[ghc-hetmet.git] / compiler / coreSyn / CoreUnfold.lhs

%
% (c) The University of Glasgow 2006
% (c) The AQUA Project, Glasgow University, 1994-1998
%

Core-syntax unfoldings

Unfoldings (which can travel across module boundaries) are in Core
syntax (namely @CoreExpr@s).

The type @Unfolding@ sits ``above'' simply-Core-expressions
unfoldings, capturing ``higher-level'' things we know about a binding,
usually things that the simplifier found out (e.g., ``it's a
literal'').  In the corner of a @CoreUnfolding@ unfolding, you will
find, unsurprisingly, a Core expression.

\begin{code}
{-# OPTIONS -w #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
-- for details

module CoreUnfold (
        Unfolding, UnfoldingGuidance,   -- Abstract types

        noUnfolding, mkTopUnfolding, mkUnfolding, mkCompulsoryUnfolding, seqUnfolding,
        evaldUnfolding, mkOtherCon, otherCons,
        unfoldingTemplate, maybeUnfoldingTemplate,
        isEvaldUnfolding, isValueUnfolding, isCheapUnfolding, isCompulsoryUnfolding,
        hasUnfolding, hasSomeUnfolding, neverUnfold,

        couldBeSmallEnoughToInline, 
        certainlyWillInline, smallEnoughToInline,

        callSiteInline, CallContInfo(..)

    ) where

#include "HsVersions.h"

import StaticFlags
import DynFlags
import CoreSyn
import PprCore          ()      -- Instances
import OccurAnal
import CoreUtils
import Id
import DataCon
import Literal
import PrimOp
import IdInfo
import Type
import PrelNames
import Bag
import FastTypes
import Outputable

import GHC.Exts         ( Int# )
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Making unfoldings}
%*                                                                      *
%************************************************************************

\begin{code}
mkTopUnfolding expr = mkUnfolding True {- Top level -} expr

mkUnfolding top_lvl expr
  = CoreUnfolding (occurAnalyseExpr expr)
                  top_lvl

                  (exprIsHNF expr)
                        -- Already evaluated

                  (exprIsCheap expr)
                        -- OK to inline inside a lambda

                  (calcUnfoldingGuidance opt_UF_CreationThreshold expr)
        -- Sometimes during simplification, there's a large let-bound thing     
        -- which has been substituted, and so is now dead; so 'expr' contains
        -- two copies of the thing while the occurrence-analysed expression doesn't
        -- Nevertheless, we don't occ-analyse before computing the size because the
        -- size computation bales out after a while, whereas occurrence analysis does not.
        --
        -- This can occasionally mean that the guidance is very pessimistic;
        -- it gets fixed up next round

instance Outputable Unfolding where
  ppr NoUnfolding = ptext SLIT("No unfolding")
  ppr (OtherCon cs) = ptext SLIT("OtherCon") <+> ppr cs
  ppr (CompulsoryUnfolding e) = ptext SLIT("Compulsory") <+> ppr e
  ppr (CoreUnfolding e top hnf cheap g) 
        = ptext SLIT("Unf") <+> sep [ppr top <+> ppr hnf <+> ppr cheap <+> ppr g, 
                                     ppr e]

mkCompulsoryUnfolding expr      -- Used for things that absolutely must be unfolded
  = CompulsoryUnfolding (occurAnalyseExpr expr)
\end{code}


%************************************************************************
%*                                                                      *
\subsection{The UnfoldingGuidance type}
%*                                                                      *
%************************************************************************

\begin{code}
instance Outputable UnfoldingGuidance where
    ppr UnfoldNever     = ptext SLIT("NEVER")
    ppr (UnfoldIfGoodArgs v cs size discount)
      = hsep [ ptext SLIT("IF_ARGS"), int v,
               brackets (hsep (map int cs)),
               int size,
               int discount ]
\end{code}


\begin{code}
calcUnfoldingGuidance
        :: Int                  -- bomb out if size gets bigger than this
        -> CoreExpr             -- expression to look at
        -> UnfoldingGuidance
calcUnfoldingGuidance bOMB_OUT_SIZE expr
  = case collect_val_bndrs expr of { (inline, val_binders, body) ->
    let
        n_val_binders = length val_binders

        max_inline_size = n_val_binders+2
        -- The idea is that if there is an INLINE pragma (inline is True)
        -- and there's a big body, we give a size of n_val_binders+2.  This
        -- This is just enough to fail the no-size-increase test in callSiteInline,
        --   so that INLINE things don't get inlined into entirely boring contexts,
        --   but no more.

