/compiler/simplCore/OccurAnal.lhs

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%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
%************************************************************************
%*                                                                      *
\section[OccurAnal]{Occurrence analysis pass}
%*                                                                      *
%************************************************************************

The occurrence analyser re-typechecks a core expression, returning a new
core expression with (hopefully) improved usage information.

\begin{code}
{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

{-# LANGUAGE BangPatterns #-}
module OccurAnal (
        occurAnalysePgm, occurAnalyseExpr
    ) where

#include "HsVersions.h"

import CoreSyn
import CoreFVs
import CoreUtils        ( exprIsTrivial, isDefaultAlt, isExpandableApp, mkCast )
import Id
import Name( localiseName )
import BasicTypes
import Module( Module )
import Coercion

import VarSet
import VarEnv
import Var

import Maybes           ( orElse )
import Digraph          ( SCC(..), stronglyConnCompFromEdgedVerticesR )
import PrelNames        ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
import Unique
import UniqFM
import Util
import Bag
import Outputable
import FastString
import Data.List
\end{code}


%************************************************************************
%*                                                                      *
\subsection[OccurAnal-main]{Counting occurrences: main function}
%*                                                                      *
%************************************************************************

Here's the externally-callable interface:

\begin{code}
occurAnalysePgm :: Module	-- Used only in debug output
                -> (Activation -> Bool) 
                -> [CoreRule] -> [CoreVect]
                -> CoreProgram -> CoreProgram
occurAnalysePgm this_mod active_rule imp_rules vects binds
  | isEmptyVarEnv final_usage
  = binds'
  | otherwise	-- See Note [Glomming]
  = WARN( True, hang (text "Glomming in" <+> ppr this_mod <> colon)
                   2 (ppr final_usage ) )
    [Rec (flattenBinds binds')]	 
  where
    (final_usage, binds') = go (initOccEnv active_rule) binds

    initial_uds = addIdOccs emptyDetails 
                            (rulesFreeVars imp_rules `unionVarSet` vectsFreeVars vects)
    -- The RULES and VECTORISE declarations keep things alive!

    -- Note [Preventing loops due to imported functions rules]
    imp_rules_edges = foldr (plusVarEnv_C unionVarSet) emptyVarEnv
                            [ mapVarEnv (const maps_to) (exprFreeIds arg `delVarSetList` ru_bndrs imp_rule)
                            | imp_rule <- imp_rules
                            , let maps_to = exprFreeIds (ru_rhs imp_rule)
                                             `delVarSetList` ru_bndrs imp_rule
                            , arg <- ru_args imp_rule ]

    go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind])
    go _ []
        = (initial_uds, [])
    go env (bind:binds)
        = (final_usage, bind' ++ binds')
        where
           (bs_usage, binds')   = go env binds
           (final_usage, bind') = occAnalBind env env imp_rules_edges bind bs_usage

occurAnalyseExpr :: CoreExpr -> CoreExpr
        -- Do occurrence analysis, and discard occurence info returned
occurAnalyseExpr expr 
  = snd (occAnal (initOccEnv all_active_rules) expr)
  where
    -- To be conservative, we say that all inlines and rules are active
    all_active_rules = \_ -> True
\end{code}


%************************************************************************
%*                                                                      *
\subsection[OccurAnal-main]{Counting occurrences: main function}
%*                                                                      *
%************************************************************************

Bindings
~~~~~~~~

\begin{code}
occAnalBind :: OccEnv 		-- The incoming OccEnv
	    -> OccEnv		-- Same, but trimmed by (binderOf bind)
            -> IdEnv IdSet      -- Mapping from FVs of imported RULE LHSs to RHS FVs
            -> CoreBind
            -> UsageDetails             -- Usage details of scope
            -> (UsageDetails,           -- Of the whole let(rec)
                [CoreBind])

occAnalBind env _ imp_rules_edges (NonRec binder rhs) body_usage
  | isTyVar binder	-- A type let; we don't gather usage info
  = (body_usage, [NonRec binder rhs])

  | not (binder `usedIn` body_usage)    -- It's not mentioned
  = (body_usage, [])

  | otherwise                   -- It's mentioned in the body
  = (body_usage' +++ rhs_usage4, [NonRec tagged_binder rhs'])
  where
    (body_usage', tagged_binder) = tagBinder body_usage binder
    (rhs_usage1, rhs')           = occAnalRhs env (Just tagged_binder) rhs
    rhs_usage2 = addIdOccs rhs_usage1 (idUnfoldingVars binder)
    rhs_usage3 = addIdOccs rhs_usage2 (idRuleVars binder)
       -- See Note [Rules are extra RHSs] and Note [Rule dependency info]
    rhs_usage4 = maybe rhs_usage3 (addIdOccs rhs_usage3) $ lookupVarEnv imp_rules_edges binder
       -- See Note [Preventing loops due to imported functions rules]

occAnalBind _ env imp_rules_edges (Rec pairs) body_usage
  = foldr occAnalRec (body_usage, []) sccs
	-- For a recursive group, we 
	--	* occ-analyse all the RHSs
	--	* compute strongly-connected components
	--	* feed those components to occAnalRec
  where
    bndr_set = mkVarSet (map fst pairs)

    sccs :: [SCC (Node Details)]
    sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompFromEdgedVerticesR nodes

    nodes :: [Node Details]
    nodes = {-# SCC "occAnalBind.assoc" #-} map (makeNode env imp_rules_edges bndr_set) pairs
\end{code}

Note [Dead code]
~~~~~~~~~~~~~~~~
Dropping dead code for recursive bindings is done in a very simple way:

        the entire set of bindings is dropped if none of its binders are
        mentioned in its body; otherwise none are.

This seems to miss an obvious improvement.

        letrec  f = ...g...
                g = ...f...
        in
        ...g...
===>
        letrec f = ...g...
               g = ...(...g...)...
        in
        ...g...

Now 'f' is unused! But it's OK!  Dependency analysis will sort this
out into a letrec for 'g' and a 'let' for 'f', and then 'f' will get
dropped.  It isn't easy to do a perfect job in one blow.  Consider

        letrec f = ...g...
               g = ...h...
               h = ...k...
               k = ...m...
               m = ...m...
        in
        ...m...


