liked-named Ids bouncing around at the same time---absolute
    mischief.)

    Notice that we refrain from w/w'ing an INLINE function even if it is
    in a recursive group.  It might not be the loop breaker.  (We could
    test for loop-breaker-hood, but I'm not sure that ever matters.)

    Note [Worker/wrapper for INLINABLE functions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have
      {-# INLINABLE f #-}
      f :: Ord a => [a] -> Int -> a
      f x y = ....f....

    where f is strict in y, we might get a more efficient loop by w/w'ing
    f.  But that would make a new unfolding which would overwrite the old
    one! So the function would no longer be INLINABLE, and in particular
    will not be specialised at call sites in other modules.

    This comes up in practice (#6056).

    Solution:

    * Do the w/w for strictness analysis, even for INLINABLE functions

    * Transfer the Stable unfolding to the *worker*.  How do we "transfer
      the unfolding"? Easy: by using the old one, wrapped in work_fn! See
      GHC.Core.Unfold.Make.mkWorkerUnfolding.

    * We use the /original, user-specified/ function's InlineSpec pragma
      for both the wrapper and the worker (see `mkStrWrapperInlinePrag`).
      So if f is INLINEABLE, both worker and wrapper will get an InlineSpec
      of (Inlinable "blah").

      It's important that both get this, because the specialiser uses
      the existence of a /user-specified/ INLINE/INLINABLE pragma to
      drive specialisation of imported functions.  See  GHC.Core.Opt.Specialise
      Note [Specialising imported functions]

    * Remember, the subsequent inlining behaviour of the wrapper is expressed by
      (a) the stable unfolding
      (b) the unfolding guidance of UnfWhen
      (c) the inl_act activation (see Note [Wrapper activation]

    For our {-# INLINEABLE f #-} example above, we will get something a
    bit like like this:

      {-# Has stable unfolding, active in phase 2;
          plus InlineSpec = INLINEABLE #-}
      f :: Ord a => [a] -> Int -> a
      f d x y = case y of I# y' -> fw d x y'

      {-# Has stable unfolding, plus InlineSpec = INLINEABLE #-}
      fw :: Ord a => [a] -> Int# -> a
      fw d x y' = let y = I# y' in ...f...

    (Historical note: we used to always give the wrapper an INLINE pragma,
    but CSE will not happen if there is a user-specified pragma, but
    should happen for w/w’ed things (#14186).  But now we simply propagate
    any user-defined pragma info, so we'll defeat CSE (rightly) only when
    there is a user-supplied INLINE/INLINEABLE pragma.)

    Note [No worker/wrapper for record selectors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We sometimes generate a lot of record selectors, and generally the
    don't benefit from worker/wrapper.  Yes, mkWwBodies would find a w/w split,
    but it is then suppressed by the certainlyWillInline test in splitFun.

    The wasted effort in mkWwBodies makes a measurable difference in
    compile time (see MR !2873), so although it's a terribly ad-hoc test,
    we just check here for record selectors, and do a no-op in that case.

    I did look for a generalisation, so that it's not just record
    selectors that benefit.  But you'd need a cheap test for "this
    function will definitely get a w/w split" and that's hard to predict
    in advance...the logic in mkWwBodies is complex. So I've left the
    super-simple test, with this Note to explain.

    NB: record selectors are ordinary functions, inlined iff GHC wants to,
    so won't be caught by the preceding isInlineUnfolding test in tryWW.

    Note [Worker/wrapper for NOINLINE functions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We used to disable worker/wrapper for NOINLINE things, but it turns out
    this can cause unnecessary reboxing of values. Consider

      {-# NOINLINE f #-}
      f :: Int -> a
      f x = error (show x)

      g :: Bool -> Bool -> Int -> Int
      g True  True  p = f p
      g False True  p = p + 1
      g b     False p = g b True p

    the strictness analysis will discover f and g are strict, but because f
    has no wrapper, the worker for g will rebox p. So we get

      $wg x y p# =
        let p = I# p# in  -- Yikes! Reboxing!
        case x of
          False ->
            case y of
              False -> $wg False True p#
              True -> +# p# 1#
          True ->
            case y of
              False -> $wg True True p#
              True -> case f p of { }

      g x y p = case p of (I# p#) -> $wg x y p#

    Now, in this case the reboxing will float into the True branch, and so
    the allocation will only happen on the error path. But it won't float
    inwards if there are multiple branches that call (f p), so the reboxing
    will happen on every call of g. Disaster.

    Solution: do worker/wrapper even on NOINLINE things; but move the
    NOINLINE pragma to the worker.

    (See #13143 for a real-world example.)

    It is crucial that we do this for *all* NOINLINE functions. #10069
    demonstrates what happens when we promise to w/w a (NOINLINE) leaf
    function, but fail to deliver:

      data C = C Int# Int#

      {-# NOINLINE c1 #-}
      c1 :: C -> Int#
      c1 (C _ n) = n

      {-# NOINLINE fc #-}
      fc :: C -> Int#
      fc c = 2 *# c1 c

    Failing to w/w `c1`, but still w/wing `fc` leads to the following code:

      c1 :: C -> Int#
      c1 (C _ n) = n

      $wfc :: Int# -> Int#
      $wfc n = let c = C 0# n in 2 #* c1 c

      fc :: C -> Int#
      fc (C _ n) = $wfc n

    Yikes! The reboxed `C` in `$wfc` can't cancel out, so we are in a bad place.
    This generalises to any function that derives its strictness signature from
    its callees, so we have to make sure that when a function announces particular
    strictness properties, we have to w/w them accordingly, even if it means
    splitting a NOINLINE function.

    Note [Worker activation]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Follows on from Note [Worker/wrapper for INLINABLE functions]

    It is *vital* that if the worker gets an INLINABLE pragma (from the
    original function), then the worker has the same phase activation as
    the wrapper (or later).  That is necessary to allow the wrapper to
    inline into the worker's unfolding: see GHC.Core.Opt.Simplify.Utils
    Note [Simplifying inside stable unfoldings].

    If the original is NOINLINE, it's important that the worker inherits the
    original activation. Consider

      {-# NOINLINE expensive #-}
      expensive x = x + 1

      f y = let z = expensive y in ...