    in
    case (sizeExpr (iUnbox bOMB_OUT_SIZE) val_binders body) of

      TooBig 
        | not inline -> UnfoldNever
                -- A big function with an INLINE pragma must
                -- have an UnfoldIfGoodArgs guidance
        | otherwise  -> UnfoldIfGoodArgs n_val_binders
                                         (map (const 0) val_binders)
                                         max_inline_size 0

      SizeIs size cased_args scrut_discount
        -> UnfoldIfGoodArgs
                        n_val_binders
                        (map discount_for val_binders)
                        final_size
                        (iBox scrut_discount)
        where        
            boxed_size    = iBox size

            final_size | inline     = boxed_size `min` max_inline_size
                       | otherwise  = boxed_size

                -- Sometimes an INLINE thing is smaller than n_val_binders+2.
                -- A particular case in point is a constructor, which has size 1.
                -- We want to inline this regardless, hence the `min`

            discount_for b = foldlBag (\acc (b',n) -> if b==b' then acc+n else acc) 
                                      0 cased_args
        }
  where
    collect_val_bndrs e = go False [] e
        -- We need to be a bit careful about how we collect the
        -- value binders.  In ptic, if we see 
        --      __inline_me (\x y -> e)
        -- We want to say "2 value binders".  Why?  So that 
        -- we take account of information given for the arguments

    go inline rev_vbs (Note InlineMe e)     = go True   rev_vbs     e
    go inline rev_vbs (Lam b e) | isId b    = go inline (b:rev_vbs) e
                                | otherwise = go inline rev_vbs     e
    go inline rev_vbs e                     = (inline, reverse rev_vbs, e)
\end{code}

\begin{code}
sizeExpr :: Int#            -- Bomb out if it gets bigger than this
         -> [Id]            -- Arguments; we're interested in which of these
                            -- get case'd
         -> CoreExpr
         -> ExprSize

sizeExpr bOMB_OUT_SIZE top_args expr
  = size_up expr
  where
    size_up (Type t)          = sizeZero        -- Types cost nothing
    size_up (Var v)           = sizeOne

    size_up (Note InlineMe body) = sizeOne      -- Inline notes make it look very small
        -- This can be important.  If you have an instance decl like this:
        --      instance Foo a => Foo [a] where
        --         {-# INLINE op1, op2 #-}
        --         op1 = ...
        --         op2 = ...
        -- then we'll get a dfun which is a pair of two INLINE lambdas

    size_up (Note _        body) = size_up body -- Other notes cost nothing
    
    size_up (Cast e _)           = size_up e

    size_up (App fun (Type t)) = size_up fun
    size_up (App fun arg)      = size_up_app fun [arg]

    size_up (Lit lit)          = sizeN (litSize lit)

    size_up (Lam b e) | isId b    = lamScrutDiscount (size_up e `addSizeN` 1)
                      | otherwise = size_up e

    size_up (Let (NonRec binder rhs) body)
      = nukeScrutDiscount (size_up rhs)         `addSize`
        size_up body                            `addSizeN`
        (if isUnLiftedType (idType binder) then 0 else 1)
                -- For the allocation
                -- If the binder has an unlifted type there is no allocation

    size_up (Let (Rec pairs) body)
      = nukeScrutDiscount rhs_size              `addSize`
        size_up body                            `addSizeN`
        length pairs            -- For the allocation
      where
        rhs_size = foldr (addSize . size_up . snd) sizeZero pairs

    size_up (Case (Var v) _ _ alts) 
        | v `elem` top_args             -- We are scrutinising an argument variable
        = 
{-      I'm nuking this special case; BUT see the comment with case alternatives.