------------------------------------------------------------
Note [Forming Rec groups]
~~~~~~~~~~~~~~~~~~~~~~~~~
We put bindings {f = ef; g = eg } in a Rec group if "f uses g"
and "g uses f", no matter how indirectly.  We do a SCC analysis
with an edge f -> g if "f uses g".

More precisely, "f uses g" iff g should be in scope whereever f is.
That is, g is free in:
  a) the rhs 'ef'
  b) or the RHS of a rule for f (Note [Rules are extra RHSs])
  c) or the LHS or a rule for f (Note [Rule dependency info])

These conditions apply regardless of the activation of the RULE (eg it might be
inactive in this phase but become active later).  Once a Rec is broken up
it can never be put back together, so we must be conservative.

The principle is that, regardless of rule firings, every variale is
always in scope.

  * Note [Rules are extra RHSs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A RULE for 'f' is like an extra RHS for 'f'. That way the "parent"
    keeps the specialised "children" alive.  If the parent dies
    (because it isn't referenced any more), then the children will die
    too (unless they are already referenced directly).

    To that end, we build a Rec group for each cyclic strongly
    connected component,
        *treating f's rules as extra RHSs for 'f'*.
    More concretely, the SCC analysis runs on a graph with an edge
    from f -> g iff g is mentioned in
        (a) f's rhs
        (b) f's RULES
    These are rec_edges.

    Under (b) we include variables free in *either* LHS *or* RHS of
    the rule.  The former might seems silly, but see Note [Rule
    dependency info].  So in Example [eftInt], eftInt and eftIntFB
    will be put in the same Rec, even though their 'main' RHSs are
    both non-recursive.

  * Note [Rule dependency info]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The VarSet in a SpecInfo is used for dependency analysis in the
    occurrence analyser.  We must track free vars in *both* lhs and rhs.  
    Hence use of idRuleVars, rather than idRuleRhsVars in occAnalBind.
    Why both? Consider
        x = y
        RULE f x = v+4
    Then if we substitute y for x, we'd better do so in the
    rule's LHS too, so we'd better ensure the RULE appears to mention 'x'
    as well as 'v'

  * Note [Rules are visible in their own rec group]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want the rules for 'f' to be visible in f's right-hand side.
    And we'd like them to be visible in other functions in f's Rec
    group.  E.g. in Note [Specialisation rules] we want f' rule
    to be visible in both f's RHS, and fs's RHS.

    This means that we must simplify the RULEs first, before looking
    at any of the definitions.  This is done by Simplify.simplRecBind,
    when it calls addLetIdInfo.

------------------------------------------------------------
Note [Choosing loop breakers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Loop breaking is surprisingly subtle.  First read the section 4 of
"Secrets of the GHC inliner".  This describes our basic plan.
We avoid infinite inlinings by choosing loop breakers, and
ensuring that a loop breaker cuts each loop.  

Fundamentally, we do SCC analysis on a graph.  For each recursive
group we choose a loop breaker, delete all edges to that node, 
re-analyse the SCC, and iterate.

But what is the graph?  NOT the same graph as was used for Note
[Forming Rec groups]!  In particular, a RULE is like an equation for
'f' that is *always* inlined if it is applicable.  We do *not* disable
rules for loop-breakers.  It's up to whoever makes the rules to make
sure that the rules themselves always terminate.  See Note [Rules for
recursive functions] in Simplify.lhs

Hence, if
    f's RHS (or its INLINE template if it has one) mentions g, and
    g has a RULE that mentions h, and
    h has a RULE that mentions f

then we *must* choose f to be a loop breaker.  Example: see Note
[Specialisation rules].

In general, take the free variables of f's RHS, and augment it with
all the variables reachable by RULES from those starting points.  That
is the whole reason for computing rule_fv_env in occAnalBind.  (Of
course we only consider free vars that are also binders in this Rec
group.)  See also Note [Finding rule RHS free vars]

Note that when we compute this rule_fv_env, we only consider variables
free in the *RHS* of the rule, in contrast to the way we build the
Rec group in the first place (Note [Rule dependency info])

Note that if 'g' has RHS that mentions 'w', we should add w to
g's loop-breaker edges.  More concretely there is an edge from f -> g 
iff
	(a) g is mentioned in f's RHS `xor` f's INLINE rhs 
	    (see Note [Inline rules])
	(b) or h is mentioned in f's RHS, and 
            g appears in the RHS of an active RULE of h
            or a transitive sequence of active rules starting with h
	   
Why "active rules"?  See Note [Finding rule RHS free vars]

Note that in Example [eftInt], *neither* eftInt *nor* eftIntFB is
chosen as a loop breaker, because their RHSs don't mention each other.
And indeed both can be inlined safely.

Note again that the edges of the graph we use for computing loop breakers
are not the same as the edges we use for computing the Rec blocks.
That's why we compute

- rec_edges          for the Rec block analysis
- loop_breaker_edges for the loop breaker analysis

  * Note [Finding rule RHS free vars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this real example from Data Parallel Haskell
	 tagZero :: Array Int -> Array Tag
	 {-# INLINE [1] tagZeroes #-}
	 tagZero xs = pmap (\x -> fromBool (x==0)) xs

	 {-# RULES "tagZero" [~1] forall xs n.
	     pmap fromBool <blah blah> = tagZero xs #-}     
    So tagZero's RHS mentions pmap, and pmap's RULE mentions tagZero.
    However, tagZero can only be inlined in phase 1 and later, while
    the RULE is only active *before* phase 1.  So there's no problem.

    To make this work, we look for the RHS free vars only for
    *active* rules. That's the reason for the occ_rule_act field 
    of the OccEnv.
 
  * Note [Weak loop breakers]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    There is a last nasty wrinkle.  Suppose we have

        Rec { f = f_rhs
              RULE f [] = g

              h = h_rhs
              g = h
              ...more...
        }

    Remember that we simplify the RULES before any RHS (see Note
    [Rules are visible in their own rec group] above).

    So we must *not* postInlineUnconditionally 'g', even though
    its RHS turns out to be trivial.  (I'm assuming that 'g' is
