[project @ 2000-07-11 15:57:45 by simonmar]

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

%
% (c) The AQUA Project, Glasgow University, 1994-1998
%
\section[CoreUnfold]{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}
module CoreUnfold (
        Unfolding, UnfoldingGuidance,   -- Abstract types

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

        couldBeSmallEnoughToInline, 
        certainlyWillInline, 
        okToUnfoldInHiFile,

        callSiteInline, blackListed
    ) where

#include "HsVersions.h"

import CmdLineOpts      ( opt_UF_CreationThreshold,
                          opt_UF_UseThreshold,
                          opt_UF_ScrutConDiscount,
                          opt_UF_FunAppDiscount,
                          opt_UF_PrimArgDiscount,
                          opt_UF_KeenessFactor,
                          opt_UF_CheapOp, opt_UF_DearOp,
                          opt_UnfoldCasms, opt_PprStyle_Debug,
                          opt_D_dump_inlinings
                        )
import CoreSyn
import PprCore          ( pprCoreExpr )
import OccurAnal        ( occurAnalyseGlobalExpr )
import CoreUtils        ( exprIsValue, exprIsCheap, exprIsBottom, exprIsTrivial )
import Id               ( Id, idType, idFlavour, isId, idWorkerInfo,
                          idSpecialisation, idInlinePragma, idUnfolding,
                          isPrimOpId_maybe
                        )
import VarSet
import Literal          ( isLitLitLit )
import PrimOp           ( PrimOp(..), primOpIsDupable, primOpOutOfLine, ccallIsCasm )
import IdInfo           ( ArityInfo(..), InlinePragInfo(..), OccInfo(..), IdFlavour(..), CprInfo(..), 
                          insideLam, workerExists, isNeverInlinePrag
                        )
import Type             ( splitFunTy_maybe, isUnLiftedType )
import Unique           ( Unique, buildIdKey, augmentIdKey, hasKey )
import Bag
import Outputable

#if __GLASGOW_HASKELL__ >= 404
import GlaExts          ( fromInt )
#endif
\end{code}


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

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

mkUnfolding top_lvl expr
  = CoreUnfolding (occurAnalyseGlobalExpr expr)
                  top_lvl
                  (exprIsCheap expr)
                  (exprIsValue expr)
                  (exprIsBottom expr)
                  (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

mkCompulsoryUnfolding expr      -- Used for things that absolutely must be unfolded
  = CompulsoryUnfolding (occurAnalyseGlobalExpr 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 bOMB_OUT_SIZE val_binders body) of

      TooBig 
        | not inline -> UnfoldNever
                -- A big function with an INLINE pragma must
                -- have an UnfoldIfGoodArgs guidance
        | inline     -> 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
                        (I# scrut_discount)
        where        
            boxed_size    = I# 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 (I# 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 _ body)     = size_up body    -- Notes cost nothing

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

    size_up (Lit lit) = sizeOne

    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

        -- 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) -> ...) look free.
    size_up (Case (Var v) _ [alt]) 
        | v `elem` top_args
        = size_up_alt alt `addSize` SizeIs 0# (unitBag (v, 1)) 0#
                -- Good to inline if an arg is scrutinised, because
                -- that may eliminate allocation in the caller
                -- And it eliminates the case itself
        | otherwise     
        = size_up_alt alt

        -- Scrutinising one of the argument variables,
        -- with more than one alternative
    size_up (Case (Var v) _ alts)
        | v `elem` top_args
        = alts_size (foldr addSize sizeOne alt_sizes)   -- The 1 is for the scrutinee
                    (foldr1 maxSize alt_sizes)
        where
          v_in_args = v `elem` top_args
          alt_sizes = map size_up_alt alts

          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, I# (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 idFlavour fun of
          DataConId dc -> conSizeN (valArgCount args)

