[ghc-hetmet.git] / ghc / compiler / stranal / DmdAnal.lhs

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

                        -----------------
                        A demand analysis
                        -----------------

\begin{code}
module DmdAnal ( dmdAnalPgm, dmdAnalTopRhs, 
                 both {- needed by WwLib -}
   ) where

#include "HsVersions.h"

import CmdLineOpts      ( DynFlags, DynFlag(..), opt_MaxWorkerArgs )
import NewDemand        -- All of it
import CoreSyn
import PprCore  
import CoreUtils        ( exprIsValue, exprArity )
import DataCon          ( dataConTyCon )
import TyCon            ( isProductTyCon, isRecursiveTyCon )
import Id               ( Id, idType, idDemandInfo, idInlinePragma,
                          isDataConId, isGlobalId, idArity,
                          idNewStrictness, idNewStrictness_maybe, getNewStrictness, setIdNewStrictness,
                          idNewDemandInfo, setIdNewDemandInfo, newStrictnessFromOld )
import IdInfo           ( newDemand )
import Var              ( Var )
import VarEnv
import UniqFM           ( plusUFM_C, addToUFM_Directly, lookupUFM_Directly,
                          keysUFM, minusUFM, ufmToList, filterUFM )
import Type             ( isUnLiftedType )
import CoreLint         ( showPass, endPass )
import Util             ( mapAndUnzip, mapAccumL, mapAccumR, lengthIs, equalLength )
import BasicTypes       ( Arity, TopLevelFlag(..), isTopLevel, isNeverActive )
import Maybes           ( orElse, expectJust )
import Outputable
\end{code}

To think about

* set a noinline pragma on bottoming Ids

* Consider f x = x+1 `fatbar` error (show x)
  We'd like to unbox x, even if that means reboxing it in the error case.

\begin{code}
instance Outputable TopLevelFlag where
  ppr flag = empty
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Top level stuff}
%*                                                                      *
%************************************************************************

\begin{code}
dmdAnalPgm :: DynFlags -> [CoreBind] -> IO [CoreBind]
dmdAnalPgm dflags binds
  = do {
        showPass dflags "Demand analysis" ;
        let { binds_plus_dmds = do_prog binds ;
              dmd_changes = get_changes binds_plus_dmds } ;
        endPass dflags "Demand analysis" 
                Opt_D_dump_stranal binds_plus_dmds ;
#ifdef DEBUG
        -- Only if DEBUG is on, because only then is the old strictness analyser run
        printDump (text "Changes in demands" $$ dmd_changes) ;
#endif
        return binds_plus_dmds
    }
  where
    do_prog :: [CoreBind] -> [CoreBind]
    do_prog binds = snd $ mapAccumL dmdAnalTopBind emptySigEnv binds

dmdAnalTopBind :: SigEnv
               -> CoreBind 
               -> (SigEnv, CoreBind)
dmdAnalTopBind sigs (NonRec id rhs)
  = let
        (    _, _, (_,   rhs1)) = dmdAnalRhs TopLevel sigs (id, rhs)
        (sigs2, _, (id2, rhs2)) = dmdAnalRhs TopLevel sigs (id, rhs1)
                -- Do two passes to improve CPR information
                -- See the comments with mkSigTy.ignore_cpr_info below
    in
    (sigs2, NonRec id2 rhs2)    

dmdAnalTopBind sigs (Rec pairs)
  = let
        (sigs', _, pairs')  = dmdFix TopLevel sigs pairs
                -- We get two iterations automatically
    in
    (sigs', Rec pairs')
\end{code}

\begin{code}
dmdAnalTopRhs :: CoreExpr -> (StrictSig, CoreExpr)
-- Analyse the RHS and return
--      a) appropriate strictness info
--      b) the unfolding (decorated with stricntess info)
dmdAnalTopRhs rhs
  = (sig, rhs')
  where
    arity          = exprArity rhs
    (rhs_ty, rhs') = dmdAnal emptySigEnv (vanillaCall arity) rhs
    sig            = mkTopSigTy rhs rhs_ty
\end{code}

%************************************************************************
%*                                                                      *
\subsection{The analyser itself}        
%*                                                                      *
%************************************************************************

\begin{code}
dmdAnal :: SigEnv -> Demand -> CoreExpr -> (DmdType, CoreExpr)

dmdAnal sigs Abs  e = (topDmdType, e)
dmdAnal sigs Bot  e = (botDmdType, e)

dmdAnal sigs Lazy e = let 
                        (res_ty, e') = dmdAnal sigs Eval e
                      in
                      (deferType res_ty, e')
        -- It's important not to analyse e with a lazy demand because
        -- a) When we encounter   case s of (a,b) -> 
        --      we demand s with U(d1d2)... but if the overall demand is lazy
        --      that is wrong, and we'd need to reduce the demand on s,
        --      which is inconvenient
        -- b) More important, consider
        --      f (let x = R in x+x), where f is lazy
        --    We still want to mark x as demanded, because it will be when we
        --    enter the let.  If we analyse f's arg with a Lazy demand, we'll
        --    just mark x as Lazy
        -- c) The application rule wouldn't be right either
        --    Evaluating (f x) in a L demand does *not* cause
        --    evaluation of f in a C(L) demand!


