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

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

{-# LANGUAGE CPP #-}
module GHC.Core.Opt.WorkWrap ( wwTopBinds ) where

import GHC.Prelude

import GHC.Core.Opt.Arity  ( manifestArity )
import GHC.Core
import GHC.Core.Unfold
import GHC.Core.Unfold.Make
import GHC.Core.Utils  ( exprType, exprIsHNF )
import GHC.Core.FVs    ( exprFreeVars )
import GHC.Types.Var
import GHC.Types.Id
import GHC.Types.Id.Info
import GHC.Core.Type
import GHC.Types.Unique.Supply
import GHC.Types.Basic
import GHC.Driver.Session
import GHC.Driver.Ppr
import GHC.Driver.Config
import GHC.Types.Demand
import GHC.Types.Cpr
import GHC.Types.SourceText
import GHC.Core.Opt.WorkWrap.Utils
import GHC.Utils.Misc
import GHC.Utils.Outputable
import GHC.Types.Unique
import GHC.Utils.Panic
import GHC.Core.FamInstEnv
import GHC.Utils.Monad

#include "HsVersions.h"

{-
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 :: DynFlags -> FamInstEnvs -> UniqSupply -> CoreProgram -> CoreProgram

wwTopBinds :: DynFlags -> FamInstEnvs -> UniqSupply -> CoreProgram -> CoreProgram
wwTopBinds DynFlags
dflags FamInstEnvs
fam_envs 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
$ do
    [CoreProgram]
top_binds' <- (CoreBind -> UniqSM CoreProgram)
-> CoreProgram -> UniqSM [CoreProgram]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM (DynFlags -> FamInstEnvs -> CoreBind -> UniqSM CoreProgram
wwBind DynFlags
dflags FamInstEnvs
fam_envs) CoreProgram
top_binds
    CoreProgram -> UniqSM CoreProgram
forall (m :: * -> *) a. Monad m => a -> m a
return ([CoreProgram] -> CoreProgram
forall (t :: * -> *) a. Foldable t => t [a] -> [a]
concat [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  :: DynFlags
        -> FamInstEnvs
        -> CoreBind
        -> UniqSM [CoreBind]    -- returns a WwBinding intermediate form;
                                -- the caller will convert to Expr/Binding,
                                -- as appropriate.

wwBind :: DynFlags -> FamInstEnvs -> CoreBind -> UniqSM CoreProgram
wwBind DynFlags
dflags FamInstEnvs
fam_envs (NonRec Var
binder Expr Var
rhs) = do
    Expr Var
new_rhs <- DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
rhs
    [(Var, Expr Var)]
new_pairs <- DynFlags
-> FamInstEnvs
-> RecFlag
-> Var
-> Expr Var
-> UniqSM [(Var, Expr Var)]
tryWW DynFlags
dflags FamInstEnvs
fam_envs RecFlag
NonRecursive Var
binder Expr Var
new_rhs
    CoreProgram -> UniqSM CoreProgram
forall (m :: * -> *) a. Monad m => a -> m a
return [Var -> Expr Var -> CoreBind
forall b. b -> Expr b -> Bind b
NonRec Var
b Expr Var
e | (Var
b,Expr Var
e) <- [(Var, Expr Var)]
new_pairs]
      -- Generated bindings must be non-recursive
      -- because the original binding was.

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

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

wwExpr DynFlags
dflags FamInstEnvs
fam_envs (Lam Var
binder Expr Var
expr)
  = Var -> Expr Var -> Expr Var
forall b. b -> Expr b -> Expr b
Lam Var
new_binder (Expr Var -> Expr Var) -> UniqSM (Expr Var) -> UniqSM (Expr Var)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
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 DynFlags
dflags FamInstEnvs
fam_envs (App Expr Var
f Expr Var
a)
  = Expr Var -> Expr Var -> Expr Var
forall b. Expr b -> Expr b -> Expr b
App (Expr Var -> Expr Var -> Expr Var)
-> UniqSM (Expr Var) -> UniqSM (Expr Var -> Expr Var)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
f UniqSM (Expr Var -> Expr Var)
-> UniqSM (Expr Var) -> UniqSM (Expr Var)
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
a