    not choosen as a loop breaker.)  Why not?  Because then we
    drop the binding for 'g', which leaves it out of scope in the
    RULE!
  
    Here's a somewhat different example of the same thing
        Rec { g = h
            ; h = ...f...
            ; f = f_rhs
              RULE f [] = g }
    Here the RULE is "below" g, but we *still* can't postInlineUnconditionally
    g, because the RULE for f is active throughout.  So the RHS of h
    might rewrite to 	 h = ...g...
    So g must remain in scope in the output program!
    
    We "solve" this by:

        Make g a "weak" loop breaker (OccInfo = IAmLoopBreaker True)
        iff g is a "missing free variable" of the Rec group

    A "missing free variable" x is one that is mentioned in an RHS or
    INLINE or RULE of a binding in the Rec group, but where the
    dependency on x may not show up in the loop_breaker_edges (see
    note [Choosing loop breakers} above).

    A normal "strong" loop breaker has IAmLoopBreaker False.  So

                                Inline  postInlineUnconditionally
        IAmLoopBreaker False    no      no
        IAmLoopBreaker True     yes     no
        other                   yes     yes

    The **sole** reason for this kind of loop breaker is so that
    postInlineUnconditionally does not fire.  Ugh.  (Typically it'll
    inline via the usual callSiteInline stuff, so it'll be dead in the
    next pass, so the main Ugh is the tiresome complication.)

Note [Rules for imported functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this
   f = /\a. B.g a
   RULE B.g Int = 1 + f Int
Note that 
  * The RULE is for an imported function.  
  * f is non-recursive
Now we
can get
   f Int --> B.g Int	  Inlining f
         --> 1 + f Int    Firing RULE
and so the simplifier goes into an infinite loop. This 
would not happen if the RULE was for a local function,
because we keep track of dependencies through rules.  But
that is pretty much impossible to do for imported Ids.  Suppose
f's definition had been
   f = /\a. C.h a
where (by some long and devious process), C.h eventually inlines to
B.g.  We could only spot such loops by exhaustively following
unfoldings of C.h etc, in case we reach B.g, and hence (via the RULE)
f.

Note that RULES for imported functions are important in practice; they
occur a lot in the libraries.

We regard this potential infinite loop as a *programmer* error.
It's up the programmer not to write silly rules like
     RULE f x = f x
and the example above is just a more complicated version. 

Note [Preventing loops due to imported functions rules]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider:
  import GHC.Base (foldr)
  
  {-# RULES "filterList" forall p. foldr (filterFB (:) p) [] = filter p #-}
  filter p xs = build (\c n -> foldr (filterFB c p) n xs)
  filterFB c p = ...

  f = filter p xs

Note that filter is not a loop-breaker, so what happens is:
  f =          filter p xs
    = {inline} build (\c n -> foldr (filterFB c p) n xs)
    = {inline} foldr (filterFB (:) p) [] xs
    = {RULE}   filter p xs

We are in an infinite loop.

A more elaborate example (that I actually saw in practice when I went to
mark GHC.List.filter as INLINABLE) is as follows. Say I have this module:
  {-# LANGUAGE Rank2Types #-}
  module GHCList where
  
  import Prelude hiding (filter)
  import GHC.Base (build)
  
  {-# INLINABLE filter #-}
  filter :: (a -> Bool) -> [a] -> [a]
  filter p [] = []
  filter p (x:xs) = if p x then x : filter p xs else filter p xs
  
  {-# NOINLINE [0] filterFB #-}
  filterFB :: (a -> b -> b) -> (a -> Bool) -> a -> b -> b
  filterFB c p x r | p x       = x `c` r
                   | otherwise = r
  
  {-# RULES
  "filter"     [~1] forall p xs.  filter p xs = build (\c n -> foldr
  (filterFB c p) n xs)
  "filterList" [1]  forall p.     foldr (filterFB (:) p) [] = filter p
   #-}

Then (because RULES are applied inside INLINABLE unfoldings, but inlinings
are not), the unfolding given to "filter" in the interface file will be:
  filter p []     = []
  filter p (x:xs) = if p x then x : build (\c n -> foldr (filterFB c p) n xs)
                           else     build (\c n -> foldr (filterFB c p) n xs

Note that because this unfolding does not mention "filter", filter is not
marked as a strong loop breaker. Therefore at a use site in another module:
  filter p xs
    = {inline}
      case xs of []     -> []
                 (x:xs) -> if p x then x : build (\c n -> foldr (filterFB c p) n xs)
                                  else     build (\c n -> foldr (filterFB c p) n xs)

  build (\c n -> foldr (filterFB c p) n xs)
    = {inline} foldr (filterFB (:) p) [] xs
    = {RULE}   filter p xs

And we are in an infinite loop again, except that this time the loop is producing an
infinitely large *term* (an unrolling of filter) and so the simplifier finally
dies with "ticks exhausted"

Because of this problem, we make a small change in the occurrence analyser
designed to mark functions like "filter" as strong loop breakers on the basis that:
  1. The RHS of filter mentions the local function "filterFB"
  2. We have a rule which mentions "filterFB" on the LHS and "filter" on the RHS

So for each RULE for an *imported* function we are going to add
dependency edges between the *local* FVS of the rule LHS and the
*local* FVS of the rule RHS. We don't do anything special for RULES on
local functions because the standard occurrence analysis stuff is
pretty good at getting loop-breakerness correct there.

It is important to note that even with this extra hack we aren't always going to get
things right. For example, it might be that the rule LHS mentions an imported Id,
and another module has a RULE that can rewrite that imported Id to one of our local
Ids.

Note [Specialising imported functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
BUT for *automatically-generated* rules, the programmer can't be
responsible for the "programmer error" in Note [Rules for imported
functions].  In paricular, consider specialising a recursive function
defined in another module.  If we specialise a recursive function B.g,
we get
	 g_spec = .....(B.g Int).....
	 RULE B.g Int = g_spec 
Here, g_spec doesn't look recursive, but when the rule fires, it
becomes so.  And if B.g was mutually recursive, the loop might
not be as obvious as it is here.

To avoid this, 
 * When specialising a function that is a loop breaker, 
   give a NOINLINE pragma to the specialised function

Note [Glomming]
~~~~~~~~~~~~~~~
RULES for imported Ids can make something at the top refer to something at the bottom:
	f = \x -> B.g (q x)
	h = \y -> 3
	