    If expensive's worker inherits the wrapper's activation,
    we'll get this (because of the compromise in point (2) of
    Note [Wrapper activation])

      {-# NOINLINE[Final] $wexpensive #-}
      $wexpensive x = x + 1
      {-# INLINE[Final] expensive #-}
      expensive x = $wexpensive x

      f y = let z = expensive y in ...

    and $wexpensive will be immediately inlined into expensive, followed by
    expensive into f. This effectively removes the original NOINLINE!

    Otherwise, nothing is lost by giving the worker the same activation as the
    wrapper, because the worker won't have any chance of inlining until the
    wrapper does; there's no point in giving it an earlier activation.

    Note [Don't w/w inline small non-loop-breaker things]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general, we refrain from w/w-ing *small* functions, which are not
    loop breakers, because they'll inline anyway.  But we must take care:
    it may look small now, but get to be big later after other inlining
    has happened.  So we take the precaution of adding a StableUnfolding
    for any such functions.

    I made this change when I observed a big function at the end of
    compilation with a useful strictness signature but no w-w.  (It was
    small during demand analysis, we refrained from w/w, and then got big
    when something was inlined in its rhs.) When I measured it on nofib,
    it didn't make much difference; just a few percent improved allocation
    on one benchmark (bspt/Euclid.space).  But nothing got worse.

    There is an infelicity though.  We may get something like
          f = g val
    ==>
          g x = case gw x of r -> I# r

          f {- InlineStable, Template = g val -}
          f = case gw x of r -> I# r

    The code for f duplicates that for g, without any real benefit. It
    won't really be executed, because calls to f will go via the inlining.

    Note [Don't w/w join points for CPR]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There's no point in exploiting CPR info on a join point. If the whole function
    is getting CPR'd, then the case expression around the worker function will get
    pushed into the join point by the simplifier, which will have the same effect
    that w/w'ing for CPR would have - the result will be returned in an unboxed
    tuple.

      f z = let join j x y = (x+1, y+1)
            in case z of A -> j 1 2
                         B -> j 2 3

      =>

      f z = case $wf z of (# a, b #) -> (a, b)
      $wf z = case (let join j x y = (x+1, y+1)
                    in case z of A -> j 1 2
                                 B -> j 2 3) of (a, b) -> (# a, b #)

      =>

      f z = case $wf z of (# a, b #) -> (a, b)
      $wf z = let join j x y = (# x+1, y+1 #)
              in case z of A -> j 1 2
                           B -> j 2 3

    Note that we still want to give `j` the CPR property, so that `f` has it. So
    CPR *analyse* join points as regular functions, but don't *transform* them.

    We could retain the CPR /signature/ on the worker after W/W, but it would
    become outright wrong if the Simplifier pushes a non-trivial continuation
    into it. For example:
        case (let $j x = (x,x) in ...) of alts
        ==>
        let $j x = case (x,x) of alts in case ... of alts
    Before pushing the case in, `$j` has the CPR property, but not afterwards.

    So we simply zap the CPR signature for join pints as part of the W/W pass.
    The signature served its purpose during CPR analysis in propagating the
    CPR property of `$j`.

    Doing W/W for returned products on a join point would be tricky anyway, as the
    worker could not be a join point because it would not be tail-called. However,
    doing the *argument* part of W/W still works for join points, since the wrapper
    body will make a tail call:

      f z = let join j x y = x + y
            in ...

      =>

      f z = let join $wj x# y# = x# +# y#
                     j x y = case x of I# x# ->
                             case y of I# y# ->
                             $wj x# y#
            in ...

    Note [Wrapper activation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    When should the wrapper inlining be active?

    1. It must not be active earlier than the current Activation of the Id,
       because we must give rewrite rules mentioning the wrapper and
       specialisation a chance to fire.
       See Note [Worker/wrapper for INLINABLE functions]
       and Note [Worker activation]

    2. It should be active at some point, despite (1) because of
       Note [Worker/wrapper for NOINLINE functions]

    3. For ordinary functions with no pragmas we want to inline the
       wrapper as early as possible (#15056).  Suppose another module
       defines    f !x xs = ... foldr k z xs ...
       and suppose we have the usual foldr/build RULE.  Then if we have
       a call `f x [1..x]`, we'd expect to inline f and the RULE will fire.
       But if f is w/w'd (which it might be), we want the inlining to
       occur just as if it hadn't been.

       (This only matters if f's RHS is big enough to w/w, but small
       enough to inline given the call site, but that can happen.)

    4. We do not want to inline the wrapper before specialisation.
             module Foo where
               f :: Num a => a -> Int -> a
               f n 0 = n              -- Strict in the Int, hence wrapper
               f n x = f (n+n) (x-1)

               g :: Int -> Int
               g x = f x x            -- Provokes a specialisation for f

             module Bar where
               import Foo

               h :: Int -> Int
               h x = f 3 x

       In module Bar we want to give specialisations a chance to fire
       before inlining f's wrapper.

       (Historical note: At one stage I tried making the wrapper inlining
       always-active, and that had a very bad effect on nofib/imaginary/x2n1;
       a wrapper was inlined before the specialisation fired.)

    4a. If we have
          {-# SPECIALISE foo :: (Int,Int) -> Bool -> Int #-}
          {-# NOINLINE [n] foo #-}
        then specialisation will generate a SPEC rule active from Phase n.
        See Note [Auto-specialisation and RULES] in GHC.Core.Opt.Specialise
        This SPEC specialisation rule will compete with inlining, but we don't
        mind that, because if inlining succeeds, it should be better.

        Now, if we w/w foo, we must ensure that the wrapper (which is very
        keen to inline) has a phase /after/ 'n', else it'll always "win" over
        the SPEC rule -- disaster (#20709).

    Conclusion: the activation for the wrapper should be the /later/ of
      (a) the current activation of the function, or FinalPhase if it is NOINLINE
      (b) one phase /after/ the activation of any rules
    This is implemented by mkStrWrapperInlinePrag.

    Reminder: Note [Don't w/w INLINE things], so we don't need to worry
              about INLINE things here.