wwExpr DynFlags
dflags FamInstEnvs
fam_envs (Tick CoreTickish
note Expr Var
expr)
  = CoreTickish -> Expr Var -> Expr Var
forall b. CoreTickish -> Expr b -> Expr b
Tick CoreTickish
note (Expr Var -> Expr Var) -> UniqSM (Expr Var) -> UniqSM (Expr Var)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
expr

wwExpr DynFlags
dflags FamInstEnvs
fam_envs (Cast Expr Var
expr CoercionR
co) = do
    Expr Var
new_expr <- DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
expr
    Expr Var -> UniqSM (Expr Var)
forall (m :: * -> *) a. Monad m => a -> m a
return (Expr Var -> CoercionR -> Expr Var
forall b. Expr b -> CoercionR -> Expr b
Cast Expr Var
new_expr CoercionR
co)

wwExpr DynFlags
dflags FamInstEnvs
fam_envs (Let CoreBind
bind Expr Var
expr)
  = CoreProgram -> Expr Var -> Expr Var
forall b. [Bind b] -> Expr b -> Expr b
mkLets (CoreProgram -> Expr Var -> Expr Var)
-> UniqSM CoreProgram -> UniqSM (Expr Var -> Expr Var)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> DynFlags -> FamInstEnvs -> CoreBind -> UniqSM CoreProgram
wwBind DynFlags
dflags FamInstEnvs
fam_envs CoreBind
bind UniqSM (Expr Var -> Expr Var)
-> UniqSM (Expr Var) -> UniqSM (Expr Var)
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
expr

wwExpr DynFlags
dflags FamInstEnvs
fam_envs (Case Expr Var
expr Var
binder Type
ty [Alt Var]
alts) = do
    Expr Var
new_expr <- DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
expr
    [Alt Var]
new_alts <- (Alt Var -> UniqSM (Alt Var)) -> [Alt Var] -> UniqSM [Alt Var]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
mapM Alt Var -> UniqSM (Alt Var)
ww_alt [Alt Var]
alts
    let new_binder :: Var
new_binder = Var -> Var
zapIdUsedOnceInfo Var
binder
      -- See Note [Zapping Used Once info in WorkWrap]
    Expr Var -> UniqSM (Expr Var)
forall (m :: * -> *) a. Monad m => a -> m a
return (Expr Var -> Var -> Type -> [Alt Var] -> Expr Var
forall b. Expr b -> b -> Type -> [Alt b] -> Expr b
Case Expr Var
new_expr Var
new_binder Type
ty [Alt Var]
new_alts)
  where
    ww_alt :: Alt Var -> UniqSM (Alt Var)
ww_alt (Alt AltCon
con [Var]
binders Expr Var
rhs) = do
        Expr Var
new_rhs <- DynFlags -> FamInstEnvs -> Expr Var -> UniqSM (Expr Var)
wwExpr DynFlags
dflags FamInstEnvs
fam_envs Expr Var
rhs
        let new_binders :: [Var]
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]
        Alt Var -> UniqSM (Alt Var)
forall (m :: * -> *) a. Monad m => a -> m a
return (AltCon -> [Var] -> Expr Var -> Alt Var
forall b. AltCon -> [b] -> Expr b -> Alt b
Alt AltCon
con [Var]
new_binders Expr Var
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 INLNABLE, and in particular
will not be specialised at call sites in other modules.

This comes in practice (#6056).

Solution: do the w/w for strictness analysis, but transfer the Stable
unfolding to the *worker*.  So we will get something like this:

  {-# INLINE[0] f #-}
  f :: Ord a => [a] -> Int -> a
  f d x y = case y of I# y' -> fw d x y'

  {-# INLINABLE[0] fw #-}
  fw :: Ord a => [a] -> Int# -> a
  fw d x y' = let y = I# y' in ...f...

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

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.


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 work inherit 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[0] $wexpensive #-}
  $wexpensive x = x + 1
  {-# INLINE[0] 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.

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

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 = g x x
   and suppose there is some RULE for (g True True).  Then if we have
   a call (f True), 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.

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

Conclusion:
  - If the user said NOINLINE[n], respect that

  - If the user said NOINLINE, inline the wrapper only after
    phase 0, the last user-visible phase.  That means that all
    rules will have had a chance to fire.

    What phase is after phase 0?  Answer: FinalPhase, that's the reason it
    exists. NB: Similar to InitialPhase, users can't write INLINE[Final] f;
    it's syntactically illegal.

