[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, -- types

        noUnfolding, mkUnfolding, 
        mkOtherCon, otherCons,
        unfoldingTemplate, maybeUnfoldingTemplate,
        isEvaldUnfolding, isCheapUnfolding,
        hasUnfolding, hasSomeUnfolding,

        couldBeSmallEnoughToInline, 
        certainlySmallEnoughToInline, 
        okToUnfoldInHiFile,

        calcUnfoldingGuidance,

        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_UF_NoRepLit,
                          opt_UnfoldCasms, opt_PprStyle_Debug,
                          opt_D_dump_inlinings
                        )
import CoreSyn
import PprCore          ( pprCoreExpr )
import OccurAnal        ( occurAnalyseGlobalExpr )
import BinderInfo       ( )
import CoreUtils        ( coreExprType, exprIsTrivial, exprIsValue, exprIsCheap )
import Id               ( Id, idType, idUnique, isId, 
                          getIdSpecialisation, getInlinePragma, getIdUnfolding
                        )
import VarSet
import Name             ( isLocallyDefined )
import Const            ( Con(..), isLitLitLit, isWHNFCon )
import PrimOp           ( PrimOp(..), primOpIsDupable )
import IdInfo           ( ArityInfo(..), InlinePragInfo(..), OccInfo(..) )
import TyCon            ( tyConFamilySize )
import Type             ( splitAlgTyConApp_maybe, splitFunTy_maybe, isUnLiftedType )
import Const            ( isNoRepLit )
import Unique           ( Unique, buildIdKey, augmentIdKey, runSTRepIdKey )
import Maybes           ( maybeToBool )
import Bag
import Util             ( isIn, lengthExceeds )
import Outputable
\end{code}

%************************************************************************
%*                                                                      *
\subsection{@Unfolding@ and @UnfoldingGuidance@ types}
%*                                                                      *
%************************************************************************

\begin{code}
data Unfolding
  = NoUnfolding

  | OtherCon [Con]              -- It ain't one of these
                                -- (OtherCon xs) also indicates that something has been evaluated
                                -- and hence there's no point in re-evaluating it.
                                -- OtherCon [] is used even for non-data-type values
                                -- to indicated evaluated-ness.  Notably:
                                --      data C = C !(Int -> Int)
                                --      case x of { C f -> ... }
                                -- Here, f gets an OtherCon [] unfolding.

  | CoreUnfolding                       -- An unfolding with redundant cached information
                CoreExpr                -- Template; binder-info is correct
                Bool                    -- exprIsCheap template (cached); it won't duplicate (much) work 
                                        --      if you inline this in more than one place
                Bool                    -- exprIsValue template (cached); it is ok to discard a `seq` on
                                        --      this variable
                UnfoldingGuidance       -- Tells about the *size* of the template.
\end{code}

\begin{code}
noUnfolding = NoUnfolding
mkOtherCon  = OtherCon

mkUnfolding expr
  = CoreUnfolding (occurAnalyseGlobalExpr expr)
                  (exprIsCheap expr)
                  (exprIsValue expr)
                  (calcUnfoldingGuidance opt_UF_CreationThreshold expr)

unfoldingTemplate :: Unfolding -> CoreExpr
unfoldingTemplate (CoreUnfolding expr _ _ _) = expr
unfoldingTemplate other = panic "getUnfoldingTemplate"

maybeUnfoldingTemplate :: Unfolding -> Maybe CoreExpr
maybeUnfoldingTemplate (CoreUnfolding expr _ _ _) = Just expr
maybeUnfoldingTemplate other                      = Nothing

otherCons (OtherCon cons) = cons
otherCons other           = []

isEvaldUnfolding :: Unfolding -> Bool
isEvaldUnfolding (OtherCon _)                   = True
isEvaldUnfolding (CoreUnfolding _ _ is_evald _) = is_evald
isEvaldUnfolding other                          = False