    What if `foo` has no specialisations, is worker/wrappered (with the
    wrapper inlining very early), and exported; and then in an importing
    module we have {-# SPECIALISE foo : ... #-}?

    Well then, we'll specialise foo's wrapper, which will expose a
    specialisation for foo's worker, which we will do too.  That seems
    fine.  (To work reliably, `foo` would need an INLINABLE pragma,
    in which case we don't unpack dictionaries for the worker; see
    see Note [Do not unbox class dictionaries].)

    Note [Drop absent bindings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (#19824):
       let t = ...big...
       in ...(f t x)...

    were `f` ignores its first argument.  With luck f's wrapper will inline
    thereby dropping `t`, but maybe not: the arguments to f all look boring.

    So we pre-empt the problem by replacing t's RHS with an absent filler.
    Simple and effective.
    -}

    tryWW   :: WwOpts
            -> RecFlag
            -> Id                           -- The fn binder
            -> CoreExpr                     -- The bound rhs; its innards
                                            --   are already ww'd
            -> UniqSM [(Id, CoreExpr)]      -- either *one* or *two* pairs;
                                            -- if one, then no worker (only
                                            -- the orig "wrapper" lives on);
                                            -- if two, then a worker and a
                                            -- wrapper.
    tryWW :: WwOpts -> RecFlag -> Var -> CoreExpr -> UniqSM [(Var, CoreExpr)]
    tryWW WwOpts
    ww_opts RecFlag
    is_rec Var
    fn_id CoreExpr
    rhs
      -- See Note [Drop absent bindings]
      | Demand -> Bool
    isAbsDmd (IdInfo -> Demand
    demandInfo IdInfo
    fn_info)
      , Bool -> Bool
    not (Var -> Bool
    isJoinId Var
    fn_id)
      , Just CoreExpr
    filler <- WwOpts -> Var -> StrictnessMark -> Maybe CoreExpr
    mkAbsentFiller WwOpts
    ww_opts Var
    fn_id StrictnessMark
    NotMarkedStrict
      = [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [(Var
    new_fn_id, CoreExpr
    filler)]

      -- See Note [Don't w/w INLINE things]
      | IdInfo -> Bool
    hasInlineUnfolding IdInfo
    fn_info
      = [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [(Var
    new_fn_id, CoreExpr
    rhs)]

      -- See Note [No worker/wrapper for record selectors]
      | Var -> Bool
    isRecordSelector Var
    fn_id
      = [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [ (Var
    new_fn_id, CoreExpr
    rhs ) ]

      -- Don't w/w OPAQUE things
      -- See Note [OPAQUE pragma]
      --
      -- Whilst this check might seem superfluous, since we strip boxity
      -- information in GHC.Core.Opt.DmdAnal.finaliseArgBoxities and
      -- CPR information in GHC.Core.Opt.CprAnal.cprAnalBind, it actually
      -- isn't. That is because we would still perform w/w when:
      --
      -- - An argument is used strictly, and -fworker-wrapper-cbv is
      --   enabled, or,
      -- - When demand analysis marks an argument as absent.
      --
      -- In a debug build we do assert that boxity and CPR information
      -- are actually stripped, since we want to prevent callers of OPAQUE
      -- things to do reboxing. See:
      -- - Note [The OPAQUE pragma and avoiding the reboxing of arguments]
      -- - Note [The OPAQUE pragma and avoiding the reboxing of results]
      | InlinePragma -> Bool
    isOpaquePragma (IdInfo -> InlinePragma
    inlinePragInfo IdInfo
    fn_info)
      = Bool
    -> SDoc -> UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (DmdSig -> Bool
    onlyBoxedArguments (IdInfo -> DmdSig
    dmdSigInfo IdInfo
    fn_info) Bool -> Bool -> Bool
    &&
                   CprSig -> Bool
    isTopCprSig (IdInfo -> CprSig
    cprSigInfo IdInfo
    fn_info))
                  (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "OPAQUE fun with boxity" SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$
                   Var -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Var
    new_fn_id SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$
                   DmdSig -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (IdInfo -> DmdSig
    dmdSigInfo IdInfo
    fn_info) SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$
                   CprSig -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (IdInfo -> CprSig
    cprSigInfo IdInfo
    fn_info) SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$
                   CoreExpr -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CoreExpr
    rhs) (UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)])
    -> UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a b. (a -> b) -> a -> b
    $
        [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [ (Var
    new_fn_id, CoreExpr
    rhs) ]

      -- Do this even if there is a NOINLINE pragma
      -- See Note [Worker/wrapper for NOINLINE functions]
      | Bool
    is_fun
      = WwOpts -> Var -> CoreExpr -> UniqSM [(Var, CoreExpr)]
    splitFun WwOpts
    ww_opts Var
    new_fn_id CoreExpr
    rhs

      -- See Note [Thunk splitting]
      | RecFlag -> Bool
    isNonRec RecFlag
    is_rec, Bool
    is_thunk
      = WwOpts -> RecFlag -> Var -> CoreExpr -> UniqSM [(Var, CoreExpr)]
    splitThunk WwOpts
    ww_opts RecFlag
    is_rec Var
    new_fn_id CoreExpr
    rhs

      | Bool
    otherwise
      = [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [ (Var
    new_fn_id, CoreExpr
    rhs) ]

      where
        fn_info :: IdInfo
    fn_info        = HasDebugCallStack => Var -> IdInfo
    Var -> IdInfo
    idInfo Var
    fn_id
        ([Demand]
    wrap_dmds, Divergence
    _) = DmdSig -> ([Demand], Divergence)
    splitDmdSig (IdInfo -> DmdSig
    dmdSigInfo IdInfo
    fn_info)
        new_fn_id :: Var
    new_fn_id      = Var -> Var
    zap_join_cpr (Var -> Var) -> Var -> Var
    forall a b. (a -> b) -> a -> b
    $ Var -> Var
    zap_usage Var
    fn_id

        zap_usage :: Var -> Var
    zap_usage = Var -> Var
    zapIdUsedOnceInfo (Var -> Var) -> (Var -> Var) -> Var -> Var
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Var -> Var
    zapIdUsageEnvInfo
            -- See Note [Zapping DmdEnv after Demand Analyzer] and
            -- See Note [Zapping Used Once info in WorkWrap]