  - Otherwise inline wrapper in phase 2.  That allows the
    'gentle' simplification pass to apply specialisation rules

Note [Wrapper NoUserInlinePrag]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We use NoUserInlinePrag on the wrapper, to say that there is no
user-specified inline pragma. (The worker inherits that; see Note
[Worker-wrapper for INLINABLE functions].)  The wrapper has no pragma
given by the user.

(Historical note: we used to 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).  We don't need a pragma, because
everything we needs is expressed by (a) the stable unfolding and (b)
the inl_act activation.)

-}

tryWW   :: DynFlags
        -> FamInstEnvs
        -> 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 :: DynFlags
-> FamInstEnvs
-> RecFlag
-> Var
-> Expr Var
-> UniqSM [(Var, Expr Var)]
tryWW DynFlags
dflags FamInstEnvs
fam_envs RecFlag
is_rec Var
fn_id Expr Var
rhs
  -- See Note [Worker-wrapper for NOINLINE functions]

  | Just Unfolding
stable_unf <- UnfoldingOpts -> IdInfo -> Maybe Unfolding
certainlyWillInline UnfoldingOpts
uf_opts IdInfo
fn_info
  = [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return [ (Var
fn_id Var -> Unfolding -> Var
`setIdUnfolding` Unfolding
stable_unf, Expr Var
rhs) ]
        -- See Note [Don't w/w INLINE things]
        -- See Note [Don't w/w inline small non-loop-breaker things]

  | Bool
is_fun Bool -> Bool -> Bool
&& Bool
is_eta_exp
  = DynFlags
-> FamInstEnvs
-> Var
-> IdInfo
-> [Demand]
-> Divergence
-> Cpr
-> Expr Var
-> UniqSM [(Var, Expr Var)]
splitFun DynFlags
dflags FamInstEnvs
fam_envs Var
new_fn_id IdInfo
fn_info [Demand]
wrap_dmds Divergence
div Cpr
cpr Expr Var
rhs

  | RecFlag -> Bool
isNonRec RecFlag
is_rec, Bool
is_thunk                        -- See Note [Thunk splitting]
  = DynFlags
-> FamInstEnvs
-> RecFlag
-> Var
-> Expr Var
-> UniqSM [(Var, Expr Var)]
splitThunk DynFlags
dflags FamInstEnvs
fam_envs RecFlag
is_rec Var
new_fn_id Expr Var
rhs

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

  where
    uf_opts :: UnfoldingOpts
uf_opts      = DynFlags -> UnfoldingOpts
unfoldingOpts DynFlags
dflags
    fn_info :: IdInfo
fn_info      = HasDebugCallStack => Var -> IdInfo
Var -> IdInfo
idInfo Var
fn_id
    ([Demand]
wrap_dmds, Divergence
div) = StrictSig -> ([Demand], Divergence)
splitStrictSig (IdInfo -> StrictSig
strictnessInfo IdInfo
fn_info)

    cpr_ty :: CprType
cpr_ty       = CprSig -> CprType
getCprSig (IdInfo -> CprSig
cprInfo 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          = ASSERT2( isJoinId fn_id || cpr_ty == topCprType || ct_arty cpr_ty == arityInfo fn_info
                          , ppr fn_id <> colon <+> text "ct_arty:" <+> int (ct_arty cpr_ty) <+> text "arityInfo:" <+> ppr (arityInfo fn_info))
                   CprType -> Cpr
ct_cpr CprType
cpr_ty

    new_fn_id :: Var
new_fn_id = Var -> Var
zapIdUsedOnceInfo (Var -> Var
zapIdUsageEnvInfo Var
fn_id)
        -- See Note [Zapping DmdEnv after Demand Analyzer] and
        -- See Note [Zapping Used Once info WorkWrap]

    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
    -- See Note [Don't eta expand in w/w]
    is_eta_exp :: Bool
is_eta_exp = [Demand] -> ArityInfo
forall (t :: * -> *) a. Foldable t => t a -> ArityInfo
length [Demand]
wrap_dmds ArityInfo -> ArityInfo -> Bool
forall a. Eq a => a -> a -> Bool
== Expr Var -> ArityInfo
manifestArity Expr Var
rhs
    is_thunk :: Bool
is_thunk   = Bool -> Bool
not Bool
is_fun Bool -> Bool -> Bool
&& Bool -> Bool
not (Expr Var -> Bool
exprIsHNF Expr Var
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]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the worker-wrapper pass we zap the used once info in demands and in
strictness signatures.