dmdAnal sigs dmd (Lit lit)
  = (topDmdType, Lit lit)

dmdAnal sigs dmd (Var var)
  = (dmdTransform sigs var dmd, Var var)

dmdAnal sigs dmd (Note n e)
  = (dmd_ty, Note n e')
  where
    (dmd_ty, e') = dmdAnal sigs dmd' e  
    dmd' = case n of
             Coerce _ _ -> Eval   -- This coerce usually arises from a recursive
             other      -> dmd    -- newtype, and we don't want to look inside them
                                  -- for exactly the same reason that we don't look
                                  -- inside recursive products -- we might not reach
                                  -- a fixpoint.  So revert to a vanilla Eval demand

dmdAnal sigs dmd (App fun (Type ty))
  = (fun_ty, App fun' (Type ty))
  where
    (fun_ty, fun') = dmdAnal sigs dmd fun

-- Lots of the other code is there to make this
-- beautiful, compositional, application rule :-)
dmdAnal sigs dmd e@(App fun arg)        -- Non-type arguments
  = let                         -- [Type arg handled above]
        (fun_ty, fun')    = dmdAnal sigs (Call dmd) fun
        (arg_ty, arg')    = dmdAnal sigs arg_dmd arg
        (arg_dmd, res_ty) = splitDmdTy fun_ty
    in
    (res_ty `bothType` arg_ty, App fun' arg')

dmdAnal sigs dmd (Lam var body)
  | isTyVar var
  = let   
        (body_ty, body') = dmdAnal sigs dmd body
    in
    (body_ty, Lam var body')

  | Call body_dmd <- dmd        -- A call demand: good!
  = let 
        (body_ty, body') = dmdAnal sigs body_dmd body
        (lam_ty, var')   = annotateLamIdBndr body_ty var
    in
    (lam_ty, Lam var' body')

  | otherwise   -- Not enough demand on the lambda; but do the body
  = let         -- anyway to annotate it and gather free var info
        (body_ty, body') = dmdAnal sigs Eval body
        (lam_ty, var')   = annotateLamIdBndr body_ty var
    in
    (deferType lam_ty, Lam var' body')

dmdAnal sigs dmd (Case scrut case_bndr [alt@(DataAlt dc,bndrs,rhs)])
  | let tycon = dataConTyCon dc,
    isProductTyCon tycon,
    not (isRecursiveTyCon tycon)
  = let
        sigs_alt              = extendSigEnv NotTopLevel sigs case_bndr case_bndr_sig
        (alt_ty, alt')        = dmdAnalAlt sigs_alt dmd alt
        (alt_ty1, case_bndr') = annotateBndr alt_ty case_bndr
        (_, bndrs', _)        = alt'
        case_bndr_sig         = StrictSig (mkDmdType emptyVarEnv [] RetCPR)
                -- Inside the alternative, the case binder has the CPR property.
                -- Meaning that a case on it will successfully cancel.
                -- Example:
                --      f True  x = case x of y { I# x' -> if x' ==# 3 then y else I# 8 }
                --      f False x = I# 3
                --      
                -- We want f to have the CPR property:
                --      f b x = case fw b x of { r -> I# r }
                --      fw True  x = case x of y { I# x' -> if x' ==# 3 then x' else 8 }
                --      fw False x = 3

        -- Figure out whether the demand on the case binder is used, and use
        -- that to set the scrut_dmd.  This is utterly essential.
        -- Consider     f x = case x of y { (a,b) -> k y a }
        -- If we just take scrut_demand = U(L,A), then we won't pass x to the
        -- worker, so the worker will rebuild 
        --      x = (a, absent-error)
        -- and that'll crash.
        -- So at one stage I had:
        --      dead_case_bndr           = isAbsentDmd (idNewDemandInfo case_bndr')
        --      keepity | dead_case_bndr = Drop
        --              | otherwise      = Keep         
        --
        -- But then consider
        --      case x of y { (a,b) -> h y + a }
        -- where h : U(LL) -> T
        -- The above code would compute a Keep for x, since y is not Abs, which is silly
        -- The insight is, of course, that a demand on y is a demand on the
        -- scrutinee, so we need to `both` it with the scrut demand

        scrut_dmd          = mkSeq Drop [idNewDemandInfo b | b <- bndrs', isId b]
                                   `both`
                             idNewDemandInfo case_bndr'

        (scrut_ty, scrut') = dmdAnal sigs scrut_dmd scrut
    in
    (alt_ty1 `bothType` scrut_ty, Case scrut' case_bndr' [alt'])

dmdAnal sigs dmd (Case scrut case_bndr alts)
  = let
        (alt_tys, alts')        = mapAndUnzip (dmdAnalAlt sigs dmd) alts
        (scrut_ty, scrut')      = dmdAnal sigs Eval scrut
        (alt_ty, case_bndr')    = annotateBndr (foldr1 lubType alt_tys) case_bndr
    in
--    pprTrace "dmdAnal:Case" (ppr alts $$ ppr alt_tys)
    (alt_ty `bothType` scrut_ty, Case scrut' case_bndr' alts')

dmdAnal sigs dmd (Let (NonRec id rhs) body) 
  = let
        (sigs', lazy_fv, (id1, rhs')) = dmdAnalRhs NotTopLevel sigs (id, rhs)
        (body_ty, body')              = dmdAnal sigs' dmd body
        (body_ty1, id2)               = annotateBndr body_ty id1
        body_ty2                      = addLazyFVs body_ty1 lazy_fv
    in
    (let vanilla_dmd = vanillaCall (idArity id)
         actual_dmd  = idNewDemandInfo id2
     in
     if not (vanilla_dmd `betterDemand` actual_dmd) then
        pprTrace "dmdLet: better demand" (ppr id <+> vcat [text "vanilla" <+> ppr vanilla_dmd,
                                                           text "actual" <+> ppr actual_dmd])
     else \x -> x)
    (body_ty2, Let (NonRec id2 rhs') body')    