        zap_join_cpr :: Var -> Var
    zap_join_cpr Var
    id
          | Var -> Bool
    isJoinId Var
    id = Var
    id Var -> CprSig -> Var
    `setIdCprSig` CprSig
    topCprSig
          | Bool
    otherwise   = Var
    id
            -- See Note [Don't w/w join points for CPR]

        is_fun :: Bool
    is_fun     = [Demand] -> Bool
    forall (f :: * -> *) a. Foldable f => f a -> Bool
    notNull [Demand]
    wrap_dmds Bool -> Bool -> Bool
    || Var -> Bool
    isJoinId Var
    fn_id
        is_thunk :: Bool
    is_thunk   = Bool -> Bool
    not Bool
    is_fun Bool -> Bool -> Bool
    && Bool -> Bool
    not (CoreExpr -> Bool
    exprIsHNF CoreExpr
    rhs) Bool -> Bool -> Bool
    && Bool -> Bool
    not (Var -> Bool
    isJoinId Var
    fn_id)
                                Bool -> Bool -> Bool
    && Bool -> Bool
    not (HasDebugCallStack => Type -> Bool
    Type -> Bool
    isUnliftedType (Var -> Type
    idType Var
    fn_id))

    {-
    Note [Zapping DmdEnv after Demand Analyzer]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In the worker-wrapper pass we zap the DmdEnv.  Why?
     (a) it is never used again
     (b) it wastes space
     (c) it becomes incorrect as things are cloned, because
         we don't push the substitution into it

    Why here?
     * Because we don’t want to do it in the Demand Analyzer, as we never know
       there when we are doing the last pass.
     * We want them to be still there at the end of DmdAnal, so that
       -ddump-str-anal contains them.
     * We don’t want a second pass just for that.
     * WorkWrap looks at all bindings anyway.

    We also need to do it in TidyCore.tidyLetBndr to clean up after the
    final, worker/wrapper-less run of the demand analyser (see
    Note [Final Demand Analyser run] in GHC.Core.Opt.DmdAnal).

    Note [Zapping Used Once info in WorkWrap]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    During the work/wrap pass, using zapIdUsedOnceInfo, we zap the "used once" info
    * on every binder (let binders, case binders, lambda binders)
    * in both demands and in strictness signatures
    * recursively

    Why?
     * The simplifier may happen to transform code in a way that invalidates the
       data (see #11731 for an example).
     * It is not used in later passes, up to code generation.

    At first it's hard to see how the simplifier might invalidate it (and
    indeed for a while I thought it couldn't: #19482), but it's not quite
    as simple as I thought.  Consider this:
      {-# STRICTNESS SIG <SP(M,A)> #-}
      f p = let v = case p of (a,b) -> a
            in p `seq` (v,v)

    I think we'll give `f` the strictness signature `<SP(M,A)>`, where the
    `M` says that we'll evaluate the first component of the pair at most
    once.  Why?  Because the RHS of the thunk `v` is evaluated at most
    once.

    But now let's worker/wrapper f:
      {-# STRICTNESS SIG <M> #-}
      $wf p1 = let p2 = absentError "urk" in
               let p = (p1,p2) in
               let v = case p of (a,b) -> a
               in p `seq` (v,v)

    where I've gotten the demand on `p1` by decomposing the P(M,A) argument demand.
    This rapidly simplifies to
      {-# STRICTNESS SIG <M> #-}
      $wf p1 = let v = p1 in
               (v,v)

    and thence to `(p1,p1)` by inlining the trivial let. Now the demand on `p1` should
    not be at most once!!

    Conclusion: used-once info is fragile to simplification, because of
    the non-monotonic behaviour of let's, which turn used-many into
    used-once.  So indeed we should zap this info in worker/wrapper.

    Conclusion: kill it during worker/wrapper, using `zapUsedOnceInfo`.
    Both the *demand signature* of the binder, and the *demand-info* of
    the binder.  Moreover, do so recursively.

    You might wonder: why do we generate used-once info if we then throw
    it away.  The main reason is that we do a final run of the demand analyser,
    immediately before CoreTidy, which is /not/ followed by worker/wrapper; it
    is there only to generate used-once info for single-entry thunks.

    Note [Don't eta expand in w/w]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A binding where the manifestArity of the RHS is less than idArity of
    the binder means GHC.Core.Opt.Arity didn't eta expand that binding
    When this happens, it does so for a reason (see Note [Arity invariants for bindings]
    in GHC.Core.Opt.Arity) and we probably have a PAP, cast or trivial expression
    as RHS.

    Below is a historical account of what happened when w/w still did eta expansion.
    Nowadays, it doesn't do that, but will simply w/w for the wrong arity, unleashing
    a demand signature meant for e.g. 2 args to be unleashed for e.g. 1 arg
    (manifest arity). That's at least as terrible as doing eta expansion, so don't
    do it.
    ---
    When worker/wrapper did eta expansion, it implictly eta expanded the binding to
    idArity, overriding GHC.Core.Opt.Arity's decision. Other than playing fast and loose with
    divergence, it's also broken for newtypes:

      f = (\xy.blah) |> co
        where
          co :: (Int -> Int -> Char) ~ T

    Then idArity is 2 (despite the type T), and it can have a DmdSig based on a
    threshold of 2. But we can't w/w it without a type error.

    The situation is less grave for PAPs, but the implicit eta expansion caused a
    compiler allocation regression in T15164, where huge recursive instance method
    groups, mostly consisting of PAPs, got w/w'd. This caused great churn in the
    simplifier, when simply waiting for the PAPs to inline arrived at the same
    output program.

    Note there is the worry here that such PAPs and trivial RHSs might not *always*
    be inlined. That would lead to reboxing, because the analysis tacitly assumes
    that we W/W'd for idArity and will propagate analysis information under that
    assumption. So far, this doesn't seem to matter in practice.
    See https://gitlab.haskell.org/ghc/ghc/merge_requests/312#note_192064.