isCheapUnfolding :: Unfolding -> Bool
isCheapUnfolding (CoreUnfolding _ is_cheap _ _) = is_cheap
isCheapUnfolding other                          = False

hasUnfolding :: Unfolding -> Bool
hasUnfolding (CoreUnfolding _ _ _ _) = True
hasUnfolding other                   = False

hasSomeUnfolding :: Unfolding -> Bool
hasSomeUnfolding NoUnfolding = False
hasSomeUnfolding other       = True

data UnfoldingGuidance
  = UnfoldNever
  | UnfoldAlways                -- There is no "original" definition,
                                -- so you'd better unfold.  Or: something
                                -- so cheap to unfold (e.g., 1#) that
                                -- you should do it absolutely always.

  | UnfoldIfGoodArgs    Int     -- and "n" value args

                        [Int]   -- Discount if the argument is evaluated.
                                -- (i.e., a simplification will definitely
                                -- be possible).  One elt of the list per *value* arg.

                        Int     -- The "size" of the unfolding; to be elaborated
                                -- later. ToDo

                        Int     -- Scrutinee discount: the discount to substract if the thing is in
                                -- a context (case (thing args) of ...),
                                -- (where there are the right number of arguments.)
\end{code}

\begin{code}
instance Outputable UnfoldingGuidance where
    ppr UnfoldAlways    = ptext SLIT("ALWAYS")
    ppr UnfoldNever     = ptext SLIT("NEVER")
    ppr (UnfoldIfGoodArgs v cs size discount)
      = hsep [ptext SLIT("IF_ARGS"), int v,
               if null cs       -- always print *something*
                then char 'X'
                else hcat (map (text . show) cs),
               int size,
               int discount ]
\end{code}


%************************************************************************
%*                                                                      *
\subsection[calcUnfoldingGuidance]{Calculate ``unfolding guidance'' for an expression}
%*                                                                      *
%************************************************************************

\begin{code}
calcUnfoldingGuidance
        :: Int                  -- bomb out if size gets bigger than this
        -> CoreExpr             -- expression to look at
        -> UnfoldingGuidance
calcUnfoldingGuidance bOMB_OUT_SIZE expr
  | exprIsTrivial expr          -- Often trivial expressions are never bound
                                -- to an expression, but it can happen.  For
                                -- example, the Id for a nullary constructor has
                                -- a trivial expression as its unfolding, and
                                -- we want to make sure that we always unfold it.
  = UnfoldAlways
 
  | otherwise
  = case collectBinders expr of { (binders, body) ->
    let
        val_binders = filter isId binders
    in
    case (sizeExpr bOMB_OUT_SIZE val_binders body) of

      TooBig -> UnfoldNever

      SizeIs size cased_args scrut_discount
        -> UnfoldIfGoodArgs
                        (length val_binders)
                        (map discount_for val_binders)
                        (I# size)
                        (I# scrut_discount)
        where        
            discount_for b 
                | num_cases == 0 = 0
                | is_fun_ty      = num_cases * opt_UF_FunAppDiscount
                | is_data_ty     = num_cases * tyConFamilySize tycon * opt_UF_ScrutConDiscount
                | otherwise      = num_cases * opt_UF_PrimArgDiscount
                where
                  num_cases           = foldlBag (\n b' -> if b==b' then n+1 else n) 0 cased_args
                                        -- Count occurrences of b in cased_args
                  arg_ty              = idType b
                  is_fun_ty           = maybeToBool (splitFunTy_maybe arg_ty)
                  (is_data_ty, tycon) = case (splitAlgTyConApp_maybe (idType b)) of
                                          Nothing       -> (False, panic "discount")
                                          Just (tc,_,_) -> (True,  tc)
        }
\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) args expr
  = size_up expr
  where
    size_up (Type t)          = sizeZero        -- Types cost nothing
    size_up (Var v)           = sizeOne

    size_up (Note InlineMe _) = sizeTwo         -- The idea is that this is one more
                                                -- than the size of the "call" (i.e. 1)
                                                -- We want to reply "no" to noSizeIncrease
                                                -- for a bare reference (i.e. applied to no args) 
                                                -- to an INLINE thing