dmdAnal sigs dmd (Let (Rec pairs) body) 
  = let
        bndrs                    = map fst pairs
        (sigs', lazy_fv, pairs') = dmdFix NotTopLevel sigs pairs
        (body_ty, body')         = dmdAnal sigs' dmd body
        body_ty1                 = addLazyFVs body_ty lazy_fv
    in
    sigs' `seq` body_ty `seq`
    let
        (body_ty2, _) = annotateBndrs body_ty1 bndrs
                -- Don't bother to add demand info to recursive
                -- binders as annotateBndr does; 
                -- being recursive, we can't treat them strictly.
                -- But we do need to remove the binders from the result demand env
    in
    (body_ty2,  Let (Rec pairs') body')


dmdAnalAlt sigs dmd (con,bndrs,rhs) 
  = let 
        (rhs_ty, rhs')   = dmdAnal sigs dmd rhs
        (alt_ty, bndrs') = annotateBndrs rhs_ty bndrs
    in
    (alt_ty, (con, bndrs', rhs'))
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Bindings}
%*                                                                      *
%************************************************************************

\begin{code}
dmdFix :: TopLevelFlag
       -> SigEnv                -- Does not include bindings for this binding
       -> [(Id,CoreExpr)]
       -> (SigEnv, DmdEnv,
           [(Id,CoreExpr)])     -- Binders annotated with stricness info

dmdFix top_lvl sigs orig_pairs
  = loop 1 initial_sigs orig_pairs
  where
    bndrs        = map fst orig_pairs
    initial_sigs = extendSigEnvList sigs [(id, (initial_sig id, top_lvl)) | id <- bndrs]
    
    loop :: Int
         -> SigEnv                      -- Already contains the current sigs
         -> [(Id,CoreExpr)]             
         -> (SigEnv, DmdEnv, [(Id,CoreExpr)])
    loop n sigs pairs
      | all (same_sig sigs sigs') bndrs 
      = (sigs', lazy_fv, pairs')
                -- Note: use pairs', not pairs.   pairs' is the result of 
                -- processing the RHSs with sigs (= sigs'), whereas pairs 
                -- is the result of processing the RHSs with the *previous* 
                -- iteration of sigs.
      | n >= 10       = pprTrace "dmdFix loop" (ppr n <+> (vcat 
                                [ text "Sigs:" <+> ppr [(id,lookup sigs id, lookup sigs' id) | (id,_) <- pairs],
                                  text "env:" <+> ppr (ufmToList sigs),
                                  text "binds:" <+> pprCoreBinding (Rec pairs)]))
                              (emptySigEnv, emptyDmdEnv, orig_pairs)    -- Safe output
      | otherwise    = loop (n+1) sigs' pairs'
      where
                -- Use the new signature to do the next pair
                -- The occurrence analyser has arranged them in a good order
                -- so this can significantly reduce the number of iterations needed
        ((sigs',lazy_fv), pairs') = mapAccumL (my_downRhs top_lvl) (sigs, emptyDmdEnv) pairs
        
    my_downRhs top_lvl (sigs,lazy_fv) (id,rhs)
        = -- pprTrace "downRhs {" (ppr id <+> (ppr old_sig))
          -- (new_sig `seq` 
          --    pprTrace "downRhsEnd" (ppr id <+> ppr new_sig <+> char '}' ) 
          ((sigs', lazy_fv'), pair')
          --     )
        where
          (sigs', lazy_fv1, pair') = dmdAnalRhs top_lvl sigs (id,rhs)
          lazy_fv'                 = plusUFM_C both lazy_fv lazy_fv1   
          -- old_sig               = lookup sigs id
          -- new_sig               = lookup sigs' id
           
        -- Get an initial strictness signature from the Id
        -- itself.  That way we make use of earlier iterations
        -- of the fixpoint algorithm.  (Cunning plan.)
        -- Note that the cunning plan extends to the DmdEnv too,
        -- since it is part of the strictness signature
    initial_sig id = idNewStrictness_maybe id `orElse` botSig

    same_sig sigs sigs' var = lookup sigs var == lookup sigs' var
    lookup sigs var = case lookupVarEnv sigs var of
                        Just (sig,_) -> sig

dmdAnalRhs :: TopLevelFlag 
        -> SigEnv -> (Id, CoreExpr)
        -> (SigEnv,  DmdEnv, (Id, CoreExpr))
-- Process the RHS of the binding, add the strictness signature
-- to the Id, and augment the environment with the signature as well.