    Note [Inline pragma for certainlyWillInline]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this (#19824 comment on 15 May 21):
      f _ (x,y) = ...big...
      v = ...big...
      g x = f v x + 1

    So `f` will generate a worker/wrapper split; and `g` (since it is small)
    will trigger the certainlyWillInline case of splitFun.  The danger is that
    we end up with
      g {- StableUnfolding = \x -> f v x + 1 -}
        = ...blah...

    Since (a) that unfolding for g is AlwaysActive
          (b) the unfolding for f's wrapper is ActiveAfterInitial
    the call of f will never inline in g's stable unfolding, thereby
    keeping `v` alive.

    I thought of changing g's unfolding to be ActiveAfterInitial, but that
    too is bad: it delays g's inlining into other modules, which makes fewer
    specialisations happen. Example in perf/should_run/DeriveNull.

    So I decided to live with the problem.  In fact v's RHS will be replaced
    by LitRubbish (see Note [Drop absent bindings]) so there is no great harm.
    -}

    ---------------------
    splitFun :: WwOpts -> Id -> CoreExpr -> UniqSM [(Id, CoreExpr)]
    splitFun :: WwOpts -> Var -> CoreExpr -> UniqSM [(Var, CoreExpr)]
    splitFun WwOpts
    ww_opts Var
    fn_id CoreExpr
    rhs
      | Just ([Var]
    arg_vars, CoreExpr
    body) <- Arity -> CoreExpr -> Maybe ([Var], CoreExpr)
    collectNValBinders_maybe ([Demand] -> Arity
    forall a. [a] -> Arity
    forall (t :: * -> *) a. Foldable t => t a -> Arity
    length [Demand]
    wrap_dmds) CoreExpr
    rhs
      = Bool
    -> String
    -> SDoc
    -> UniqSM [(Var, CoreExpr)]
    -> UniqSM [(Var, CoreExpr)]
    forall a. HasCallStack => Bool -> String -> SDoc -> a -> a
    warnPprTrace (Bool -> Bool
    not ([Demand]
    wrap_dmds [Demand] -> Arity -> Bool
    forall a. [a] -> Arity -> Bool
    `lengthIs` (IdInfo -> Arity
    arityInfo IdInfo
    fn_info)))
                     String
    "splitFun"
                     (Var -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Var
    fn_id SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> ([Demand] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Demand]
    wrap_dmds SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ Cpr -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Cpr
    cpr)) (UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)])
    -> UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a b. (a -> b) -> a -> b
    $
        do { mb_stuff <- WwOpts
    -> Var
    -> [Var]
    -> Type
    -> [Demand]
    -> Cpr
    -> UniqSM (Maybe WwResult)
    mkWwBodies WwOpts
    ww_opts Var
    fn_id [Var]
    arg_vars (HasDebugCallStack => CoreExpr -> Type
    CoreExpr -> Type
    exprType CoreExpr
    body) [Demand]
    wrap_dmds Cpr
    cpr
           ; case mb_stuff of
                Maybe WwResult
    Nothing -> -- No useful wrapper; leave the binding alone
                           [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [(Var
    fn_id, CoreExpr
    rhs)]

                Just WwResult
    stuff
                  | let opt_wwd_rhs :: CoreExpr
    opt_wwd_rhs = HasDebugCallStack => SimpleOpts -> CoreExpr -> CoreExpr
    SimpleOpts -> CoreExpr -> CoreExpr
    simpleOptExpr (WwOpts -> SimpleOpts
    wo_simple_opts WwOpts
    ww_opts) CoreExpr
    rhs
                      -- We need to stabilise the WW'd (and optimised) RHS below
                  , Just Unfolding
    stable_unf <- UnfoldingOpts -> IdInfo -> CoreExpr -> Maybe Unfolding
    certainlyWillInline UnfoldingOpts
    uf_opts IdInfo
    fn_info CoreExpr
    opt_wwd_rhs
                    -- We could make a w/w split, but in fact the RHS is small
                    -- See Note [Don't w/w inline small non-loop-breaker things]
                  , let id_w_unf :: Var
    id_w_unf = Var
    fn_id Var -> Unfolding -> Var
    `setIdUnfolding` Unfolding
    stable_unf
                    -- See Note [Inline pragma for certainlyWillInline]
                  ->  [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [ (Var
    id_w_unf, CoreExpr
    rhs) ]

                  | Bool
    otherwise
                  -> do { work_uniq <- UniqSM Unique
    forall (m :: * -> *). MonadUnique m => m Unique
    getUniqueM
                        ; return (mkWWBindPair ww_opts fn_id fn_info arg_vars body
                                               work_uniq div stuff) } }

      | Bool
    otherwise    -- See Note [Don't eta expand in w/w]
      = [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. a -> UniqSM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [(Var
    fn_id, CoreExpr
    rhs)]

      where
        uf_opts :: UnfoldingOpts
    uf_opts = SimpleOpts -> UnfoldingOpts
    so_uf_opts (WwOpts -> SimpleOpts
    wo_simple_opts WwOpts
    ww_opts)
        fn_info :: IdInfo
    fn_info = HasDebugCallStack => Var -> IdInfo
    Var -> IdInfo
    idInfo Var
    fn_id

        ([Demand]
    wrap_dmds, Divergence
    div) = DmdSig -> ([Demand], Divergence)
    splitDmdSig (IdInfo -> DmdSig
    dmdSigInfo IdInfo
    fn_info)

        cpr_ty :: CprType
    cpr_ty = CprSig -> CprType
    getCprSig (IdInfo -> CprSig
    cprSigInfo IdInfo
    fn_info)
        -- Arity of the CPR sig should match idArity when it's not a join point.
        -- See Note [Arity trimming for CPR signatures] in GHC.Core.Opt.CprAnal
        cpr :: Cpr
    cpr = Bool -> SDoc -> Cpr -> Cpr
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (Var -> Bool
    isJoinId Var
    fn_id Bool -> Bool -> Bool
    || CprType
    cpr_ty CprType -> CprType -> Bool
    forall a. Eq a => a -> a -> Bool
    == CprType
    topCprType Bool -> Bool -> Bool
    || CprType -> Arity
    ct_arty CprType
    cpr_ty Arity -> Arity -> Bool
    forall a. Eq a => a -> a -> Bool
    == IdInfo -> Arity
    arityInfo IdInfo
    fn_info)
                        (Var -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Var
    fn_id SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc
    forall doc. IsLine doc => doc
    colon SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ct_arty:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Arity -> SDoc
    forall doc. IsLine doc => Arity -> doc
    int (CprType -> Arity
    ct_arty CprType
    cpr_ty)
                          SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "arityInfo:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Arity -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (IdInfo -> Arity
    arityInfo IdInfo
    fn_info)) (Cpr -> Cpr) -> Cpr -> Cpr
    forall a b. (a -> b) -> a -> b
    $
              CprType -> Cpr
    ct_cpr CprType
    cpr_ty