          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

    ------------
        -- 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) (I# m)
      | n_tot ># bOMB_OUT_SIZE      = TooBig
      | otherwise                   = SizeIs n_tot xs d
      where
        n_tot = n +# m
    
    addSize TooBig _ = TooBig
    addSize _ TooBig = TooBig
    addSize (SizeIs n1 xs d1) (SizeIs n2 ys d2)
      | n_tot ># bOMB_OUT_SIZE = TooBig
      | otherwise              = SizeIs n_tot xys d_tot
      where
        n_tot = n1 +# n2
        d_tot = d1 +# d2
        xys   = xs `unionBags` ys
\end{code}

Code for manipulating sizes

\begin{code}

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

isTooBig TooBig = True
isTooBig _      = False

maxSize TooBig         _                                  = TooBig
maxSize _              TooBig                             = TooBig
maxSize s1@(SizeIs n1 _ _) s2@(SizeIs n2 _ _) | n1 ># n2  = s1
                                              | otherwise = s2

sizeZero        = SizeIs 0# emptyBag 0#
sizeOne         = SizeIs 1# emptyBag 0#
sizeTwo         = SizeIs 2# emptyBag 0#
sizeN (I# n)    = SizeIs n  emptyBag 0#
conSizeN (I# n) = SizeIs 1# emptyBag (n +# 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 (I# x) has size 1,
        -- which is the same as a lone variable; and hence 'v' will 
        -- always be replaced by (I# x), where v is bound to I# x.

primOpSize op n_args
 | not (primOpIsDupable op) = sizeN opt_UF_DearOp
 | not (primOpOutOfLine op) = sizeZero                  -- These are good to inline
 | 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 { I# d -> SizeIs n vs 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 :: Id -> Bool
        -- Sees if the Id is pretty certain to inline   
certainlyWillInline v
  = case idUnfolding v of

        CoreUnfolding _ _ _ is_value _ g@(UnfoldIfGoodArgs n_vals _ size _)
           ->    is_value 
              && size - (n_vals +1) <= opt_UF_UseThreshold

        other -> False
\end{code}

@okToUnfoldInHifile@ is used when emitting unfolding info into an interface
file to determine whether an unfolding candidate really should be unfolded.
The predicate is needed to prevent @_casm_@s (+ lit-lits) from being emitted
into interface files. 

The reason for inlining expressions containing _casm_s into interface files
is that these fragments of C are likely to mention functions/#defines that
will be out-of-scope when inlined into another module. This is not an
unfixable problem for the user (just need to -#include the approp. header
file), but turning it off seems to the simplest thing to do.

\begin{code}
okToUnfoldInHiFile :: CoreExpr -> Bool
okToUnfoldInHiFile e = opt_UnfoldCasms || go e
 where
    -- Race over an expression looking for CCalls..
    go (Var v)                = case isPrimOpId_maybe v of
                                  Just op -> okToUnfoldPrimOp op
                                  Nothing -> True
    go (Lit lit)              = not (isLitLitLit lit)
    go (App fun arg)          = go fun && go arg
    go (Lam _ body)           = go body
    go (Let binds body)       = and (map go (body :rhssOfBind binds))
    go (Case scrut bndr alts) = and (map go (scrut:rhssOfAlts alts)) &&
                                not (any isLitLitLit [ lit | (LitAlt lit, _, _) <- alts ])
    go (Note _ body)          = go body
    go (Type _)               = True