        (a) It's too eager.  We don't want to inline a wrapper into a
            context with no benefit.  
            E.g.  \ x. f (x+x)          no point in inlining (+) here!

        (b) It's ineffective. Once g's wrapper is inlined, its case-expressions 
            aren't scrutinising arguments any more

            case alts of

                [alt] -> size_up_alt alt `addSize` SizeIs 0# (unitBag (v, 1)) 0#
                -- We want to make wrapper-style evaluation look cheap, so that
                -- when we inline a wrapper it doesn't make call site (much) bigger
                -- Otherwise we get nasty phase ordering stuff: 
                --      f x = g x x
                --      h y = ...(f e)...
                -- If we inline g's wrapper, f looks big, and doesn't get inlined
                -- into h; if we inline f first, while it looks small, then g's 
                -- wrapper will get inlined later anyway.  To avoid this nasty
                -- ordering difference, we make (case a of (x,y) -> ...), 
                --  *where a is one of the arguments* look free.

                other -> 
-}
                         alts_size (foldr addSize sizeOne alt_sizes)    -- The 1 is for the scrutinee
                                   (foldr1 maxSize alt_sizes)

                -- Good to inline if an arg is scrutinised, because
                -- that may eliminate allocation in the caller
                -- And it eliminates the case itself

        where
          alt_sizes = map size_up_alt alts

                -- alts_size tries to compute a good discount for
                -- the case when we are scrutinising an argument variable
          alts_size (SizeIs tot tot_disc tot_scrut)             -- Size of all alternatives
                    (SizeIs max max_disc max_scrut)             -- Size of biggest alternative
                = SizeIs tot (unitBag (v, iBox (_ILIT 1 +# tot -# max)) `unionBags` max_disc) max_scrut
                        -- If the variable is known, we produce a discount that
                        -- will take us back to 'max', the size of rh largest alternative
                        -- The 1+ is a little discount for reduced allocation in the caller
          alts_size tot_size _ = tot_size

    size_up (Case e _ _ alts) = nukeScrutDiscount (size_up e) `addSize` 
                                 foldr (addSize . size_up_alt) sizeZero alts
                -- We don't charge for the case itself
                -- It's a strict thing, and the price of the call
                -- is paid by scrut.  Also consider
                --      case f x of DEFAULT -> e
                -- This is just ';'!  Don't charge for it.

    ------------ 
    size_up_app (App fun arg) args   
        | isTypeArg arg              = size_up_app fun args
        | otherwise                  = size_up_app fun (arg:args)
    size_up_app fun           args   = foldr (addSize . nukeScrutDiscount . size_up) 
                                             (size_up_fun fun args)
                                             args

        -- A function application with at least one value argument
        -- so if the function is an argument give it an arg-discount
        --
        -- Also behave specially if the function is a build
        --
        -- Also if the function is a constant Id (constr or primop)
        -- compute discounts specially
    size_up_fun (Var fun) args
      | fun `hasKey` buildIdKey   = buildSize
      | fun `hasKey` augmentIdKey = augmentSize
      | otherwise 
      = case globalIdDetails fun of
          DataConWorkId dc -> conSizeN dc (valArgCount args)

          FCallId fc   -> sizeN opt_UF_DearOp
          PrimOpId op  -> primOpSize op (valArgCount args)
                          -- foldr addSize (primOpSize op) (map arg_discount args)
                          -- At one time I tried giving an arg-discount if a primop 
                          -- is applied to one of the function's arguments, but it's
                          -- not good.  At the moment, any unlifted-type arg gets a
                          -- 'True' for 'yes I'm evald', so we collect the discount even
                          -- if we know nothing about it.  And just having it in a primop
                          -- doesn't help at all if we don't know something more.