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.

So as the data is useless and possibly wrong, we want to remove it. The most
convenient place to do that is the worker wrapper phase, as it runs after every
run of the demand analyser besides the very last one (which is the one where we
want to _keep_ the info for the code generator).

We do not do it in the demand analyser for the same reasons outlined in
Note [Zapping DmdEnv after Demand Analyzer] above.

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 [exprArity invariant] in GHC.Core.Opt.Arity) and we probably have
a PAP, cast or trivial expression as RHS.

Performing the worker/wrapper split will implicitly eta-expand 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 StrictSig 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.
-}


---------------------
splitFun :: DynFlags -> FamInstEnvs -> Id -> IdInfo -> [Demand] -> Divergence -> Cpr -> CoreExpr
         -> UniqSM [(Id, CoreExpr)]
splitFun :: DynFlags
-> FamInstEnvs
-> Var
-> IdInfo
-> [Demand]
-> Divergence
-> Cpr
-> Expr Var
-> UniqSM [(Var, Expr Var)]
splitFun DynFlags
dflags FamInstEnvs
fam_envs Var
fn_id IdInfo
fn_info [Demand]
wrap_dmds Divergence
div Cpr
cpr Expr Var
rhs
  | Var -> Bool
isRecordSelector Var
fn_id  -- See Note [No worker/wrapper for record selectors]
  = [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return [ (Var
fn_id, Expr Var
rhs ) ]

  | Bool
otherwise
  = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr cpr) )
          -- The arity should match the signature
    do { Maybe WwResult
mb_stuff <- DynFlags
-> FamInstEnvs
-> VarSet
-> Var
-> [Demand]
-> Cpr
-> UniqSM (Maybe WwResult)
mkWwBodies DynFlags
dflags FamInstEnvs
fam_envs VarSet
rhs_fvs Var
fn_id [Demand]
wrap_dmds Cpr
use_cpr_info
       ; case Maybe WwResult
mb_stuff of
            Maybe WwResult
Nothing -> [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return [(Var
fn_id, Expr Var
rhs)]

            Just WwResult
stuff
              | Just Unfolding
stable_unf <- UnfoldingOpts -> IdInfo -> Maybe Unfolding
certainlyWillInline (DynFlags -> UnfoldingOpts
unfoldingOpts DynFlags
dflags) IdInfo
fn_info
              ->  [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return [ (Var
fn_id Var -> Unfolding -> Var
`setIdUnfolding` Unfolding
stable_unf, Expr Var
rhs) ]
                  -- See Note [Don't w/w INLINE things]
                  -- See Note [Don't w/w inline small non-loop-breaker things]

              | Bool
otherwise
              -> do { Unique
work_uniq <- UniqSM Unique
forall (m :: * -> *). MonadUnique m => m Unique
getUniqueM
                    ; [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return (DynFlags
-> Var
-> IdInfo
-> ArityInfo
-> Expr Var
-> Unique
-> Divergence
-> Cpr
-> WwResult
-> [(Var, Expr Var)]
mkWWBindPair DynFlags
dflags Var
fn_id IdInfo
fn_info ArityInfo
arity Expr Var
rhs
                                           Unique
work_uniq Divergence
div Cpr
cpr WwResult
stuff) } }
  where
    rhs_fvs :: VarSet
rhs_fvs = Expr Var -> VarSet
exprFreeVars Expr Var
rhs
    arity :: ArityInfo
arity   = IdInfo -> ArityInfo
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

    -- use_cpr_info is the CPR we w/w for. Note that we kill it for join points,
    -- see Note [Don't w/w join points for CPR].
    use_cpr_info :: Cpr
use_cpr_info  | Var -> Bool
isJoinId Var
fn_id = Cpr
topCpr
                  | Bool
otherwise      = Cpr
cpr