	RULE:  B.g (q x) = h x

Applying this rule makes f refer to h, although f doesn't appear to
depend on h.  (And, as in Note [Rules for imported functions], the
dependency might be more indirect. For example, f might mention C.t
rather than B.g, where C.t eventually inlines to B.g.)

NOTICE that this cannot happen for rules whose head is a
locally-defined function, because we accurately track dependencies
through RULES.  It only happens for rules whose head is an imported
function (B.g in the example above).

Solution:
  - When simplifying, bring all top level identifiers into
    scope at the start, ignoring the Rec/NonRec structure, so 
    that when 'h' pops up in f's rhs, we find it in the in-scope set
    (as the simplifier generally expects). This happens in simplTopBinds.

  - In the occurrence analyser, if there are any out-of-scope
    occurrences that pop out of the top, which will happen after
    firing the rule:      f = \x -> h x
                          h = \y -> 3
    then just glom all the bindings into a single Rec, so that
    the *next* iteration of the occurrence analyser will sort 
    them all out.   This part happens in occurAnalysePgm.

------------------------------------------------------------
Note [Inline rules]
~~~~~~~~~~~~~~~~~~~
None of the above stuff about RULES applies to Inline Rules,
stored in a CoreUnfolding.  The unfolding, if any, is simplified
at the same time as the regular RHS of the function (ie *not* like
Note [Rules are visible in their own rec group]), so it should be
treated *exactly* like an extra RHS.

Or, rather, when computing loop-breaker edges,
  * If f has an INLINE pragma, and it is active, we treat the
    INLINE rhs as f's rhs
  * If it's inactive, we treat f as having no rhs
  * If it has no INLINE pragma, we look at f's actual rhs


There is a danger that we'll be sub-optimal if we see this
     f = ...f...
     [INLINE f = ..no f...]
where f is recursive, but the INLINE is not. This can just about
happen with a sufficiently odd set of rules; eg

	foo :: Int -> Int
	{-# INLINE [1] foo #-}
	foo x = x+1

	bar :: Int -> Int
	{-# INLINE [1] bar #-}
	bar x = foo x + 1

	{-# RULES "foo" [~1] forall x. foo x = bar x #-}

Here the RULE makes bar recursive; but it's INLINE pragma remains
non-recursive. It's tempting to then say that 'bar' should not be
a loop breaker, but an attempt to do so goes wrong in two ways:
   a) We may get
         $df = ...$cfoo...
         $cfoo = ...$df....
         [INLINE $cfoo = ...no-$df...]
      But we want $cfoo to depend on $df explicitly so that we
      put the bindings in the right order to inline $df in $cfoo
      and perhaps break the loop altogether.  (Maybe this
   b)


Example [eftInt]
~~~~~~~~~~~~~~~
Example (from GHC.Enum):

  eftInt :: Int# -> Int# -> [Int]
  eftInt x y = ...(non-recursive)...

  {-# INLINE [0] eftIntFB #-}
  eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
  eftIntFB c n x y = ...(non-recursive)...

  {-# RULES
  "eftInt"  [~1] forall x y. eftInt x y = build (\ c n -> eftIntFB c n x y)
  "eftIntList"  [1] eftIntFB  (:) [] = eftInt
   #-}

Note [Specialisation rules]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this group, which is typical of what SpecConstr builds:

   fs a = ....f (C a)....
   f  x = ....f (C a)....
   {-# RULE f (C a) = fs a #-}

So 'f' and 'fs' are in the same Rec group (since f refers to fs via its RULE).

But watch out!  If 'fs' is not chosen as a loop breaker, we may get an infinite loop:
  - the RULE is applied in f's RHS (see Note [Self-recursive rules] in Simplify
  - fs is inlined (say it's small)
  - now there's another opportunity to apply the RULE

This showed up when compiling Control.Concurrent.Chan.getChanContents.


\begin{code}
type Node details = (details, Unique, [Unique])	-- The Ints are gotten from the Unique,
						-- which is gotten from the Id.
data Details
  = ND { nd_bndr :: Id          -- Binder
       , nd_rhs  :: CoreExpr    -- RHS, already occ-analysed

       , nd_uds  :: UsageDetails  -- Usage from RHS, and RULES, and InlineRule unfolding
       	 	    		  -- ignoring phase (ie assuming all are active)
				  -- See Note [Forming Rec groups]

       , nd_inl  :: IdSet       -- Free variables of
                                --   the InlineRule (if present and active)
                                --   or the RHS (ir no InlineRule)
                                -- but excluding any RULES
                                -- This is the IdSet that may be used if the Id is inlined

       , nd_weak :: IdSet       -- Binders of this Rec that are mentioned in nd_uds
       	 	       		-- but are *not* in nd_inl.  These are the ones whose
				-- dependencies might not be respected by loop_breaker_edges
				-- See Note [Weak loop breakers]
  
       , nd_active_rule_fvs :: IdSet   -- Free variables of the RHS of active RULES
  }

instance Outputable Details where
   ppr nd = ptext (sLit "ND") <> braces 
             (sep [ ptext (sLit "bndr =") <+> ppr (nd_bndr nd)
                  , ptext (sLit "uds =") <+> ppr (nd_uds nd)
                  , ptext (sLit "inl =") <+> ppr (nd_inl nd)
                  , ptext (sLit "weak =") <+> ppr (nd_weak nd)
                  , ptext (sLit "rule =") <+> ppr (nd_active_rule_fvs nd)
	     ])

makeNode :: OccEnv -> IdEnv IdSet -> VarSet -> (Var, CoreExpr) -> Node Details
makeNode env imp_rules_edges bndr_set (bndr, rhs)
  = (details, varUnique bndr, keysUFM node_fvs)
  where
    details = ND { nd_bndr = bndr
                 , nd_rhs  = rhs'
                 , nd_uds  = rhs_usage3
		 , nd_weak = node_fvs `minusVarSet` inl_fvs
                 , nd_inl  = inl_fvs
                 , nd_active_rule_fvs = active_rule_fvs }

    -- Constructing the edges for the main Rec computation
    -- See Note [Forming Rec groups]
    (rhs_usage1, rhs') = occAnalRhs env Nothing rhs
    rhs_usage2 = addIdOccs rhs_usage1 all_rule_fvs   -- Note [Rules are extra RHSs]
                                                     -- Note [Rule dependency info]
    rhs_usage3 = case mb_unf_fvs of
                   Just unf_fvs -> addIdOccs rhs_usage2 unf_fvs
                   Nothing      -> rhs_usage2
    node_fvs = udFreeVars bndr_set rhs_usage3