    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 (Con con args) = foldr (addSize . size_up) 
                                   (size_up_con con args)
                                   args

    size_up (Lam b e) | isId b    = 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 scrut _ alts)
      = nukeScrutDiscount (size_up scrut)               `addSize`
        arg_discount scrut                              `addSize`
        foldr (addSize . size_up_alt) sizeZero alts     

-- Just charge for the alts that exist, not the ones that might exist
--      `addSizeN`
--      case (splitAlgTyConApp_maybe (coreExprType scrut)) of
--              Nothing       -> 1
--              Just (tc,_,_) -> tyConFamilySize tc

    ------------ 
    size_up_app (App fun arg) args   = size_up_app fun (arg:args)
    size_up_app fun           args   = foldr (addSize . size_up) (fun_discount fun) 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
    fun_discount (Var fun) | idUnique fun == buildIdKey   = buildSize
                           | idUnique fun == augmentIdKey = augmentSize
                           | fun `is_elem` args         = scrutArg fun
    fun_discount other                                  = sizeZero

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

    ------------
    size_up_con (Literal lit) args | isNoRepLit lit = sizeN opt_UF_NoRepLit
                                   | otherwise      = sizeOne

    size_up_con (DataCon dc) args = conSizeN (valArgCount args)
                             
    size_up_con (PrimOp op) args = foldr addSize (sizeN op_cost) (map arg_discount args)
                -- Give an arg-discount if a primop is applies to
                -- one of the function's arguments
      where
        op_cost | primOpIsDupable op = opt_UF_CheapOp
                | otherwise          = opt_UF_DearOp

        -- We want to record if we're case'ing, or applying, an argument
    arg_discount (Var v) | v `is_elem` args = scrutArg v
    arg_discount other                      = sizeZero

    ------------
    is_elem :: Id -> [Id] -> Bool
    is_elem = isIn "size_up_scrut"

    ------------
        -- 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 -# d <# bOMB_OUT_SIZE = SizeIs n_tot xs d
      | otherwise                   = TooBig
      where
        n_tot = n +# m
    
    addSize TooBig _ = TooBig
    addSize _ TooBig = TooBig
    addSize (SizeIs n1 xs d1) (SizeIs n2 ys d2)
      | (n_tot -# d_tot) <# bOMB_OUT_SIZE = SizeIs n_tot xys d_tot
      | otherwise                         = TooBig
      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) -- Arguments cased herein
                       Int#     -- Size to subtract if result is scrutinised 
                                -- by a case expression

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, that unfoldAlways responsds 'False'
        -- when asked about 'x' when x is bound to (C 3#).
        -- This avoids gratuitous 'ticks' when x itself appears as an
        -- atomic constructor argument.

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 
                                                
scrutArg v      = SizeIs 0# (unitBag v) 0#

nukeScrutDiscount (SizeIs n vs d) = SizeIs n vs 0#
nukeScrutDiscount 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 :: UnfoldingGuidance -> Bool
couldBeSmallEnoughToInline UnfoldNever = False
couldBeSmallEnoughToInline other       = True

certainlySmallEnoughToInline :: UnfoldingGuidance -> Bool
certainlySmallEnoughToInline UnfoldNever                   = False
certainlySmallEnoughToInline UnfoldAlways                  = True
certainlySmallEnoughToInline (UnfoldIfGoodArgs _ _ size _) = size <= opt_UF_UseThreshold
\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 _)                = True
    go (Con (Literal lit) _)  = not (isLitLitLit lit)
    go (Con (PrimOp op) args) = okToUnfoldPrimOp op && all go args
    go (Con con args)         = True -- con args are always atomic
    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))
    go (Note _ body)          = go body
    go (Type _)               = True