mkWWBindPair :: DynFlags -> Id -> IdInfo -> Arity
             -> CoreExpr -> Unique -> Divergence -> Cpr
             -> ([Demand], JoinArity, Id -> CoreExpr, Expr CoreBndr -> CoreExpr)
             -> [(Id, CoreExpr)]
mkWWBindPair :: DynFlags
-> Var
-> IdInfo
-> ArityInfo
-> Expr Var
-> Unique
-> Divergence
-> Cpr
-> WwResult
-> [(Var, Expr Var)]
mkWWBindPair DynFlags
dflags Var
fn_id IdInfo
fn_info ArityInfo
arity Expr Var
rhs Unique
work_uniq Divergence
div Cpr
cpr
             ([Demand]
work_demands, ArityInfo
join_arity, Var -> Expr Var
wrap_fn, Expr Var -> Expr Var
work_fn)
  = [(Var
work_id, Expr Var
work_rhs), (Var
wrap_id, Expr Var
wrap_rhs)]
     -- Worker first, because wrapper mentions it
  where
    simpl_opts :: SimpleOpts
simpl_opts = DynFlags -> SimpleOpts
initSimpleOpts DynFlags
dflags

    work_rhs :: Expr Var
work_rhs = Expr Var -> Expr Var
work_fn Expr Var
rhs
    work_act :: Activation
work_act = case InlineSpec
fn_inline_spec of  -- See Note [Worker activation]
                   InlineSpec
NoInline -> 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 = String -> SourceText
SourceText String
"{-# INLINE"
                             , inl_inline :: InlineSpec
inl_inline = InlineSpec
fn_inline_spec
                             , inl_sat :: Maybe ArityInfo
inl_sat    = Maybe ArityInfo
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 :: Maybe ArityInfo
work_join_arity | Var -> Bool
isJoinId Var
fn_id = ArityInfo -> Maybe ArityInfo
forall a. a -> Maybe a
Just ArityInfo
join_arity
                    | Bool
otherwise      = Maybe ArityInfo
forall a. Maybe a
Nothing
      -- worker is join point iff wrapper is join point
      -- (see Note [Don't w/w join points for CPR])

    work_id :: Var
work_id  = Unique -> Var -> Type -> Var
mkWorkerId Unique
work_uniq Var
fn_id (Expr Var -> Type
exprType Expr Var
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 -> (Expr Var -> Expr Var) -> Unfolding -> Unfolding
mkWorkerUnfolding SimpleOpts
simpl_opts Expr Var -> Expr Var
work_fn Unfolding
fn_unfolding
                        -- See Note [Worker-wrapper for INLINABLE functions]

                Var -> StrictSig -> Var
`setIdStrictness` [Demand] -> Divergence -> StrictSig
mkClosedStrictSig [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
`setIdCprInfo` ArityInfo -> Cpr -> CprSig
mkCprSig ArityInfo
work_arity Cpr
work_cpr_info

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

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

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

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

    wrap_rhs :: Expr Var
wrap_rhs  = Var -> Expr Var
wrap_fn Var
work_id
    wrap_prag :: InlinePragma
wrap_prag = InlinePragma -> InlinePragma
mkStrWrapperInlinePrag InlinePragma
fn_inl_prag

    wrap_id :: Var
wrap_id   = Var
fn_id Var -> Unfolding -> Var
`setIdUnfolding`  SimpleOpts -> Expr Var -> ArityInfo -> Unfolding
mkWwInlineRule SimpleOpts
simpl_opts Expr Var
wrap_rhs ArityInfo
arity
                      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
unfoldingInfo IdInfo
fn_info

    -- Even if we don't w/w join points for CPR, we might still do so for
    -- strictness. In which case a join point worker keeps its original CPR
    -- property; see Note [Don't w/w join points for CPR]. Otherwise, the worker
    -- doesn't have the CPR property anymore.
    work_cpr_info :: Cpr
work_cpr_info | Var -> Bool
isJoinId Var
fn_id = Cpr
cpr
                  | Bool
otherwise      = Cpr
topCpr