    -- Finding the free variables of the rules
    is_active = occ_rule_act env :: Activation -> Bool
    rules = filterOut isBuiltinRule (idCoreRules bndr)
    rules_w_fvs :: [(Activation, VarSet)]    -- Find the RHS fvs
    rules_w_fvs = maybe id (\ids -> ((AlwaysActive, ids):)) (lookupVarEnv imp_rules_edges bndr)
                   -- See Note [Preventing loops due to imported functions rules]
                  [ (ru_act rule, fvs)
                  | rule <- rules
                  , let fvs = exprFreeVars (ru_rhs rule)
		    	      `delVarSetList` ru_bndrs rule
                  , not (isEmptyVarSet fvs) ]
    all_rule_fvs = foldr (unionVarSet . snd) rule_lhs_fvs rules_w_fvs
    rule_lhs_fvs = foldr (unionVarSet . (\ru -> exprsFreeVars (ru_args ru)
                                                `delVarSetList` ru_bndrs ru))
                         emptyVarSet rules
    active_rule_fvs = unionVarSets [fvs | (a,fvs) <- rules_w_fvs, is_active a]

    -- Finding the free variables of the INLINE pragma (if any)
    unf        = realIdUnfolding bndr     -- Ignore any current loop-breaker flag
    mb_unf_fvs = stableUnfoldingVars isLocalId unf

    -- Find the "nd_inl" free vars; for the loop-breaker phase
    inl_fvs = case mb_unf_fvs of
                Nothing	-> udFreeVars bndr_set rhs_usage1 -- No INLINE, use RHS
                Just unf_fvs -> unf_fvs	
                      -- We could check for an *active* INLINE (returning
		      -- emptyVarSet for an inactive one), but is_active
		      -- isn't the right thing (it tells about
		      -- RULE activation), so we'd need more plumbing

-----------------------------
occAnalRec :: SCC (Node Details)
           -> (UsageDetails, [CoreBind])
	   -> (UsageDetails, [CoreBind])

	-- The NonRec case is just like a Let (NonRec ...) above
occAnalRec (AcyclicSCC (ND { nd_bndr = bndr, nd_rhs = rhs, nd_uds = rhs_uds}, _, _))
           (body_uds, binds)
  | not (bndr `usedIn` body_uds) 
  = (body_uds, binds)

  | otherwise			-- It's mentioned in the body
  = (body_uds' +++ rhs_uds,	
     NonRec tagged_bndr rhs : binds)
  where
    (body_uds', tagged_bndr) = tagBinder body_uds bndr

	-- The Rec case is the interesting one
	-- See Note [Loop breaking]
occAnalRec (CyclicSCC nodes) (body_uds, binds)
  | not (any (`usedIn` body_uds) bndrs)	-- NB: look at body_uds, not total_uds
  = (body_uds, binds)				-- Dead code

  | otherwise	-- At this point we always build a single Rec
  = -- pprTrace "occAnalRec" (vcat
    --   [ text "tagged nodes" <+> ppr tagged_nodes
    --   , text "lb edges" <+> ppr loop_breaker_edges])
    (final_uds, Rec pairs : binds)

  where
    bndrs    = [b | (ND { nd_bndr = b }, _, _) <- nodes]
    bndr_set = mkVarSet bndrs

	----------------------------
	-- Tag the binders with their occurrence info
    tagged_nodes = map tag_node nodes
    total_uds = foldl add_uds body_uds nodes
    final_uds = total_uds `minusVarEnv` bndr_set
    add_uds usage_so_far (nd, _, _) = usage_so_far +++ nd_uds nd

    tag_node :: Node Details -> Node Details
    tag_node (details@ND { nd_bndr = bndr }, k, ks)
      = (details { nd_bndr = setBinderOcc total_uds bndr }, k, ks)

    ---------------------------
    -- Now reconstruct the cycle
    pairs :: [(Id,CoreExpr)]
    pairs | isEmptyVarSet weak_fvs = reOrderNodes   0 bndr_set weak_fvs tagged_nodes       []
          | otherwise              = loopBreakNodes 0 bndr_set weak_fvs loop_breaker_edges []
	  -- If weak_fvs is empty, the loop_breaker_edges will include all 
	  -- the edges in tagged_nodes, so there isn't any point in doing 
	  -- a fresh SCC computation that will yield a single CyclicSCC result.

    weak_fvs :: VarSet
    weak_fvs = foldr (unionVarSet . nd_weak . fstOf3) emptyVarSet nodes

	-- See Note [Choosing loop breakers] for loop_breaker_edges
    loop_breaker_edges = map mk_node tagged_nodes
    mk_node (details@(ND { nd_inl = inl_fvs }), k, _) 
      = (details, k, keysUFM (extendFvs_ rule_fv_env inl_fvs))

    ------------------------------------
    rule_fv_env :: IdEnv IdSet  
        -- Maps a variable f to the variables from this group 
        --      mentioned in RHS of active rules for f
        -- Domain is *subset* of bound vars (others have no rule fvs)
    rule_fv_env = transClosureFV (mkVarEnv init_rule_fvs)
    init_rule_fvs   -- See Note [Finding rule RHS free vars]
      = [ (b, trimmed_rule_fvs)
        | (ND { nd_bndr = b, nd_active_rule_fvs = rule_fvs },_,_) <- nodes
        , let trimmed_rule_fvs = rule_fvs `intersectVarSet` bndr_set
        , not (isEmptyVarSet trimmed_rule_fvs)]
\end{code}

@loopBreakSCC@ is applied to the list of (binder,rhs) pairs for a cyclic
strongly connected component (there's guaranteed to be a cycle).  It returns the
same pairs, but
        a) in a better order,
        b) with some of the Ids having a IAmALoopBreaker pragma

The "loop-breaker" Ids are sufficient to break all cycles in the SCC.  This means
that the simplifier can guarantee not to loop provided it never records an inlining
for these no-inline guys.