    mkWWBindPair :: WwOpts -> Id -> IdInfo
                 -> [Var] -> CoreExpr -> Unique -> Divergence
                 -> ([Demand],JoinArity, Id -> CoreExpr, Expr CoreBndr -> CoreExpr)
                 -> [(Id, CoreExpr)]
    mkWWBindPair :: WwOpts
    -> Var
    -> IdInfo
    -> [Var]
    -> CoreExpr
    -> Unique
    -> Divergence
    -> WwResult
    -> [(Var, CoreExpr)]
    mkWWBindPair WwOpts
    ww_opts Var
    fn_id IdInfo
    fn_info [Var]
    fn_args CoreExpr
    fn_body Unique
    work_uniq Divergence
    div
                 ([Demand]
    work_demands, Arity
    join_arity, Var -> CoreExpr
    wrap_fn, CoreExpr -> CoreExpr
    work_fn)
      = -- pprTrace "mkWWBindPair" (ppr fn_id <+> ppr wrap_id <+> ppr work_id $$ ppr wrap_rhs) $
        [(Var
    work_id, CoreExpr
    work_rhs), (Var
    wrap_id, CoreExpr
    wrap_rhs)]
         -- Worker first, because wrapper mentions it
      where
        arity :: Arity
    arity = IdInfo -> Arity
    arityInfo IdInfo
    fn_info
                -- The arity is set by the simplifier using exprEtaExpandArity
                -- So it may be more than the number of top-level-visible lambdas

        simpl_opts :: SimpleOpts
    simpl_opts = WwOpts -> SimpleOpts
    wo_simple_opts WwOpts
    ww_opts

        work_rhs :: CoreExpr
    work_rhs = CoreExpr -> CoreExpr
    work_fn ([Var] -> CoreExpr -> CoreExpr
    forall b. [b] -> Expr b -> Expr b
    mkLams [Var]
    fn_args CoreExpr
    fn_body)
        work_act :: Activation
    work_act = case InlineSpec
    fn_inline_spec of  -- See Note [Worker activation]
                       NoInline SourceText
    _  -> InlinePragma -> Activation
    inl_act InlinePragma
    fn_inl_prag
                       InlineSpec
    _           -> InlinePragma -> Activation
    inl_act InlinePragma
    wrap_prag

        work_prag :: InlinePragma
    work_prag = InlinePragma { inl_src :: SourceText
    inl_src = FastString -> SourceText
    SourceText (FastString -> SourceText) -> FastString -> SourceText
    forall a b. (a -> b) -> a -> b
    $ String -> FastString
    fsLit String
    "{-# INLINE"
                                 , inl_inline :: InlineSpec
    inl_inline = InlineSpec
    fn_inline_spec
                                 , inl_sat :: Maybe Arity
    inl_sat    = Maybe Arity
    forall a. Maybe a
    Nothing
                                 , inl_act :: Activation
    inl_act    = Activation
    work_act
                                 , inl_rule :: RuleMatchInfo
    inl_rule   = RuleMatchInfo
    FunLike }
          -- inl_inline: copy from fn_id; see Note [Worker/wrapper for INLINABLE functions]
          -- inl_act:    see Note [Worker activation]
          -- inl_rule:   it does not make sense for workers to be constructorlike.

        work_join_arity :: JoinPointHood
    work_join_arity | Var -> Bool
    isJoinId Var
    fn_id = Arity -> JoinPointHood
    JoinPoint Arity
    join_arity
                        | Bool
    otherwise      = JoinPointHood
    NotJoinPoint
          -- worker is join point iff wrapper is join point
          -- (see Note [Don't w/w join points for CPR])

        work_id :: Var
    work_id  = Var -> Var
    asWorkerLikeId (Var -> Var) -> Var -> Var
    forall a b. (a -> b) -> a -> b
    $
                   Unique -> Var -> Type -> Var
    mkWorkerId Unique
    work_uniq Var
    fn_id (HasDebugCallStack => CoreExpr -> Type
    CoreExpr -> Type
    exprType CoreExpr
    work_rhs)
                    Var -> OccInfo -> Var
    `setIdOccInfo` IdInfo -> OccInfo
    occInfo IdInfo
    fn_info
                            -- Copy over occurrence info from parent
                            -- Notably whether it's a loop breaker
                            -- Doesn't matter much, since we will simplify next, but
                            -- seems right-er to do so

                    Var -> InlinePragma -> Var
    `setInlinePragma` InlinePragma
    work_prag

                    Var -> Unfolding -> Var
    `setIdUnfolding` SimpleOpts -> (CoreExpr -> CoreExpr) -> Unfolding -> Unfolding
    mkWorkerUnfolding SimpleOpts
    simpl_opts CoreExpr -> CoreExpr
    work_fn Unfolding
    fn_unfolding
                            -- See Note [Worker/wrapper for INLINABLE functions]

                    Var -> DmdSig -> Var
    `setIdDmdSig` [Demand] -> Divergence -> DmdSig
    mkClosedDmdSig [Demand]
    work_demands Divergence
    div
                            -- Even though we may not be at top level,
                            -- it's ok to give it an empty DmdEnv

                    Var -> CprSig -> Var
    `setIdCprSig` CprSig
    topCprSig

                    Var -> Demand -> Var
    `setIdDemandInfo` Demand
    worker_demand

                    Var -> Arity -> Var
    `setIdArity` Arity
    work_arity
                            -- Set the arity so that the Core Lint check that the
                            -- arity is consistent with the demand type goes
                            -- through