dmdAnalRhs top_lvl sigs (id, rhs)
 = (sigs', lazy_fv, (id', rhs'))
 where
  arity              = idArity id   -- The idArity should be up to date
                                    -- The simplifier was run just beforehand
  (rhs_dmd_ty, rhs') = dmdAnal sigs (vanillaCall arity) rhs
  (lazy_fv, sig_ty)  = WARN( arity /= dmdTypeDepth rhs_dmd_ty, ppr id )
                       mkSigTy id rhs rhs_dmd_ty
  id'                = id `setIdNewStrictness` sig_ty
  sigs'              = extendSigEnv top_lvl sigs id sig_ty
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Strictness signatures and types}
%*                                                                      *
%************************************************************************

\begin{code}
mkTopSigTy :: CoreExpr -> DmdType -> StrictSig
        -- Take a DmdType and turn it into a StrictSig
        -- NB: not used for never-inline things; hence False
mkTopSigTy rhs dmd_ty = snd (mk_sig_ty False False rhs dmd_ty)

mkSigTy :: Id -> CoreExpr -> DmdType -> (DmdEnv, StrictSig)
mkSigTy id rhs dmd_ty = mk_sig_ty (isNeverActive (idInlinePragma id))
                                  (isStrictDmd (idNewDemandInfo id))
                                  rhs dmd_ty

mk_sig_ty never_inline strictly_demanded rhs (DmdType fv dmds res) 
  | never_inline && not (isBotRes res)
        --                      HACK ALERT
        -- Don't strictness-analyse NOINLINE things.  Why not?  Because
        -- the NOINLINE says "don't expose any of the inner workings at the call 
        -- site" and the strictness is certainly an inner working.
        --
        -- More concretely, the demand analyser discovers the following strictness
        -- for unsafePerformIO:  C(U(AV))
        -- But then consider
        --      unsafePerformIO (\s -> let r = f x in 
        --                             case writeIORef v r s of (# s1, _ #) ->
        --                             (# s1, r #)
        -- The strictness analyser will find that the binding for r is strict,
        -- (becuase of uPIO's strictness sig), and so it'll evaluate it before 
        -- doing the writeIORef.  This actually makes tests/lib/should_run/memo002
        -- get a deadlock!  
        --
        -- Solution: don't expose the strictness of unsafePerformIO.
        --
        -- But we do want to expose the strictness of error functions, 
        -- which are also often marked NOINLINE
        --      {-# NOINLINE foo #-}
        --      foo x = error ("wubble buggle" ++ x)
        -- So (hack, hack) we only drop the strictness for non-bottom things
        -- This is all very unsatisfactory.
  = (deferEnv fv, topSig)

  | otherwise
  = (lazy_fv, mkStrictSig dmd_ty)
  where
    dmd_ty = DmdType strict_fv final_dmds res'

    lazy_fv   = filterUFM (not . isStrictDmd) fv
    strict_fv = filterUFM isStrictDmd         fv
        -- We put the strict FVs in the DmdType of the Id, so 
        -- that at its call sites we unleash demands on its strict fvs.
        -- An example is 'roll' in imaginary/wheel-sieve2
        -- Something like this:
        --      roll x = letrec 
        --                   go y = if ... then roll (x-1) else x+1
        --               in 
        --               go ms
        -- We want to see that roll is strict in x, which is because
        -- go is called.   So we put the DmdEnv for x in go's DmdType.
        --
        -- Another example:
        --      f :: Int -> Int -> Int
        --      f x y = let t = x+1
        --          h z = if z==0 then t else 
        --                if z==1 then x+1 else
        --                x + h (z-1)
        --      in
        --      h y
        -- Calling h does indeed evaluate x, but we can only see
        -- that if we unleash a demand on x at the call site for t.
        --
        -- Incidentally, here's a place where lambda-lifting h would
        -- lose the cigar --- we couldn't see the joint strictness in t/x
        --
        --      ON THE OTHER HAND
        -- We don't want to put *all* the fv's from the RHS into the
        -- DmdType, because that makes fixpointing very slow --- the 
        -- DmdType gets full of lazy demands that are slow to converge.

    final_dmds = setUnpackStrategy dmds
        -- Set the unpacking strategy
        
    res' = case res of
                RetCPR | ignore_cpr_info -> TopRes
                other                    -> res
    ignore_cpr_info = is_thunk && not strictly_demanded
    is_thunk        = not (exprIsValue rhs)
        -- If the rhs is a thunk, we forget the CPR info, because
        -- it is presumably shared (else it would have been inlined, and 
        -- so we'd lose sharing if w/w'd it into a function.
        --
        -- Also, if the strictness analyser has figured out (in a previous iteration)
        -- that it's strict, the let-to-case transformation will happen, so again 
        -- it's good.
        -- This made a big difference to PrelBase.modInt, which had something like
        --      modInt = \ x -> let r = ... -> I# v in
        --                      ...body strict in r...
        -- r's RHS isn't a value yet; but modInt returns r in various branches, so
        -- if r doesn't have the CPR property then neither does modInt
        -- Another case I found in practice (in Complex.magnitude), looks like this:
        --              let k = if ... then I# a else I# b
        --              in ... body strict in k ....
        -- (For this example, it doesn't matter whether k is returned as part of
        -- the overall result.)  Left to itself, the simplifier will make a join
        -- point thus:
        --              let $j k = ...body strict in k...
        --              if ... then $j (I# a) else $j (I# b)
        -- 
        --
        -- The difficulty with this is that we need the strictness type to
        -- look at the body... but we now need the body to calculate the demand
        -- on the variable, so we can decide whether its strictness type should
        -- have a CPR in it or not.  Simple solution: 
        --      a) use strictness info from the previous iteration
        --      b) make sure we do at least 2 iterations, by doing a second
        --         round for top-level non-recs.  Top level recs will get at
        --         least 2 iterations except for totally-bottom functions
        --         which aren't very interesting anyway.
        --
        -- NB: strictly_demanded is never true of a top-level Id, or of a recursive Id.
\end{code}

The unpack strategy determines whether we'll *really* unpack the argument,
or whether we'll just remember its strictness.  If unpacking would give
rise to a *lot* of worker args, we may decide not to unpack after all.