    -- ok to unfold a PrimOp as long as it's not a _casm_
    okToUnfoldPrimOp (CCallOp _ is_casm _ _) = not is_casm
    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

\begin{code}
callSiteInline :: Bool                  -- True <=> the Id is black listed
               -> Bool                  -- 'inline' note at call site
               -> 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 id arg_infos interesting_cont
  = case getIdUnfolding id of {
        NoUnfolding -> Nothing ;
        OtherCon _  -> Nothing ;
        CoreUnfolding unf_template is_cheap _ guidance ->

    let
        result | yes_or_no = Just unf_template
               | otherwise = Nothing

        inline_prag = getInlinePragma id
        n_val_args  = length arg_infos

        yes_or_no =
            case inline_prag of
                IAmDead           -> pprTrace "callSiteInline: dead" (ppr id) False
                IMustNotBeINLINEd -> False
                IAmALoopBreaker   -> False
                IMustBeINLINEd    -> True       -- Overrides absolutely everything, including the black list
                ICanSafelyBeINLINEd in_lam one_br -> consider in_lam    True  one_br
                NoInlinePragInfo                  -> consider InsideLam False False

        consider in_lam once once_in_one_branch
          | black_listed = False
          | inline_call  = True
          | once_in_one_branch  -- Be very keen to inline something if this is its unique occurrence; that
                                -- gives a good chance of eliminating the original binding for the thing.
                                -- 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.
          = WARN( case in_lam of { NotInsideLam -> True; other -> False },
                  text "callSiteInline:oneOcc" <+> ppr id )
                -- If it has one occurrence, not inside a lambda, PreInlineUnconditionally
                -- should have zapped it already
            is_cheap && (not (null arg_infos) || interesting_cont)

          | otherwise   -- Occurs (textually) more than once, so look at its size
          = case guidance of
              UnfoldAlways -> True
              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)
                -> case in_lam of
                        NotInsideLam -> True
                        InsideLam    -> is_cheap

                | not (or arg_infos || really_interesting_cont || once)
                        -- If it occurs more than once, there must be something interesting 
                        -- about some argument, or the result, to make it worth inlining
                        -- We also drop this case if the thing occurs once, although perhaps in 
                        -- several branches.  In this case we are keener about inlining in the hope
                        -- that we'll be able to drop the allocation for the function altogether.
                -> False
  
                | otherwise
                -> case in_lam of
                        NotInsideLam -> small_enough
                        InsideLam    -> is_cheap && small_enough

                where
                  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
                        -- This rather elaborate defn for really_interesting_cont is important
                        -- Consider an I# = INLINE (\x -> I# {x})
                        -- The unfolding guidance deems it to have size 2, and no arguments.
                        -- So in an application (I# y) we must take the extra arg 'y' as
                        -- evidence of 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 "inline prag:" <+> ppr inline_prag,
                                   text "arg infos" <+> ppr arg_infos,
                                   text "interesting continuation" <+> ppr interesting_cont,
                                   text "is cheap" <+> ppr is_cheap,
                                   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).
  = 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.

\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.

-- Phase 0: used for 'no imported inlinings please'
-- This prevents wrappers getting inlined which in turn is bad for full laziness
blackListed rule_vars (Just 0)
  = \v -> not (isLocallyDefined v)

-- Phase 1: don't inline any rule-y things or things with specialisations
blackListed rule_vars (Just 1)
  = \v -> let v_uniq = idUnique v
          in v `elemVarSet` rule_vars
          || not (isEmptyCoreRules (getIdSpecialisation v))
          || v_uniq == runSTRepIdKey

-- Phase 2: allow build/augment to inline, and specialisations
blackListed rule_vars (Just 2)
  = \v -> let v_uniq = idUnique v
          in (v `elemVarSet` rule_vars && not (v_uniq == buildIdKey || 
                                               v_uniq == augmentIdKey))
          || v_uniq == runSTRepIdKey

-- Otherwise just go for it
blackListed rule_vars phase
  = \v -> False
\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.