Furthermore, the order of the binds is such that if we neglect dependencies
on the no-inline Ids then the binds are topologically sorted.  This means
that the simplifier will generally do a good job if it works from top bottom,
recording inlinings for any Ids which aren't marked as "no-inline" as it goes.

\begin{code}
type Binding = (Id,CoreExpr)

mk_loop_breaker :: Node Details -> Binding
mk_loop_breaker (ND { nd_bndr = bndr, nd_rhs = rhs}, _, _) 
  = (setIdOccInfo bndr strongLoopBreaker, rhs)

mk_non_loop_breaker :: VarSet -> Node Details -> Binding
-- See Note [Weak loop breakers]
mk_non_loop_breaker used_in_rules (ND { nd_bndr = bndr, nd_rhs = rhs}, _, _) 
  | bndr `elemVarSet` used_in_rules = (setIdOccInfo bndr weakLoopBreaker, rhs)
  | otherwise                       = (bndr, rhs)

udFreeVars :: VarSet -> UsageDetails -> VarSet
-- Find the subset of bndrs that are mentioned in uds
udFreeVars bndrs uds = intersectUFM_C (\b _ -> b) bndrs uds

loopBreakNodes :: Int 
	       -> VarSet	-- All binders
               -> VarSet	-- Binders whose dependencies may be "missing"
	       	  		-- See Note [Weak loop breakers]
               -> [Node Details]
               -> [Binding]	        -- Append these to the end
               -> [Binding]
-- Return the bindings sorted into a plausible order, and marked with loop breakers.  
loopBreakNodes depth bndr_set weak_fvs nodes binds
  = go (stronglyConnCompFromEdgedVerticesR nodes) binds
  where
    go []         binds = binds
    go (scc:sccs) binds = loop_break_scc scc (go sccs binds)

    loop_break_scc scc binds
      = case scc of
          AcyclicSCC node  -> mk_non_loop_breaker weak_fvs node : binds
          CyclicSCC [node] -> mk_loop_breaker node : binds
          CyclicSCC nodes  -> reOrderNodes depth bndr_set weak_fvs nodes binds

reOrderNodes :: Int -> VarSet -> VarSet -> [Node Details] -> [Binding] -> [Binding]
    -- Choose a loop breaker, mark it no-inline,
    -- do SCC analysis on the rest, and recursively sort them out
reOrderNodes _ _ _ [] _  = panic "reOrderNodes"
reOrderNodes depth bndr_set weak_fvs (node : nodes) binds
  = -- pprTrace "reOrderNodes" (text "unchosen" <+> ppr unchosen $$ 
    --                           text "chosen" <+> ppr chosen_nodes) $
    loopBreakNodes new_depth bndr_set weak_fvs unchosen $
    (map mk_loop_breaker chosen_nodes ++ binds)
  where
    (chosen_nodes, unchosen) = choose_loop_breaker (score node) [node] [] nodes

    approximate_loop_breaker = depth >= 2
    new_depth | approximate_loop_breaker = 0
	      | otherwise		 = depth+1
	-- After two iterations (d=0, d=1) give up
	-- and approximate, returning to d=0

    choose_loop_breaker :: Int			-- Best score so far
                        -> [Node Details]	-- Nodes with this score
                        -> [Node Details] 	-- Nodes with higher scores
                        -> [Node Details]	-- Unprocessed nodes
                        -> ([Node Details], [Node Details])
        -- This loop looks for the bind with the lowest score
        -- to pick as the loop  breaker.  The rest accumulate in
    choose_loop_breaker _ loop_nodes acc []
        = (loop_nodes, acc)        -- Done

	-- If approximate_loop_breaker is True, we pick *all*
	-- nodes with lowest score, else just one
	-- See Note [Complexity of loop breaking]
    choose_loop_breaker loop_sc loop_nodes acc (node : nodes)
        | sc < loop_sc  -- Lower score so pick this new one
        = choose_loop_breaker sc [node] (loop_nodes ++ acc) nodes

	| approximate_loop_breaker && sc == loop_sc
	= choose_loop_breaker loop_sc (node : loop_nodes) acc nodes
	
        | otherwise     -- Higher score so don't pick it
        = choose_loop_breaker loop_sc loop_nodes (node : acc) nodes
        where
          sc = score node

    score :: Node Details -> Int        -- Higher score => less likely to be picked as loop breaker
    score (ND { nd_bndr = bndr, nd_rhs = rhs }, _, _)
        | not (isId bndr) = 100	    -- A type or cercion variable is never a loop breaker

        | isDFunId bndr = 9   -- Never choose a DFun as a loop breaker
	   	     	      -- Note [DFuns should not be loop breakers]

        | Just inl_source <- isStableCoreUnfolding_maybe (idUnfolding bndr)
	= case inl_source of
	     InlineWrapper {} -> 10  -- Note [INLINE pragmas]
	     _other	      ->  3  -- Data structures are more important than this
	     		             -- so that dictionary/method recursion unravels
		-- Note that this case hits all InlineRule things, so we
		-- never look at 'rhs' for InlineRule stuff. That's right, because
		-- 'rhs' is irrelevant for inlining things with an InlineRule
                
        | is_con_app rhs = 5  -- Data types help with cases: Note [Constructor applications]
                
        | exprIsTrivial rhs = 10  -- Practically certain to be inlined
                -- Used to have also: && not (isExportedId bndr)
                -- But I found this sometimes cost an extra iteration when we have
                --      rec { d = (a,b); a = ...df...; b = ...df...; df = d }
                -- where df is the exported dictionary. Then df makes a really
                -- bad choice for loop breaker

	
-- If an Id is marked "never inline" then it makes a great loop breaker
-- The only reason for not checking that here is that it is rare
-- and I've never seen a situation where it makes a difference,
-- so it probably isn't worth the time to test on every binder
--	| isNeverActive (idInlinePragma bndr) = -10

        | isOneOcc (idOccInfo bndr) = 2  -- Likely to be inlined

        | canUnfold (realIdUnfolding bndr) = 1
                -- The Id has some kind of unfolding
		-- Ignore loop-breaker-ness here because that is what we are setting!

        | otherwise = 0

	-- Checking for a constructor application
        -- Cheap and cheerful; the simplifer moves casts out of the way
        -- The lambda case is important to spot x = /\a. C (f a)
        -- which comes up when C is a dictionary constructor and
        -- f is a default method.
        -- Example: the instance for Show (ST s a) in GHC.ST
        --
        -- However we *also* treat (\x. C p q) as a con-app-like thing,
        --      Note [Closure conversion]
    is_con_app (Var v)    = isConLikeId v
    is_con_app (App f _)  = is_con_app f
    is_con_app (Lam _ e)  = is_con_app e
    is_con_app (Tick _ e) = is_con_app e
    is_con_app _          = False
\end{code}

Note [Complexity of loop breaking]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The loop-breaking algorithm knocks out one binder at a time, and 
performs a new SCC analysis on the remaining binders.  That can
behave very badly in tightly-coupled groups of bindings; in the
worst case it can be (N**2)*log N, because it does a full SCC
on N, then N-1, then N-2 and so on.