                    Var -> JoinPointHood -> Var
    `asJoinId_maybe` JoinPointHood
    work_join_arity

        work_arity :: Arity
    work_arity = [Demand] -> Arity
    forall a. [a] -> Arity
    forall (t :: * -> *) a. Foldable t => t a -> Arity
    length [Demand]
    work_demands :: Int

        -- See Note [Demand on the worker]
        single_call :: Bool
    single_call = Arity -> Demand -> Bool
    saturatedByOneShots Arity
    arity (IdInfo -> Demand
    demandInfo IdInfo
    fn_info)
        worker_demand :: Demand
    worker_demand | Bool
    single_call = Arity -> Demand
    mkWorkerDemand Arity
    work_arity
                      | Bool
    otherwise   = Demand
    topDmd

        wrap_rhs :: CoreExpr
    wrap_rhs  = Var -> CoreExpr
    wrap_fn Var
    work_id
        wrap_prag :: InlinePragma
    wrap_prag = InlinePragma -> [CoreRule] -> InlinePragma
    mkStrWrapperInlinePrag InlinePragma
    fn_inl_prag [CoreRule]
    fn_rules
        wrap_unf :: Unfolding
    wrap_unf  = SimpleOpts -> CoreExpr -> Arity -> Unfolding
    mkWrapperUnfolding SimpleOpts
    simpl_opts CoreExpr
    wrap_rhs Arity
    arity

        wrap_id :: Var
    wrap_id   = Var
    fn_id Var -> Unfolding -> Var
    `setIdUnfolding`  Unfolding
    wrap_unf
                          Var -> InlinePragma -> Var
    `setInlinePragma` InlinePragma
    wrap_prag
                          Var -> OccInfo -> Var
    `setIdOccInfo`    OccInfo
    noOccInfo
                            -- Zap any loop-breaker-ness, to avoid bleating from Lint
                            -- about a loop breaker with an INLINE rule

        fn_inl_prag :: InlinePragma
    fn_inl_prag     = IdInfo -> InlinePragma
    inlinePragInfo IdInfo
    fn_info
        fn_inline_spec :: InlineSpec
    fn_inline_spec  = InlinePragma -> InlineSpec
    inl_inline InlinePragma
    fn_inl_prag
        fn_unfolding :: Unfolding
    fn_unfolding    = IdInfo -> Unfolding
    realUnfoldingInfo IdInfo
    fn_info
        fn_rules :: [CoreRule]
    fn_rules        = RuleInfo -> [CoreRule]
    ruleInfoRules (IdInfo -> RuleInfo
    ruleInfo IdInfo
    fn_info)

    mkStrWrapperInlinePrag :: InlinePragma -> [CoreRule] -> InlinePragma
    mkStrWrapperInlinePrag :: InlinePragma -> [CoreRule] -> InlinePragma
    mkStrWrapperInlinePrag (InlinePragma { inl_inline :: InlinePragma -> InlineSpec
    inl_inline = InlineSpec
    fn_inl
                                         , inl_act :: InlinePragma -> Activation
    inl_act    = Activation
    fn_act
                                         , inl_rule :: InlinePragma -> RuleMatchInfo
    inl_rule   = RuleMatchInfo
    rule_info }) [CoreRule]
    rules
      = InlinePragma { inl_src :: SourceText
    inl_src    = FastString -> SourceText
    SourceText (FastString -> SourceText) -> FastString -> SourceText
    forall a b. (a -> b) -> a -> b
    $ String -> FastString
    fsLit String
    "{-# INLINE"
                     , inl_sat :: Maybe Arity
    inl_sat    = Maybe Arity
    forall a. Maybe a
    Nothing

                     , inl_inline :: InlineSpec
    inl_inline = InlineSpec
    fn_inl
                          -- See Note [Worker/wrapper for INLINABLE functions]

                     , inl_act :: Activation
    inl_act    = CompilerPhase -> Activation
    activeAfter CompilerPhase
    wrapper_phase
                          -- See Note [Wrapper activation]

                     , inl_rule :: RuleMatchInfo
    inl_rule   = RuleMatchInfo
    rule_info }  -- RuleMatchInfo is (and must be) unaffected
      where
        -- See Note [Wrapper activation]
        wrapper_phase :: CompilerPhase
    wrapper_phase = (CoreRule -> CompilerPhase -> CompilerPhase)
    -> CompilerPhase -> [CoreRule] -> CompilerPhase
    forall a b. (a -> b -> b) -> b -> [a] -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr (CompilerPhase -> CompilerPhase -> CompilerPhase
    laterPhase (CompilerPhase -> CompilerPhase -> CompilerPhase)
    -> (CoreRule -> CompilerPhase)
    -> CoreRule
    -> CompilerPhase
    -> CompilerPhase
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . CoreRule -> CompilerPhase
    get_rule_phase) CompilerPhase
    earliest_inline_phase [CoreRule]
    rules
        earliest_inline_phase :: CompilerPhase
    earliest_inline_phase = Activation -> CompilerPhase
    beginPhase Activation
    fn_act CompilerPhase -> CompilerPhase -> CompilerPhase
    `laterPhase` CompilerPhase -> CompilerPhase
    nextPhase CompilerPhase
    InitialPhase
              -- laterPhase (nextPhase InitialPhase) is a temporary hack
              -- to inline no earlier than phase 2.  I got regressions in
              -- 'mate', due to changes in full laziness due to Note [Case
              -- MFEs], when I did earlier inlining.

        get_rule_phase :: CoreRule -> CompilerPhase
        -- The phase /after/ the rule is first active
        get_rule_phase :: CoreRule -> CompilerPhase
    get_rule_phase CoreRule
    rule = CompilerPhase -> CompilerPhase
    nextPhase (Activation -> CompilerPhase
    beginPhase (CoreRule -> Activation
    ruleActivation CoreRule
    rule))

    {-
    Note [Demand on the worker]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~

    If the original function is called once, according to its demand info, then
    so is the worker. This is important so that the occurrence analyser can
    attach OneShot annotations to the worker’s lambda binders.