\begin{code}
setUnpackStrategy :: [Demand] -> [Demand]
setUnpackStrategy ds
  = snd (go (opt_MaxWorkerArgs - nonAbsentArgs ds) ds)
  where
    go :: Int                   -- Max number of args available for sub-components of [Demand]
       -> [Demand]
       -> (Int, [Demand])       -- Args remaining after subcomponents of [Demand] are unpacked

    go n (Seq keep cs : ds) 
        | n' >= 0    = Seq keep cs' `cons` go n'' ds
        | otherwise  = Eval `cons` go n ds
        where
          (n'',cs') = go n' cs
          n' = n + box - non_abs_args
          box = case keep of
                   Keep -> 0
                   Drop -> 1    -- Add one to the budget if we drop the top-level arg
          non_abs_args = nonAbsentArgs cs
                -- Delete # of non-absent args to which we'll now be committed
                                
    go n (d:ds) = d `cons` go n ds
    go n []     = (n,[])

    cons d (n,ds) = (n, d:ds)

nonAbsentArgs :: [Demand] -> Int
nonAbsentArgs []         = 0
nonAbsentArgs (Abs : ds) = nonAbsentArgs ds
nonAbsentArgs (d   : ds) = 1 + nonAbsentArgs ds
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Strictness signatures and types}
%*                                                                      *
%************************************************************************

\begin{code}
splitDmdTy :: DmdType -> (Demand, DmdType)
-- Split off one function argument
-- We already have a suitable demand on all
-- free vars, so no need to add more!
splitDmdTy (DmdType fv (dmd:dmds) res_ty) = (dmd, DmdType fv dmds res_ty)
splitDmdTy ty@(DmdType fv [] TopRes)      = (Lazy, ty)
splitDmdTy ty@(DmdType fv [] BotRes)      = (Bot,  ty)
        -- NB: Bot not Abs
splitDmdTy ty@(DmdType fv [] RetCPR)      = panic "splitDmdTy"
        -- We should not be applying a product as a function!
\end{code}

\begin{code}
unitVarDmd var dmd = DmdType (unitVarEnv var dmd) [] TopRes

addVarDmd top_lvl dmd_ty@(DmdType fv ds res) var dmd
  | isTopLevel top_lvl = dmd_ty         -- Don't record top level things
  | otherwise          = DmdType (extendVarEnv fv var dmd) ds res

addLazyFVs (DmdType fv ds res) lazy_fvs
  = DmdType both_fv1 ds res
  where
    both_fv = (plusUFM_C both fv lazy_fvs)
    both_fv1 = modifyEnv (isBotRes res) (`both` Bot) lazy_fvs fv both_fv
        -- This modifyEnv is vital.  Consider
        --      let f = \x -> (x,y)
        --      in  error (f 3)
        -- Here, y is treated as a lazy-fv of f, but we must `both` that L
        -- demand with the bottom coming up from 'error'
        -- 
        -- I got a loop in the fixpointer without this, due to an interaction
        -- with the lazy_fv filtering in mkSigTy.  Roughly, it was
        --      letrec f n x 
        --          = letrec g y = x `fatbar` 
        --                         letrec h z = z + ...g...
        --                         in h (f (n-1) x)
        --      in ...
        -- In the initial iteration for f, f=Bot
        -- Suppose h is found to be strict in z, but the occurrence of g in its RHS
        -- is lazy.  Now consider the fixpoint iteration for g, esp the demands it
        -- places on its free variables.  Suppose it places none.  Then the
        --      x `fatbar` ...call to h...
        -- will give a x->V demand for x.  That turns into a L demand for x,
        -- which floats out of the defn for h.  Without the modifyEnv, that
        -- L demand doesn't get both'd with the Bot coming up from the inner
        -- call to f.  So we just get an L demand for x for g.
        --
        -- A better way to say this is that the lazy-fv filtering should give the
        -- same answer as putting the lazy fv demands in the function's type.

annotateBndr :: DmdType -> Var -> (DmdType, Var)
-- The returned env has the var deleted
-- The returned var is annotated with demand info
-- No effect on the argument demands
annotateBndr dmd_ty@(DmdType fv ds res) var
  | isTyVar var = (dmd_ty, var)
  | otherwise   = (DmdType fv' ds res, 
                   setIdNewDemandInfo var (argDemand var dmd))
  where
    (fv', dmd) = removeFV fv var res

annotateBndrs = mapAccumR annotateBndr

annotateLamIdBndr dmd_ty@(DmdType fv ds res) id
-- For lambdas we add the demand to the argument demands
-- Only called for Ids
  = ASSERT( isId id )
    (DmdType fv' (hacked_dmd:ds) res, setIdNewDemandInfo id hacked_dmd)
  where
    (fv', dmd) = removeFV fv id res
    hacked_dmd = argDemand id dmd
        -- This call to argDemand is vital, because otherwise we label
        -- a lambda binder with demand 'B'.  But in terms of calling
        -- conventions that's Abs, because we don't pass it.  But
        -- when we do a w/w split we get
        --      fw x = (\x y:B -> ...) x (error "oops")
        -- And then the simplifier things the 'B' is a strict demand
        -- and evaluates the (error "oops").  Sigh

removeFV fv var res = (fv', dmd)
                where
                  fv' = fv `delVarEnv` var
                  dmd = lookupVarEnv fv var `orElse` deflt
                  deflt | isBotRes res = Bot
                        | otherwise    = Abs
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Strictness signatures}
%*                                                                      *
%************************************************************************