    -- ok to unfold a PrimOp as long as it's not a _casm_
    okToUnfoldPrimOp (CCallOp ccall) = not (ccallIsCasm ccall)
    okToUnfoldPrimOp _               = True
\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 :: Bool                  -- True <=> the Id is black listed
               -> Bool                  -- 'inline' note at call site
               -> OccInfo
               -> Id                    -- The Id
               -> [Bool]                -- One for each value arg; True if it is interesting
               -> Bool                  -- True <=> continuation is interesting
               -> Maybe CoreExpr        -- Unfolding, if any


callSiteInline black_listed inline_call occ id arg_infos interesting_cont
  = case idUnfolding id of {
        NoUnfolding -> Nothing ;
        OtherCon cs -> Nothing ;
        CompulsoryUnfolding unf_template | black_listed -> Nothing 
                                         | otherwise    -> Just unf_template ;
                -- Constructors have compulsory unfoldings, but
                -- may have rules, in which case they are 
                -- black listed till later
        CoreUnfolding unf_template is_top is_cheap is_value is_bot guidance ->

    let
        result | yes_or_no = Just unf_template
               | otherwise = Nothing

        n_val_args  = length arg_infos

        ok_inside_lam = is_value || is_bot || (is_cheap && not is_top)
                                -- I'm experimenting with is_cheap && not is_top

        yes_or_no 
          | black_listed = False
          | otherwise    = case occ of
                                IAmDead              -> pprTrace "callSiteInline: dead" (ppr id) False
                                IAmALoopBreaker      -> False
                                OneOcc in_lam one_br -> (not in_lam || ok_inside_lam) && consider_safe in_lam True  one_br
                                NoOccInfo            -> ok_inside_lam                 && consider_safe True   False False

        consider_safe in_lam once once_in_one_branch
                -- consider_safe decides whether it's a good idea to inline something,
                -- given that there's no work-duplication issue (the caller checks that).
                -- once_in_one_branch = True means there's a unique textual occurrence
          | inline_call  = True

          | once_in_one_branch
                -- Be very keen to inline something if this is its unique occurrence:
                --
                --   a) Inlining gives a good chance of eliminating the original 
                --      binding (and hence the allocation) for the thing.  
                --      (Provided it's not a top level binding, in which case the 
                --       allocation costs nothing.)
                --
                --   b) Inlining a function that is called only once exposes the 
                --      body function to the call site.
                --
                -- The only time we hold back is when substituting inside a lambda;
                -- then if the context is totally uninteresting (not applied, not scrutinised)
                -- there is no point in substituting because it might just increase allocation,
                -- by allocating the function itself many times
                --
                -- Note: there used to be a '&& not top_level' in the guard above,
                --       but that stopped us inlining top-level functions used only once,
                --       which is stupid
          = not in_lam || not (null arg_infos) || interesting_cont

          | otherwise
          = case guidance of
              UnfoldNever  -> False ;
              UnfoldIfGoodArgs n_vals_wanted arg_discounts size res_discount

                  | enough_args && size <= (n_vals_wanted + 1)
                        -- No size increase
                        -- Size of call is n_vals_wanted (+1 for the function)
                  -> True

                  | otherwise
                  -> some_benefit && small_enough

                  where
                    some_benefit = or arg_infos || really_interesting_cont || 
                                   (not is_top && (once || (n_vals_wanted > 0 && enough_args)))
                        -- If it occurs more than once, there must be something interesting 
                        -- about some argument, or the result context, to make it worth inlining
                        --
                        -- 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.
            
                    enough_args           = n_val_args >= n_vals_wanted
                    really_interesting_cont | n_val_args <  n_vals_wanted = False       -- Too few args
                                            | n_val_args == n_vals_wanted = interesting_cont
                                            | otherwise                   = True        -- Extra args
                        -- really_interesting_cont tells if the result of the
                        -- call is in an interesting context.