          other        -> fun_discount fun `addSizeN` 
                          (1 + length (filter (not . exprIsTrivial) args))
                                -- The 1+ is for the function itself
                                -- Add 1 for each non-trivial arg;
                                -- the allocation cost, as in let(rec)
                                -- Slight hack here: for constructors the args are almost always
                                --      trivial; and for primops they are almost always prim typed
                                --      We should really only count for non-prim-typed args in the
                                --      general case, but that seems too much like hard work

    size_up_fun other args = size_up other

    ------------ 
    size_up_alt (con, bndrs, rhs) = size_up rhs
        -- Don't charge for args, so that wrappers look cheap
        -- (See comments about wrappers with Case)

    ------------
        -- We want to record if we're case'ing, or applying, an argument
    fun_discount v | v `elem` top_args = SizeIs 0# (unitBag (v, opt_UF_FunAppDiscount)) 0#
    fun_discount other                 = sizeZero

    ------------
        -- These addSize things have to be here because
        -- I don't want to give them bOMB_OUT_SIZE as an argument

    addSizeN TooBig          _  = TooBig
    addSizeN (SizeIs n xs d) m  = mkSizeIs bOMB_OUT_SIZE (n +# iUnbox m) xs d
    
    addSize TooBig            _                 = TooBig
    addSize _                 TooBig            = TooBig
    addSize (SizeIs n1 xs d1) (SizeIs n2 ys d2) 
        = mkSizeIs bOMB_OUT_SIZE (n1 +# n2) (xs `unionBags` ys) (d1 +# d2)
\end{code}

Code for manipulating sizes

\begin{code}
data ExprSize = TooBig
              | SizeIs FastInt          -- Size found
                       (Bag (Id,Int))   -- Arguments cased herein, and discount for each such
                       FastInt          -- Size to subtract if result is scrutinised 
                                        -- by a case expression

-- subtract the discount before deciding whether to bale out. eg. we
-- want to inline a large constructor application into a selector:
--      tup = (a_1, ..., a_99)
--      x = case tup of ...
--
mkSizeIs max n xs d | (n -# d) ># max = TooBig
                    | otherwise       = SizeIs n xs d
 
maxSize TooBig         _                                  = TooBig
maxSize _              TooBig                             = TooBig
maxSize s1@(SizeIs n1 _ _) s2@(SizeIs n2 _ _) | n1 ># n2  = s1
                                              | otherwise = s2

sizeZero        = SizeIs (_ILIT 0)  emptyBag (_ILIT 0)
sizeOne         = SizeIs (_ILIT 1)  emptyBag (_ILIT 0)
sizeN n         = SizeIs (iUnbox n) emptyBag (_ILIT 0)
conSizeN dc n   
  | isUnboxedTupleCon dc = SizeIs (_ILIT 0) emptyBag (iUnbox n +# _ILIT 1)
  | otherwise            = SizeIs (_ILIT 1) emptyBag (iUnbox n +# _ILIT 1)
        -- Treat constructors as size 1; we are keen to expose them
        -- (and we charge separately for their args).  We can't treat
        -- them as size zero, else we find that (iBox x) has size 1,
        -- which is the same as a lone variable; and hence 'v' will 
        -- always be replaced by (iBox x), where v is bound to iBox x.
        --
        -- However, unboxed tuples count as size zero
        -- I found occasions where we had 
        --      f x y z = case op# x y z of { s -> (# s, () #) }
        -- and f wasn't getting inlined

primOpSize op n_args
 | not (primOpIsDupable op) = sizeN opt_UF_DearOp
 | not (primOpOutOfLine op) = sizeN (2 - n_args)
        -- Be very keen to inline simple primops.
        -- We give a discount of 1 for each arg so that (op# x y z) costs 2.
        -- We can't make it cost 1, else we'll inline let v = (op# x y z) 
        -- at every use of v, which is excessive.
        --
        -- A good example is:
        --      let x = +# p q in C {x}
        -- Even though x get's an occurrence of 'many', its RHS looks cheap,
        -- and there's a good chance it'll get inlined back into C's RHS. Urgh!
 | otherwise                = sizeOne

buildSize = SizeIs (-2#) emptyBag 4#
        -- We really want to inline applications of build
        -- build t (\cn -> e) should cost only the cost of e (because build will be inlined later)
        -- Indeed, we should add a result_discount becuause build is 
        -- very like a constructor.  We don't bother to check that the
        -- build is saturated (it usually is).  The "-2" discounts for the \c n, 
        -- The "4" is rather arbitrary.