\begin{code}
type SigEnv  = VarEnv (StrictSig, TopLevelFlag)
        -- We use the SigEnv to tell us whether to
        -- record info about a variable in the DmdEnv
        -- We do so if it's a LocalId, but not top-level
        --
        -- The DmdEnv gives the demand on the free vars of the function
        -- when it is given enough args to satisfy the strictness signature

emptySigEnv  = emptyVarEnv

extendSigEnv :: TopLevelFlag -> SigEnv -> Id -> StrictSig -> SigEnv
extendSigEnv top_lvl env var sig = extendVarEnv env var (sig, top_lvl)

extendSigEnvList = extendVarEnvList

dmdTransform :: SigEnv          -- The strictness environment
             -> Id              -- The function
             -> Demand          -- The demand on the function
             -> DmdType         -- The demand type of the function in this context
        -- Returned DmdEnv includes the demand on 
        -- this function plus demand on its free variables

dmdTransform sigs var dmd

------  DATA CONSTRUCTOR
  | isDataConId var,            -- Data constructor
    Seq k ds <- res_dmd         -- and the demand looks inside its fields
  = let 
        StrictSig dmd_ty    = idNewStrictness var       -- It must have a strictness sig
        DmdType _ _ con_res = dmd_ty
        arity               = idArity var
    in
    if arity == call_depth then         -- Saturated, so unleash the demand
        let 
                -- ds can be empty, when we are just seq'ing the thing
                -- If so we must make up a suitable bunch of demands
           dmd_ds | null ds   = replicate arity Abs
                  | otherwise = ASSERT( ds `lengthIs` arity ) ds

           arg_ds = case k of
                        Keep  -> bothLazy_s dmd_ds
                        Drop  -> dmd_ds
                        Defer -> pprTrace "dmdTransform: surprising!" (ppr var) 
                                        -- I don't think this can happen
                                 dmd_ds
                -- Important!  If we Keep the constructor application, then
                -- we need the demands the constructor places (always lazy)
                -- If not, we don't need to.  For example:
                --      f p@(x,y) = (p,y)       -- S(AL)
                --      g a b     = f (a,b)
                -- It's vital that we don't calculate Absent for a!
        in
        mkDmdType emptyDmdEnv arg_ds con_res
                -- Must remember whether it's a product, hence con_res, not TopRes
    else
        topDmdType

------  IMPORTED FUNCTION
  | isGlobalId var,             -- Imported function
    let StrictSig dmd_ty = getNewStrictness var
  = if dmdTypeDepth dmd_ty <= call_depth then   -- Saturated, so unleash the demand
        dmd_ty
    else
        topDmdType

------  LOCAL LET/REC BOUND THING
  | Just (StrictSig dmd_ty, top_lvl) <- lookupVarEnv sigs var
  = let
        fn_ty | dmdTypeDepth dmd_ty <= call_depth = dmd_ty 
              | otherwise                         = deferType dmd_ty
        -- NB: it's important to use deferType, and not just return topDmdType
        -- Consider     let { f x y = p + x } in f 1
        -- The application isn't saturated, but we must nevertheless propagate 
        --      a lazy demand for p!  
    in
    addVarDmd top_lvl fn_ty var dmd

------  LOCAL NON-LET/REC BOUND THING
  | otherwise                   -- Default case
  = unitVarDmd var dmd

  where
    (call_depth, res_dmd) = splitCallDmd dmd
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Demands}
%*                                                                      *
%************************************************************************

\begin{code}
splitCallDmd :: Demand -> (Int, Demand)
splitCallDmd (Call d) = case splitCallDmd d of
                          (n, r) -> (n+1, r)
splitCallDmd d        = (0, d)

vanillaCall :: Arity -> Demand
vanillaCall 0 = Eval
vanillaCall n = Call (vanillaCall (n-1))

deferType :: DmdType -> DmdType
deferType (DmdType fv _ _) = DmdType (deferEnv fv) [] TopRes
        -- Notice that we throw away info about both arguments and results
        -- For example,   f = let ... in \x -> x
        -- We don't want to get a stricness type V->T for f.
        -- Peter??

deferEnv :: DmdEnv -> DmdEnv
deferEnv fv = mapVarEnv defer fv

---------------
bothLazy :: Demand -> Demand
bothLazy   = both Lazy
bothLazy_s :: [Demand] -> [Demand]
bothLazy_s = map bothLazy


----------------
argDemand :: Id -> Demand -> Demand
argDemand id dmd | isUnLiftedType (idType id) = unliftedArgDemand dmd
                 | otherwise                  = liftedArgDemand   dmd

liftedArgDemand :: Demand -> Demand
-- The 'Defer' demands are just Lazy at function boundaries
-- Ugly!  Ask John how to improve it.
liftedArgDemand (Seq Defer ds) = Lazy
liftedArgDemand (Seq k     ds) = Seq k (map liftedArgDemand ds)
                                        -- Urk! Don't have type info here
liftedArgDemand Err            = Eval   -- Args passed to a bottoming function
liftedArgDemand Bot            = Abs    -- Don't pass args that are consumed by bottom/err
liftedArgDemand d              = d

unliftedArgDemand :: Demand -> Demand
-- Same idea, but for unlifted types the domain is much simpler:
-- Either we use it (Lazy) or we don't (Abs)
unliftedArgDemand Bot   = Abs
unliftedArgDemand Abs   = Abs
unliftedArgDemand other = Lazy
\end{code}

\begin{code}
betterStrictness :: StrictSig -> StrictSig -> Bool
betterStrictness (StrictSig t1) (StrictSig t2) = betterDmdType t1 t2