To avoid this, we switch plans after 2 (or whatever) attempts:
  Plan A: pick one binder with the lowest score, make it
	  a loop breaker, and try again
  Plan B: pick *all* binders with the lowest score, make them
	  all loop breakers, and try again 
Since there are only a small finite number of scores, this will
terminate in a constant number of iterations, rather than O(N)
iterations.

You might thing that it's very unlikely, but RULES make it much
more likely.  Here's a real example from Trac #1969:
  Rec { $dm = \d.\x. op d
	{-# RULES forall d. $dm Int d  = $s$dm1
		  forall d. $dm Bool d = $s$dm2 #-}
	
	dInt = MkD .... opInt ...
	dInt = MkD .... opBool ...
	opInt  = $dm dInt
	opBool = $dm dBool

	$s$dm1 = \x. op dInt
	$s$dm2 = \x. op dBool }
The RULES stuff means that we can't choose $dm as a loop breaker
(Note [Choosing loop breakers]), so we must choose at least (say)
opInt *and* opBool, and so on.  The number of loop breakders is
linear in the number of instance declarations.

Note [INLINE pragmas]
~~~~~~~~~~~~~~~~~~~~~
Avoid choosing a function with an INLINE pramga as the loop breaker!  
If such a function is mutually-recursive with a non-INLINE thing,
then the latter should be the loop-breaker.

Usually this is just a question of optimisation. But a particularly
bad case is wrappers generated by the demand analyser: if you make
then into a loop breaker you may get an infinite inlining loop.  For
example:
  rec {
        $wfoo x = ....foo x....

        {-loop brk-} foo x = ...$wfoo x...
  }
The interface file sees the unfolding for $wfoo, and sees that foo is
strict (and hence it gets an auto-generated wrapper).  Result: an
infinite inlining in the importing scope.  So be a bit careful if you
change this.  A good example is Tree.repTree in
nofib/spectral/minimax. If the repTree wrapper is chosen as the loop
breaker then compiling Game.hs goes into an infinite loop.  This
happened when we gave is_con_app a lower score than inline candidates:

  Tree.repTree
    = __inline_me (/\a. \w w1 w2 -> 
                   case Tree.$wrepTree @ a w w1 w2 of
                    { (# ww1, ww2 #) -> Branch @ a ww1 ww2 })
  Tree.$wrepTree
    = /\a w w1 w2 -> 
      (# w2_smP, map a (Tree a) (Tree.repTree a w1 w) (w w2) #)

Here we do *not* want to choose 'repTree' as the loop breaker.

Note [DFuns should not be loop breakers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's particularly bad to make a DFun into a loop breaker.  See
Note [How instance declarations are translated] in TcInstDcls

We give DFuns a higher score than ordinary CONLIKE things because 
if there's a choice we want the DFun to be the non-looop breker. Eg
 
rec { sc = /\ a \$dC. $fBWrap (T a) ($fCT @ a $dC)

      $fCT :: forall a_afE. (Roman.C a_afE) => Roman.C (Roman.T a_afE)
      {-# DFUN #-}
      $fCT = /\a \$dC. MkD (T a) ((sc @ a $dC) |> blah) ($ctoF @ a $dC)
    }

Here 'sc' (the superclass) looks CONLIKE, but we'll never get to it
if we can't unravel the DFun first.

Note [Constructor applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's really really important to inline dictionaries.  Real
example (the Enum Ordering instance from GHC.Base):

     rec     f = \ x -> case d of (p,q,r) -> p x
             g = \ x -> case d of (p,q,r) -> q x
             d = (v, f, g)

Here, f and g occur just once; but we can't inline them into d.
On the other hand we *could* simplify those case expressions if
we didn't stupidly choose d as the loop breaker.
But we won't because constructor args are marked "Many".
Inlining dictionaries is really essential to unravelling
the loops in static numeric dictionaries, see GHC.Float.

Note [Closure conversion]
~~~~~~~~~~~~~~~~~~~~~~~~~
We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm.
The immediate motivation came from the result of a closure-conversion transformation
which generated code like this:

    data Clo a b = forall c. Clo (c -> a -> b) c

    ($:) :: Clo a b -> a -> b
    Clo f env $: x = f env x

    rec { plus = Clo plus1 ()

        ; plus1 _ n = Clo plus2 n

        ; plus2 Zero     n = n
        ; plus2 (Succ m) n = Succ (plus $: m $: n) }

If we inline 'plus' and 'plus1', everything unravels nicely.  But if
we choose 'plus1' as the loop breaker (which is entirely possible
otherwise), the loop does not unravel nicely.


@occAnalRhs@ deals with the question of bindings where the Id is marked
by an INLINE pragma.  For these we record that anything which occurs
in its RHS occurs many times.  This pessimistically assumes that ths
inlined binder also occurs many times in its scope, but if it doesn't
we'll catch it next time round.  At worst this costs an extra simplifier pass.
ToDo: try using the occurrence info for the inline'd binder.

[March 97] We do the same for atomic RHSs.  Reason: see notes with loopBreakSCC.
[June 98, SLPJ]  I've undone this change; I don't understand it.  See notes with loopBreakSCC.


\begin{code}
occAnalRhs :: OccEnv
           -> Maybe Id -> CoreExpr    -- Binder and rhs
                 -- Just b  => non-rec, and alrady tagged with occurrence info
                 -- Nothing => Rec, no occ info
           -> (UsageDetails, CoreExpr)
              -- Returned usage details covers only the RHS,
              -- and *not* the RULE or INLINE template for the Id
occAnalRhs env mb_bndr rhs
  = occAnal ctxt rhs
  where
    -- See Note [Cascading inlines]
    ctxt = case mb_bndr of
             Just b | certainly_inline b -> env
             _other                      -> rhsCtxt env

    certainly_inline bndr  -- See Note [Cascading inlines]
      = case idOccInfo bndr of
          OneOcc in_lam one_br _ -> not in_lam && one_br && active && not_stable
          _                      -> False
      where
        active     = isAlwaysActive (idInlineActivation bndr)
        not_stable = not (isStableUnfolding (idUnfolding bndr))

addIdOccs :: UsageDetails -> VarSet -> UsageDetails
addIdOccs usage id_set = foldVarSet add usage id_set
  where
    add v u | isId v    = addOneOcc u v NoOccInfo
            | otherwise = u
	-- Give a non-committal binder info (i.e NoOccInfo) because
	--   a) Many copies of the specialised thing can appear
	--   b) We don't want to substitute a BIG expression inside a RULE
	--	even if that's the only occurrence of the thing
	--	(Same goes for INLINE.)
\end{code}