augmentSize = SizeIs (-2#) emptyBag 4#
        -- Ditto (augment t (\cn -> e) ys) should cost only the cost of
        -- e plus ys. The -2 accounts for the \cn 
                                                
nukeScrutDiscount (SizeIs n vs d) = SizeIs n vs 0#
nukeScrutDiscount TooBig          = TooBig

-- When we return a lambda, give a discount if it's used (applied)
lamScrutDiscount  (SizeIs n vs d) = case opt_UF_FunAppDiscount of { d -> SizeIs n vs (iUnbox d) }
lamScrutDiscount TooBig           = TooBig
\end{code}


%************************************************************************
%*                                                                      *
\subsection[considerUnfolding]{Given all the info, do (not) do the unfolding}
%*                                                                      *
%************************************************************************

We have very limited information about an unfolding expression: (1)~so
many type arguments and so many value arguments expected---for our
purposes here, we assume we've got those.  (2)~A ``size'' or ``cost,''
a single integer.  (3)~An ``argument info'' vector.  For this, what we
have at the moment is a Boolean per argument position that says, ``I
will look with great favour on an explicit constructor in this
position.'' (4)~The ``discount'' to subtract if the expression
is being scrutinised. 

Assuming we have enough type- and value arguments (if not, we give up
immediately), then we see if the ``discounted size'' is below some
(semi-arbitrary) threshold.  It works like this: for every argument
position where we're looking for a constructor AND WE HAVE ONE in our
hands, we get a (again, semi-arbitrary) discount [proportion to the
number of constructors in the type being scrutinized].

If we're in the context of a scrutinee ( \tr{(case <expr > of A .. -> ...;.. )})
and the expression in question will evaluate to a constructor, we use
the computed discount size *for the result only* rather than
computing the argument discounts. Since we know the result of
the expression is going to be taken apart, discounting its size
is more accurate (see @sizeExpr@ above for how this discount size
is computed).

We use this one to avoid exporting inlinings that we ``couldn't possibly
use'' on the other side.  Can be overridden w/ flaggery.
Just the same as smallEnoughToInline, except that it has no actual arguments.

\begin{code}
couldBeSmallEnoughToInline :: Int -> CoreExpr -> Bool
couldBeSmallEnoughToInline threshold rhs = case calcUnfoldingGuidance threshold rhs of
                                                UnfoldNever -> False
                                                other       -> True

certainlyWillInline :: Unfolding -> Bool
  -- Sees if the unfolding is pretty certain to inline  
certainlyWillInline (CoreUnfolding _ _ _ is_cheap (UnfoldIfGoodArgs n_vals _ size _))
  = is_cheap && size - (n_vals +1) <= opt_UF_UseThreshold
certainlyWillInline other
  = False

smallEnoughToInline :: Unfolding -> Bool
smallEnoughToInline (CoreUnfolding _ _ _ _ (UnfoldIfGoodArgs _ _ size _))
  = size <= opt_UF_UseThreshold
smallEnoughToInline other
  = False
\end{code}

%************************************************************************
%*                                                                      *
\subsection{callSiteInline}
%*                                                                      *
%************************************************************************

This is the key function.  It decides whether to inline a variable at a call site

callSiteInline is used at call sites, so it is a bit more generous.
It's a very important function that embodies lots of heuristics.
A non-WHNF can be inlined if it doesn't occur inside a lambda,
and occurs exactly once or 
    occurs once in each branch of a case and is small

If the thing is in WHNF, there's no danger of duplicating work, 
so we can inline if it occurs once, or is small