betterDmdType t1 t2 = (t1 `lubType` t2) == t2

betterDemand :: Demand -> Demand -> Bool
-- If d1 `better` d2, and d2 `better` d2, then d1==d2
betterDemand d1 d2 = (d1 `lub` d2) == d2

squashDmdEnv (StrictSig (DmdType fv ds res)) = StrictSig (DmdType emptyDmdEnv ds res)
\end{code}

\begin{code}
-------------------------
-- Consider (if x then y else []) with demand V
-- Then the first branch gives {y->V} and the second
-- *implicitly* has {y->A}.  So we must put {y->(V `lub` A)}
-- in the result env.
lubType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
  = DmdType lub_fv2 (zipWith lub ds1 ds2) (r1 `lubRes` r2)
  where
    lub_fv  = plusUFM_C lub fv1 fv2
    lub_fv1 = modifyEnv (not (isBotRes r1)) defer fv2 fv1 lub_fv
    lub_fv2 = modifyEnv (not (isBotRes r2)) defer fv1 fv2 lub_fv1
        -- lub is the identity for Bot

-----------------------------------
-- (t1 `bothType` t2) takes the argument/result info from t1,
-- using t2 just for its free-var info
-- NB: Don't forget about r2!  It might be BotRes, which is
--     a bottom demand on all the in-scope variables.
-- Peter: can this be done more neatly?
bothType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
  = DmdType both_fv2 ds1 (r1 `bothRes` r2)
  where
    both_fv  = plusUFM_C both fv1 fv2
    both_fv1 = modifyEnv (isBotRes r1) (`both` Bot) fv2 fv1 both_fv
    both_fv2 = modifyEnv (isBotRes r2) (`both` Bot) fv1 fv2 both_fv1
        -- both is the identity for Abs
\end{code}


\begin{code}
lubRes BotRes r      = r
lubRes r      BotRes = r
lubRes RetCPR RetCPR = RetCPR
lubRes r1     r2     = TopRes

-- If either diverges, the whole thing does
-- Otherwise take CPR info from the first
bothRes r1 BotRes = BotRes
bothRes r1 r2     = r1
\end{code}

\begin{code}
-- A Seq can have an empty list of demands, in the polymorphic case.
lubs [] ds2 = ds2
lubs ds1 [] = ds1
lubs ds1 ds2 = ASSERT( equalLength ds1 ds2 ) zipWith lub ds1 ds2

-----------------------------------
-- A Seq can have an empty list of demands, in the polymorphic case.
boths [] ds2  = ds2
boths ds1 []  = ds1
boths ds1 ds2 = ASSERT( equalLength ds1 ds2 ) zipWith both ds1 ds2
\end{code}

\begin{code}
modifyEnv :: Bool                       -- No-op if False
          -> (Demand -> Demand)         -- The zapper
          -> DmdEnv -> DmdEnv           -- Env1 and Env2
          -> DmdEnv -> DmdEnv           -- Transform this env
        -- Zap anything in Env1 but not in Env2
        -- Assume: dom(env) includes dom(Env1) and dom(Env2)

modifyEnv need_to_modify zapper env1 env2 env
  | need_to_modify = foldr zap env (keysUFM (env1 `minusUFM` env2))
  | otherwise      = env
  where
    zap uniq env = addToUFM_Directly env uniq (zapper current_val)
                 where
                   current_val = expectJust "modifyEnv" (lookupUFM_Directly env uniq)
\end{code}


%************************************************************************
%*                                                                      *
\subsection{LUB and BOTH}
%*                                                                      *
%************************************************************************


\begin{code}
lub :: Demand -> Demand -> Demand

lub Bot d = d

lub Err Bot        = Err 
lub Err Abs        = Lazy       -- E.g. f x = if ... then True else error x
lub Err (Seq k ds) 
  | null ds        = Seq (case k of { Drop -> Keep; other -> k }) []
                        -- Yuk
  | not (null ds)  = Seq k [Err `lub` d | d <- ds]
                        -- E.g. f x = if ... then fst x else error x
                        -- We *cannot* use the (lub Err d = d) case,
                        -- else we'd get U(VA) for x's demand!!
lub Err d          = d 

lub Lazy d = Lazy

lub Abs  d = defer d

lub Eval Abs                           = Lazy
lub Eval Lazy                          = Lazy
lub Eval (Seq Defer ds)                = Lazy   -- Essential!
lub Eval (Seq Drop ds) | not (null ds) = Seq Drop [Lazy | d <- ds]
lub Eval d                             = Eval
        -- For the Seq Drop case, consider
        --      f n []     = n
        --      f n (x:xs) = f (n+x) xs
        -- Here we want to do better than just V for n.  It's
        -- unboxed in the (x:xs) case, and we might be prepared to
        -- rebox it in the [] case.
        -- But if we don't use *any* of the components, give up
        -- and revert to V

lub (Call d1) (Call d2) = Call (lub d1 d2)
lub d1@(Call _) d2      = d2 `lub` d1

lub (Seq k1 ds1) (Seq k2 ds2)
  = Seq (k1 `lub_keep` k2) (lub_ds k1 ds1 k2 ds2)
  where
        ------------------
    lub_ds Keep ds1 Keep ds2                 = ds1 `lubs` ds2
    lub_ds Keep ds1 non_keep ds2 | null ds1  = [Lazy | d <- ds2]
                                 | otherwise = bothLazy_s ds1 `lubs` ds2