                    small_enough = (size - discount) <= opt_UF_UseThreshold
                    discount     = computeDiscount n_vals_wanted arg_discounts res_discount 
                                                 arg_infos really_interesting_cont
                
    in    
#ifdef DEBUG
    if opt_D_dump_inlinings then
        pprTrace "Considering inlining"
                 (ppr id <+> vcat [text "black listed" <+> ppr black_listed,
                                   text "occ info:" <+> ppr occ,
                                   text "arg infos" <+> ppr arg_infos,
                                   text "interesting continuation" <+> ppr interesting_cont,
                                   text "is value:" <+> ppr is_value,
                                   text "is cheap:" <+> ppr is_cheap,
                                   text "is bottom:" <+> ppr is_bot,
                                   text "is top-level:"    <+> ppr is_top,
                                   text "guidance" <+> ppr guidance,
                                   text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO",
                                   if yes_or_no then
                                        text "Unfolding =" <+> pprCoreExpr unf_template
                                   else empty])
                  result
    else
#endif
    result
    }

computeDiscount :: Int -> [Int] -> Int -> [Bool] -> Bool -> Int
computeDiscount n_vals_wanted arg_discounts res_discount arg_infos result_used
        -- 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 * 
           fromInt (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

        -- Don't give a result discount unless there are enough args
    result_discount | result_used = res_discount        -- Over-applied, or case scrut
                    | otherwise   = 0
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Black-listing}
%*                                                                      *
%************************************************************************

Inlining is controlled by the "Inline phase" number, which is set
by the per-simplification-pass '-finline-phase' flag.

For optimisation we use phase 1,2 and nothing (i.e. no -finline-phase flag)
in that order.  The meanings of these are determined by the @blackListed@ function
here.

The final simplification doesn't have a phase number.

Pragmas
~~~~~~~
        Pragma          Black list if

(least black listing, most inlining)
        INLINE n foo    phase is Just p *and* p<n *and* foo appears on LHS of rule
        INLINE foo      phase is Just p *and*           foo appears on LHS of rule
        NOINLINE n foo  phase is Just p *and* (p<n *or* foo appears on LHS of rule)
        NOINLINE foo    always
(most black listing, least inlining)

\begin{code}
blackListed :: IdSet            -- Used in transformation rules
            -> Maybe Int        -- Inline phase
            -> Id -> Bool       -- True <=> blacklisted
        
-- The blackListed function sees whether a variable should *not* be 
-- inlined because of the inline phase we are in.  This is the sole
-- place that the inline phase number is looked at.

blackListed rule_vars Nothing           -- Last phase
  = \v -> isNeverInlinePrag (idInlinePragma v)

blackListed rule_vars (Just phase)
  = \v -> normal_case rule_vars phase v

normal_case rule_vars phase v 
  = case idInlinePragma v of
        NoInlinePragInfo -> has_rules

        IMustNotBeINLINEd from_INLINE Nothing
          | from_INLINE -> has_rules    -- Black list until final phase
          | otherwise   -> True         -- Always blacklisted

        IMustNotBeINLINEd from_inline (Just threshold)
          | from_inline -> (phase < threshold && has_rules)
          | otherwise   -> (phase < threshold || has_rules)
  where
    has_rules =  v `elemVarSet` rule_vars
              || not (isEmptyCoreRules (idSpecialisation v))
\end{code}


SLPJ 95/04: Why @runST@ must be inlined very late:
\begin{verbatim}
f x =
  runST ( \ s -> let
                    (a, s')  = newArray# 100 [] s
                    (_, s'') = fill_in_array_or_something a x s'
                  in
                  freezeArray# a s'' )
\end{verbatim}
If we inline @runST@, we'll get:
\begin{verbatim}
f x = let
        (a, s')  = newArray# 100 [] realWorld#{-NB-}
        (_, s'') = fill_in_array_or_something a x s'
      in
      freezeArray# a s''
\end{verbatim}
And now the @newArray#@ binding can be floated to become a CAF, which
is totally and utterly wrong:
\begin{verbatim}
f = let
    (a, s')  = newArray# 100 [] realWorld#{-NB-} -- YIKES!!!
    in
    \ x ->
        let (_, s'') = fill_in_array_or_something a x s' in
        freezeArray# a s''
\end{verbatim}
All calls to @f@ will share a {\em single} array!  

Yet we do want to inline runST sometime, so we can avoid
needless code.  Solution: black list it until the last moment.