NOTE: we don't want to inline top-level functions that always diverge.
It just makes the code bigger.  Tt turns out that the convenient way to prevent
them inlining is to give them a NOINLINE pragma, which we do in 
StrictAnal.addStrictnessInfoToTopId

\begin{code}
callSiteInline :: DynFlags
               -> Bool                  -- True <=> the Id can be inlined
               -> Id                    -- The Id
               -> Bool                  -- True if there are are no arguments at all (incl type args)
               -> [Bool]                -- One for each value arg; True if it is interesting
               -> CallContInfo          -- True <=> continuation is interesting
               -> Maybe CoreExpr        -- Unfolding, if any


data CallContInfo = BoringCont          
                  | InterestingCont     -- Somewhat interesting
                  | CaseCont            -- Very interesting; the argument of a case
                                        -- that decomposes its scrutinee

instance Outputable CallContInfo where
  ppr BoringCont      = ptext SLIT("BoringCont")
  ppr InterestingCont = ptext SLIT("InterestingCont")
  ppr CaseCont        = ptext SLIT("CaseCont")

callSiteInline dflags active_inline id lone_variable arg_infos cont_info
  = case idUnfolding id of {
        NoUnfolding -> Nothing ;
        OtherCon cs -> Nothing ;

        CompulsoryUnfolding unf_template -> Just unf_template ;
                -- CompulsoryUnfolding => there is no top-level binding
                -- for these things, so we must inline it.
                -- Only a couple of primop-like things have 
                -- compulsory unfoldings (see MkId.lhs).
                -- We don't allow them to be inactive

        CoreUnfolding unf_template is_top is_value is_cheap guidance ->

    let
        result | yes_or_no = Just unf_template
               | otherwise = Nothing

        n_val_args  = length arg_infos

        yes_or_no = active_inline && is_cheap && consider_safe
                -- We consider even the once-in-one-branch
                -- occurrences, because they won't all have been
                -- caught by preInlineUnconditionally.  In particular,
                -- if the occurrence is once inside a lambda, and the
                -- rhs is cheap but not a manifest lambda, then
                -- pre-inline will not have inlined it for fear of
                -- invalidating the occurrence info in the rhs.

        consider_safe
                -- consider_safe decides whether it's a good idea to
                -- inline something, given that there's no
                -- work-duplication issue (the caller checks that).
          = case guidance of
              UnfoldNever  -> False
              UnfoldIfGoodArgs n_vals_wanted arg_discounts size res_discount
                  | enough_args && size <= (n_vals_wanted + 1)
                        -- Inline unconditionally if there no size increase
                        -- Size of call is n_vals_wanted (+1 for the function)
                  -> True

                  | otherwise
                  -> some_benefit && small_enough

                  where
                    enough_args = n_val_args >= n_vals_wanted

                    some_benefit = or arg_infos || really_interesting_cont
                                -- There must be something interesting
                                -- about some argument, or the result
                                -- context, to make it worth inlining

                    really_interesting_cont 
                        | n_val_args <  n_vals_wanted = False   -- Too few args
                        | n_val_args == n_vals_wanted = interesting_saturated_call
                        | otherwise                   = True    -- Extra args
                        -- really_interesting_cont tells if the result of the
                        -- call is in an interesting context.

                    interesting_saturated_call 
                        = case cont_info of
                            BoringCont -> not is_top && n_vals_wanted > 0       -- Note [Nested functions] 
                            CaseCont   -> not lone_variable || not is_value     -- Note [Lone variables]
                            InterestingCont -> n_vals_wanted > 0

                    small_enough = (size - discount) <= opt_UF_UseThreshold
                    discount = computeDiscount n_vals_wanted arg_discounts 
                                               res_discount' arg_infos
                    res_discount' = case cont_info of
                                        BoringCont      -> 0
                                        CaseCont        -> res_discount
                                        InterestingCont -> 4 `min` res_discount
                        -- res_discount can be very large when a function returns
                        -- construtors; but we only want to invoke that large discount
                        -- when there's a case continuation.
                        -- Otherwise we, rather arbitrarily, threshold it.  Yuk.
                        -- But we want to aovid inlining large functions that return 
                        -- constructors into contexts that are simply "interesting"
                
    in    
    if dopt Opt_D_dump_inlinings dflags then
        pprTrace "Considering inlining"
                 (ppr id <+> vcat [text "active:" <+> ppr active_inline,
                                   text "arg infos" <+> ppr arg_infos,
                                   text "interesting continuation" <+> ppr cont_info,
                                   text "is value:" <+> ppr is_value,
                                   text "is cheap:" <+> ppr is_cheap,
                                   text "guidance" <+> ppr guidance,
                                   text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"])
                  result
    else
    result
    }
\end{code}