    lub_ds non_keep ds1 Keep ds2 | null ds2  = [Lazy | d <- ds1]
                                 | otherwise = ds1 `lubs` bothLazy_s ds2

    lub_ds k1 ds1 k2 ds2                     = ds1 `lubs` ds2

        ------------------
        -- Note that (Keep `lub` Drop) is Drop, not Keep
        -- Why not?  See the example above with (lub Eval d).
    lub_keep Keep k     = k

    lub_keep Drop Defer = Defer
    lub_keep Drop k     = Drop

    lub_keep Defer k    = Defer

lub d1@(Seq _ _) d2 = d2 `lub` d1


defer :: Demand -> Demand
-- Computes (Abs `lub` d)
-- For the Bot case consider
--      f x y = if ... then x else error x
--   Then for y we get Abs `lub` Bot, and we really
--   want Abs overall
defer Bot           = Abs
defer Abs           = Abs
defer (Seq Keep ds) = Lazy
defer (Seq _    ds) = Seq Defer ds
defer d             = Lazy

---------------
both :: Demand -> Demand -> Demand

both Bot Bot        = Bot
both Bot Abs        = Bot
both Bot (Seq k ds) 
  | not (null ds)   = Seq (case k of { Defer -> Drop; other -> k })
                          [both Bot d | d <- ds]
        -- E.g. f x = if ... then error (fst x) else fst x
        -- This equation helps results slightly, 
        -- but is not necessary for soundness
both Bot d          = Err

both Err d = Err

both Abs d   = d

both Lazy Bot            = Err
both Lazy Err            = Err
both Lazy Eval           = Eval
both Lazy (Call d)       = Call d
both Lazy (Seq Defer ds) = Lazy
both Lazy (Seq k ds)     = Seq Keep ds
both Lazy d              = Lazy

-- For the (Eval `both` Bot) case, consider
--      f x = error x
-- From 'error' itself we get demand Bot on x
-- From the arg demand on x we get Eval
-- So we want Eval `both` Bot to be Err.
-- That's what Err is *for*
both Eval Bot        = Err
both Eval Err        = Err
both Eval (Seq k ds) = Seq Keep ds
both Eval d          = Eval

both (Call d1)   (Call d2) = Call (d1 `both` d2)
both d1@(Call _) d2        = d2 `both` d1

both (Seq k1 ds1) (Seq k2 ds2)
  = Seq (k1 `both_keep` k2) (both_ds k1 ds1 k2 ds2)
  where
        ----------------
    both_keep Keep k2 = Keep

    both_keep Drop Keep = Keep
    both_keep Drop k2   = Drop

    both_keep Defer k2  = k2

        ----------------
    both_ds Defer ds1 Defer     ds2 = ds1 `boths` ds2
    both_ds Defer ds1 non_defer ds2 = map defer ds1 `boths` ds2

    both_ds non_defer ds1 Defer ds2 = ds1 `boths` map defer ds2

    both_ds k1 ds1 k2 ds2           = ds1 `boths` ds2

both d1@(Seq _ _) d2 = d2 `both` d1
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Miscellaneous
%*                                                                      *
%************************************************************************


\begin{code}
get_changes binds = vcat (map get_changes_bind binds)

get_changes_bind (Rec pairs) = vcat (map get_changes_pr pairs)
get_changes_bind (NonRec id rhs) = get_changes_pr (id,rhs)

get_changes_pr (id,rhs) 
  = get_changes_var id $$ get_changes_expr rhs

get_changes_var var
  | isId var  = get_changes_str var $$ get_changes_dmd var
  | otherwise = empty

get_changes_expr (Type t)     = empty
get_changes_expr (Var v)      = empty
get_changes_expr (Lit l)      = empty
get_changes_expr (Note n e)   = get_changes_expr e
get_changes_expr (App e1 e2)  = get_changes_expr e1 $$ get_changes_expr e2
get_changes_expr (Lam b e)    = {- get_changes_var b $$ -} get_changes_expr e
get_changes_expr (Let b e)    = get_changes_bind b $$ get_changes_expr e
get_changes_expr (Case e b a) = get_changes_expr e $$ {- get_changes_var b $$ -} vcat (map get_changes_alt a)

get_changes_alt (con,bs,rhs) = {- vcat (map get_changes_var bs) $$ -} get_changes_expr rhs

get_changes_str id
  | new_better && old_better = empty
  | new_better               = message "BETTER"
  | old_better               = message "WORSE"
  | otherwise                = message "INCOMPARABLE" 
  where
    message word = text word <+> text "strictness for" <+> ppr id <+> info
    info = (text "Old" <+> ppr old) $$ (text "New" <+> ppr new)
    new = squashDmdEnv (idNewStrictness id)     -- Don't report diffs in the env
    old = newStrictnessFromOld id
    old_better = old `betterStrictness` new
    new_better = new `betterStrictness` old

get_changes_dmd id
  | isUnLiftedType (idType id) = empty  -- Not useful
  | new_better && old_better = empty
  | new_better               = message "BETTER"
  | old_better               = message "WORSE"
  | otherwise                = message "INCOMPARABLE" 
  where
    message word = text word <+> text "demand for" <+> ppr id <+> info
    info = (text "Old" <+> ppr old) $$ (text "New" <+> ppr new)
    new = liftedArgDemand (idNewDemandInfo id)  -- To avoid spurious improvements
    old = newDemand (idDemandInfo id)
    new_better = new `betterDemand` old 
    old_better = old `betterDemand` new
\end{code}