    Example:

      -- Original function
      f [Demand=<L,1*C(1,U)>] :: (a,a) -> a
      f = \p -> ...

      -- Wrapper
      f [Demand=<L,1*C(1,U)>] :: a -> a -> a
      f = \p -> case p of (a,b) -> $wf a b

      -- Worker
      $wf [Demand=<L,1*C(1,C(1,U))>] :: Int -> Int
      $wf = \a b -> ...

    We need to check whether the original function is called once, with
    sufficiently many arguments. This is done using saturatedByOneShots, which
    takes the arity of the original function (resp. the wrapper) and the demand on
    the original function.

    The demand on the worker is then calculated using mkWorkerDemand, and always of
    the form [Demand=<L,1*(C(1,...(C(1,U))))>]

    Note [Thunk splitting]
    ~~~~~~~~~~~~~~~~~~~~~~
    Suppose x is used strictly; never mind whether it has the CPR
    property.  I'll use a '*' to mean "x* is demanded strictly".

          let
            x* = x-rhs
          in body

    splitThunk transforms like this:
          let
            x* = let x = x-rhs in
                 case x of { I# a -> I# a }
          in body

    This is a little strange: we are re-using the same `x` in the RHS; and
    the RHS takes `x` apart and reboxes it. But because the outer 'let' is
    strict, and the inner let mentions `x` only once, the simplifier
    transform it to
          case x-rhs of
            I# a -> let x* = I# a
                    in body

    That is good: in `body` we know the form of `x`, which
      * gives the CPR property, and
      * allows case-of-case to happen on x

    Notes
    * I tried transforming like this:
          let
            x* = let x = x-rhs in
                 case x of { I# a -> x }
          in body
      where I return `x` itself, rather than reboxing it.  But this
      turned out to cause some regressions, which I never fully
      investigated.

    * Suppose x-rhs is itself a case:
            x-rhs = case e of { T -> I# e1; F -> I# e2 }
      Then we'll get
          join j a = let x* = I# a in body
          in case e of { T -> j e1; F -> j e2 }
      which is good (no boxing).  But in the original, unsplit program
      we would transform
          let x* = case e of ... in body
      ==> join j2 x = body
          in case e of { T -> j2 (I# e1); F -> j (I# e2) }
      which is not good (boxing).

    * In fact, splitThunk uses the function argument w/w splitting
      function, mkWWstr_one, so that if x's demand is deeper (say U(U(L,L),L))
      then the splitting will go deeper too.

    * For recursive thunks, the Simplifier is unable to float `x-rhs` out of
      `x*`'s RHS, because `x*` occurs freely in `x-rhs`, and will just change it
      back to the original definition, so we just split non-recursive thunks.

    Note [Thunk splitting for top-level binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Top-level bindings are never strict. Yet they can be absent, as T14270 shows:

      module T14270 (mkTrApp) where
      mkTrApp x y
        | Just ... <- ... typeRepKind x ...
        = undefined
        | otherwise
        = undefined
      typeRepKind = Tick scc undefined

    (T19180 is a profiling-free test case for this)
    Note that `typeRepKind` is not exported and its only use site in
    `mkTrApp` guards a bottoming expression. Thus, demand analysis
    figures out that `typeRepKind` is absent and splits the thunk to

      typeRepKind =
        let typeRepKind = Tick scc undefined in
        let typeRepKind = absentError in
        typeRepKind

    But now we have a local binding with an External Name
    (See Note [About the NameSorts]). That will trigger a CoreLint error, which we
    get around by localising the Id for the auxiliary bindings in 'splitThunk'.
    -}

    -- | See Note [Thunk splitting].
    --
    -- splitThunk converts the *non-recursive* binding
    --      x = e
    -- into
    --      x = let x' = e in
    --          case x' of I# y -> let x' = I# y in x'
    -- See comments above. Is it not beautifully short?
    -- Moreover, it works just as well when there are
    -- several binders, and if the binders are lifted
    -- E.g.     x = e
    --     -->  x = let x' = e in
    --              case x' of (a,b) -> let x' = (a,b)  in x'
    -- Here, x' is a localised version of x, in case x is a
    -- top-level Id with an External Name, because Lint rejects local binders with
    -- External Names; see Note [About the NameSorts] in GHC.Types.Name.
    --
    -- How can we do thunk-splitting on a top-level binder?  See
    -- Note [Thunk splitting for top-level binders].
    splitThunk :: WwOpts -> RecFlag -> Var -> Expr Var -> UniqSM [(Var, Expr Var)]
    splitThunk :: WwOpts -> RecFlag -> Var -> CoreExpr -> UniqSM [(Var, CoreExpr)]
    splitThunk WwOpts
    ww_opts RecFlag
    is_rec Var
    x CoreExpr
    rhs
      = Bool -> UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a. HasCallStack => Bool -> a -> a
    assert (Bool -> Bool
    not (Var -> Bool
    isJoinId Var
    x)) (UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)])
    -> UniqSM [(Var, CoreExpr)] -> UniqSM [(Var, CoreExpr)]
    forall a b. (a -> b) -> a -> b
    $
        do { let x' :: Var
    x' = Var -> Var
    localiseId Var
    x -- See comment above
           ; (useful,_args, wrap_fn, fn_arg)
               <- WwOpts
    -> Var
    -> StrictnessMark
    -> UniqSM
         (Bool, [(Var, StrictnessMark)], CoreExpr -> CoreExpr, CoreExpr)
    mkWWstr_one WwOpts
    ww_opts Var
    x' StrictnessMark
    NotMarkedStrict
           ; let res = [ (Var
    x, CoreBind -> CoreExpr -> CoreExpr
    forall b. Bind b -> Expr b -> Expr b
    Let (Var -> CoreExpr -> CoreBind
    forall b. b -> Expr b -> Bind b
    NonRec Var
    x' CoreExpr
    rhs) (CoreExpr -> CoreExpr
    wrap_fn CoreExpr
    fn_arg)) ]
           ; if useful then assertPpr (isNonRec is_rec) (ppr x) -- The thunk must be non-recursive
                       return res
                       else return [(x, rhs)] }