Note [Nested functions]
~~~~~~~~~~~~~~~~~~~~~~~
If a function has a nested defn we also record some-benefit, on the
grounds that we are often able to eliminate the binding, and hence the
allocation, for the function altogether; this is good for join points.
But this only makes sense for *functions*; inlining a constructor
doesn't help allocation unless the result is scrutinised.  UNLESS the
constructor occurs just once, albeit possibly in multiple case
branches.  Then inlining it doesn't increase allocation, but it does
increase the chance that the constructor won't be allocated at all in
the branches that don't use it.

Note [Lone variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The "lone-variable" case is important.  I spent ages messing about
with unsatisfactory varaints, but this is nice.  The idea is that if a
variable appears all alone
        as an arg of lazy fn, or rhs    Stop
        as scrutinee of a case          Select
        as arg of a strict fn           ArgOf
AND
        it is bound to a value
then we should not inline it (unless there is some other reason,
e.g. is is the sole occurrence).  That is what is happening at 
the use of 'lone_variable' in 'interesting_saturated_call'.

Why?  At least in the case-scrutinee situation, turning
        let x = (a,b) in case x of y -> ...
into
        let x = (a,b) in case (a,b) of y -> ...
and thence to 
        let x = (a,b) in let y = (a,b) in ...
is bad if the binding for x will remain.

Another example: I discovered that strings
were getting inlined straight back into applications of 'error'
because the latter is strict.
        s = "foo"
        f = \x -> ...(error s)...

Fundamentally such contexts should not encourage inlining because the
context can ``see'' the unfolding of the variable (e.g. case or a
RULE) so there's no gain.  If the thing is bound to a value.

However, watch out:

 * Consider this:
        foo = _inline_ (\n. [n])
        bar = _inline_ (foo 20)
        baz = \n. case bar of { (m:_) -> m + n }
   Here we really want to inline 'bar' so that we can inline 'foo'
   and the whole thing unravels as it should obviously do.  This is 
   important: in the NDP project, 'bar' generates a closure data
   structure rather than a list. 

 * Even a type application or coercion isn't a lone variable.
   Consider
        case $fMonadST @ RealWorld of { :DMonad a b c -> c }
   We had better inline that sucker!  The case won't see through it.

   For now, I'm treating treating a variable applied to types 
   in a *lazy* context "lone". The motivating example was
        f = /\a. \x. BIG
        g = /\a. \y.  h (f a)
   There's no advantage in inlining f here, and perhaps
   a significant disadvantage.  Hence some_val_args in the Stop case

\begin{code}
computeDiscount :: Int -> [Int] -> Int -> [Bool] -> Int
computeDiscount n_vals_wanted arg_discounts result_discount arg_infos
        -- We multiple the raw discounts (args_discount and result_discount)
        -- ty opt_UnfoldingKeenessFactor because the former have to do with
        --  *size* whereas the discounts imply that there's some extra 
        --  *efficiency* to be gained (e.g. beta reductions, case reductions) 
        -- by inlining.

        -- we also discount 1 for each argument passed, because these will
        -- reduce with the lambdas in the function (we count 1 for a lambda
        -- in size_up).
  = 1 +                 -- Discount of 1 because the result replaces the call
                        -- so we count 1 for the function itself
    length (take n_vals_wanted arg_infos) +
                        -- Discount of 1 for each arg supplied, because the 
                        -- result replaces the call
    round (opt_UF_KeenessFactor * 
           fromIntegral (arg_discount + result_discount))
  where
    arg_discount = sum (zipWith mk_arg_discount arg_discounts arg_infos)

    mk_arg_discount discount is_evald | is_evald  = discount
                                      | otherwise = 0
\end{code}