mkStrWrapperInlinePrag :: InlinePragma -> InlinePragma
mkStrWrapperInlinePrag :: InlinePragma -> InlinePragma
mkStrWrapperInlinePrag (InlinePragma { inl_act :: InlinePragma -> Activation
inl_act = Activation
act, inl_rule :: InlinePragma -> RuleMatchInfo
inl_rule = RuleMatchInfo
rule_info })
  = InlinePragma { inl_src :: SourceText
inl_src    = String -> SourceText
SourceText String
"{-# INLINE"
                 , inl_inline :: InlineSpec
inl_inline = InlineSpec
NoUserInlinePrag -- See Note [Wrapper NoUserInline]
                 , inl_sat :: Maybe ArityInfo
inl_sat    = Maybe ArityInfo
forall a. Maybe a
Nothing
                 , inl_act :: Activation
inl_act    = Activation
wrap_act
                 , inl_rule :: RuleMatchInfo
inl_rule   = RuleMatchInfo
rule_info }  -- RuleMatchInfo is (and must be) unaffected
  where
    wrap_act :: Activation
wrap_act  = case Activation
act of  -- See Note [Wrapper activation]
                   Activation
NeverActive     -> Activation
activateDuringFinal
                   Activation
FinalActive     -> Activation
act
                   ActiveAfter {}  -> Activation
act
                   ActiveBefore {} -> Activation
activateAfterInitial
                   Activation
AlwaysActive    -> Activation
activateAfterInitial
      -- For the last two cases, see (4) in Note [Wrapper activation]
      -- NB: the (ActiveBefore n) isn't quite right. We really want
      -- it to be active *after* Initial but *before* n.  We don't have
      -- a way to say that, alas.

{-
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*C1(U)>] :: (a,a) -> a
  f = \p -> ...

  -- Wrapper
  f [Demand=<L,1*C1(U)>] :: a -> a -> a
  f = \p -> case p of (a,b) -> $wf a b

  -- Worker
  $wf [Demand=<L,1*C1(C1(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*(C1(...(C1(U))))>]


Note [Do not split void functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this rather common form of binding:
        $j = \x:Void# -> ...no use of x...

Since x is not used it'll be marked as absent.  But there is no point
in w/w-ing because we'll simply add (\y:Void#), see GHC.Core.Opt.WorkWrap.Utils.mkWorerArgs.

If x has a more interesting type (eg Int, or Int#), there *is* a point
in w/w so that we don't pass the argument at all.

Note [Thunk splitting]
~~~~~~~~~~~~~~~~~~~~~~
Suppose x is used strictly (never mind whether it has the CPR
property).

      let
        x* = x-rhs
      in body

splitThunk transforms like this:

      let
        x* = case x-rhs of { I# a -> I# a }
      in body

Now simplifier will transform to

      case x-rhs of
        I# a -> let x* = I# a
                in body

which is what we want. Now suppose x-rhs is itself a case:

        x-rhs = case e of { T -> I# a; F -> I# b }

The join point will abstract over a, rather than over (which is
what would have happened before) which is fine.

Notice that x certainly has the CPR property now!

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

NB: 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 :: DynFlags -> FamInstEnvs -> RecFlag -> Var -> Expr Var -> UniqSM [(Var, Expr Var)]
splitThunk :: DynFlags
-> FamInstEnvs
-> RecFlag
-> Var
-> Expr Var
-> UniqSM [(Var, Expr Var)]
splitThunk DynFlags
dflags FamInstEnvs
fam_envs RecFlag
is_rec Var
x Expr Var
rhs
  = ASSERT(not (isJoinId x))
    do { let x' :: Var
x' = Var -> Var
localiseId Var
x -- See comment above
       ; (Bool
useful,[Var]
_, Expr Var -> Expr Var
wrap_fn, Expr Var -> Expr Var
work_fn) <- DynFlags
-> FamInstEnvs
-> Bool
-> [Var]
-> UniqSM (Bool, [Var], Expr Var -> Expr Var, Expr Var -> Expr Var)
mkWWstr DynFlags
dflags FamInstEnvs
fam_envs Bool
False [Var
x']
       ; let res :: [(Var, Expr Var)]
res = [ (Var
x, CoreBind -> Expr Var -> Expr Var
forall b. Bind b -> Expr b -> Expr b
Let (Var -> Expr Var -> CoreBind
forall b. b -> Expr b -> Bind b
NonRec Var
x' Expr Var
rhs) (Expr Var -> Expr Var
wrap_fn (Expr Var -> Expr Var
work_fn (Var -> Expr Var
forall b. Var -> Expr b
Var Var
x')))) ]
       ; if Bool
useful then ASSERT2( isNonRec is_rec, ppr x ) -- The thunk must be non-recursive
                   [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return [(Var, Expr Var)]
res
                   else [(Var, Expr Var)] -> UniqSM [(Var, Expr Var)]
forall (m :: * -> *) a. Monad m => a -> m a
return [(Var
x, Expr Var
rhs)] }