Note [Cascading inlines]
~~~~~~~~~~~~~~~~~~~~~~~~
By default we use an rhsCtxt for the RHS of a binding.  This tells the
occ anal n that it's looking at an RHS, which has an effect in
occAnalApp.  In particular, for constructor applications, it makes
the arguments appear to have NoOccInfo, so that we don't inline into
them. Thus    x = f y
              k = Just x
we do not want to inline x.

But there's a problem.  Consider
     x1 = a0 : []
     x2 = a1 : x1
     x3 = a2 : x2
     g  = f x3
First time round, it looks as if x1 and x2 occur as an arg of a
let-bound constructor ==> give them a many-occurrence.
But then x3 is inlined (unconditionally as it happens) and
next time round, x2 will be, and the next time round x1 will be
Result: multiple simplifier iterations.  Sigh.

So, when analysing the RHS of x3 we notice that x3 will itself
definitely inline the next time round, and so we analyse x3's rhs in
an ordinary context, not rhsCtxt.  Hence the "certainly_inline" stuff.

Annoyingly, we have to approximiate SimplUtils.preInlineUnconditionally.
If we say "yes" when preInlineUnconditionally says "no" the simplifier iterates
indefinitely:
        x = f y
        k = Just x
inline ==>
        k = Just (f y)
float ==>
        x1 = f y
        k = Just x1

This is worse than the slow cascade, so we only want to say "certainly_inline"
if it really is certain.  Look at the note with preInlineUnconditionally
for the various clauses.

Expressions
~~~~~~~~~~~
\begin{code}
occAnal :: OccEnv
        -> CoreExpr
        -> (UsageDetails,       -- Gives info only about the "interesting" Ids
            CoreExpr)

occAnal _   expr@(Type _) = (emptyDetails, 	   expr)
occAnal _   expr@(Lit _)  = (emptyDetails, 	   expr)
occAnal env expr@(Var v)  = (mkOneOcc env v False, expr)
    -- At one stage, I gathered the idRuleVars for v here too,
    -- which in a way is the right thing to do.
    -- But that went wrong right after specialisation, when
    -- the *occurrences* of the overloaded function didn't have any
    -- rules in them, so the *specialised* versions looked as if they
    -- weren't used at all.

occAnal _ (Coercion co) 
  = (addIdOccs emptyDetails (coVarsOfCo co), Coercion co)
	-- See Note [Gather occurrences of coercion veriables]
\end{code}

Note [Gather occurrences of coercion veriables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We need to gather info about what coercion variables appear, so that
we can sort them into the right place when doing dependency analysis.

\begin{code}
occAnal env (Tick tickish body)
  | Breakpoint _ ids <- tickish
  = (mapVarEnv markInsideSCC usage
         +++ mkVarEnv (zip ids (repeat NoOccInfo)), Tick tickish body')
    -- never substitute for any of the Ids in a Breakpoint

  | tickishScoped tickish
  = (mapVarEnv markInsideSCC usage, Tick tickish body')

  | otherwise
  = (usage, Tick tickish body')
  where
    !(usage,body') = occAnal env body

occAnal env (Cast expr co)
  = case occAnal env expr of { (usage, expr') ->
    let usage1 = markManyIf (isRhsEnv env) usage
        usage2 = addIdOccs usage1 (coVarsOfCo co)
          -- See Note [Gather occurrences of coercion veriables]
    in (usage2, Cast expr' co)
        -- If we see let x = y `cast` co
        -- then mark y as 'Many' so that we don't
        -- immediately inline y again.
    }
\end{code}

\begin{code}
occAnal env app@(App _ _)
  = occAnalApp env (collectArgs app)

-- Ignore type variables altogether
--   (a) occurrences inside type lambdas only not marked as InsideLam
--   (b) type variables not in environment

occAnal env (Lam x body) | isTyVar x
  = case occAnal env body of { (body_usage, body') ->
    (body_usage, Lam x body')
    }

-- For value lambdas we do a special hack.  Consider
--      (\x. \y. ...x...)
-- If we did nothing, x is used inside the \y, so would be marked
-- as dangerous to dup.  But in the common case where the abstraction
-- is applied to two arguments this is over-pessimistic.
-- So instead, we just mark each binder with its occurrence
-- info in the *body* of the multiple lambda.
-- Then, the simplifier is careful when partially applying lambdas.

occAnal env expr@(Lam _ _)
  = case occAnal env_body body of { (body_usage, body') ->
    let
        (final_usage, tagged_binders) = tagLamBinders body_usage binders'
		      -- Use binders' to put one-shot info on the lambdas

        --      URGH!  Sept 99: we don't seem to be able to use binders' here, because
        --      we get linear-typed things in the resulting program that we can't handle yet.
        --      (e.g. PrelShow)  TODO

        really_final_usage = if linear then
                                final_usage
                             else
                                mapVarEnv markInsideLam final_usage
    in
    (really_final_usage,
     mkLams tagged_binders body') }
  where
    env_body        = vanillaCtxt (trimOccEnv env binders)
		        -- Body is (no longer) an RhsContext
    (binders, body) = collectBinders expr
    binders'        = oneShotGroup env binders
    linear          = all is_one_shot binders'
    is_one_shot b   = isId b && isOneShotBndr b

occAnal env (Case scrut bndr ty alts)
  = case occ_anal_scrut scrut alts     of { (scrut_usage, scrut') ->
    case mapAndUnzip occ_anal_alt alts of { (alts_usage_s, alts')   ->
    let
        alts_usage  = foldr combineAltsUsageDetails emptyDetails alts_usage_s
        (alts_usage1, tagged_bndr) = tag_case_bndr alts_usage…