basicTypes/Demand.lhs

% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[Demand]{@Demand@: A decoupled implementation of a demand domain} \begin{code}

    module Demand (
            StrDmd, UseDmd(..), Count(..),
            countOnce, countMany,   -- cardinality

            Demand, CleanDemand,
            mkProdDmd, mkOnceUsedDmd, mkManyUsedDmd, mkHeadStrict, oneifyDmd,
            getUsage, toCleanDmd,
            absDmd, topDmd, botDmd, seqDmd,
            lubDmd, bothDmd, apply1Dmd, apply2Dmd,
            isTopDmd, isBotDmd, isAbsDmd, isSeqDmd,
            peelUseCall, cleanUseDmd_maybe, strictenDmd, bothCleanDmd,

            DmdType(..), dmdTypeDepth, lubDmdType, bothDmdType,
            nopDmdType, botDmdType, mkDmdType,
            addDemand, removeDmdTyArgs,
            BothDmdArg, mkBothDmdArg, toBothDmdArg,

            DmdEnv, emptyDmdEnv,
            peelFV,

            DmdResult, CPRResult,
            isBotRes, isTopRes,
            topRes, botRes, cprProdRes, vanillaCprProdRes, cprSumRes,
            appIsBottom, isBottomingSig, pprIfaceStrictSig,
            trimCPRInfo, returnsCPR_maybe,
            StrictSig(..), mkStrictSig, mkClosedStrictSig, nopSig, botSig, cprProdSig,
            isNopSig, splitStrictSig, increaseStrictSigArity,

            seqDemand, seqDemandList, seqDmdType, seqStrictSig,

            evalDmd, cleanEvalDmd, cleanEvalProdDmd, isStrictDmd,
            splitDmdTy, splitFVs,
            deferAfterIO,
            postProcessUnsat, postProcessDmdTypeM,

            splitProdDmd_maybe, peelCallDmd, mkCallDmd,
            dmdTransformSig, dmdTransformDataConSig, dmdTransformDictSelSig,
            argOneShots, argsOneShots,

            isSingleUsed, reuseEnv, zapDemand, zapStrictSig,

            strictifyDictDmd

         ) where

    #include "HsVersions.h"

    import StaticFlags
    import DynFlags
    import Outputable
    import Var ( Var )
    import VarEnv
    import UniqFM
    import Util
    import BasicTypes
    import Binary
    import Maybes           ( orElse )

    import Type            ( Type, isUnLiftedType )
    import TyCon           ( isNewTyCon, isClassTyCon )
    import DataCon         ( splitDataProductType_maybe )

\end{code} %************************************************************************ %* * \subsection{Strictness domain} %* * %************************************************************************ Lazy | HeadStr / \ SCall SProd \ / HyperStr \begin{code}

    -- Vanilla strictness domain
    data StrDmd
      = HyperStr             -- Hyper-strict
                             -- Bottom of the lattice
                             -- Note [HyperStr and Use demands]

      | SCall StrDmd         -- Call demand
                             -- Used only for values of function type

      | SProd [MaybeStr]     -- Product
                             -- Used only for values of product type
                             -- Invariant: not all components are HyperStr (use HyperStr)
                             --            not all components are Lazy     (use HeadStr)

      | HeadStr              -- Head-Strict
                             -- A polymorphic demand: used for values of all types,
                             --                       including a type variable

      deriving ( Eq, Show )

    data MaybeStr = Lazy            -- Lazy
                                    -- Top of the lattice
                  | Str StrDmd
      deriving ( Eq, Show )

    -- Well-formedness preserving constructors for the Strictness domain
    strBot, strTop :: MaybeStr
    strBot = Str HyperStr
    strTop = Lazy

    mkSCall :: StrDmd -> StrDmd
    mkSCall HyperStr = HyperStr
    mkSCall s        = SCall s

    mkSProd :: [MaybeStr] -> StrDmd
    mkSProd sx
      | any isHyperStr sx = HyperStr
      | all isLazy     sx = HeadStr
      | otherwise         = SProd sx

    isLazy :: MaybeStr -> Bool
    isLazy Lazy    = True
    isLazy (Str _) = False

    isHyperStr :: MaybeStr -> Bool
    isHyperStr (Str HyperStr) = True
    isHyperStr _              = False

    -- Pretty-printing
    instance Outputable StrDmd where
      ppr HyperStr      = char 'B'
      ppr (SCall s)     = char 'C' <> parens (ppr s)
      ppr HeadStr       = char 'S'
      ppr (SProd sx)    = char 'S' <> parens (hcat (map ppr sx))

    instance Outputable MaybeStr where
      ppr (Str s)       = ppr s
      ppr Lazy          = char 'L'

    lubMaybeStr :: MaybeStr -> MaybeStr -> MaybeStr
    lubMaybeStr Lazy     _        = Lazy
    lubMaybeStr _        Lazy     = Lazy
    lubMaybeStr (Str s1) (Str s2) = Str (s1 `lubStr` s2)

    lubStr :: StrDmd -> StrDmd -> StrDmd
    lubStr HyperStr s              = s
    lubStr (SCall s1) HyperStr     = SCall s1
    lubStr (SCall _)  HeadStr      = HeadStr
    lubStr (SCall s1) (SCall s2)   = SCall (s1 `lubStr` s2)
    lubStr (SCall _)  (SProd _)    = HeadStr
    lubStr (SProd sx) HyperStr     = SProd sx
    lubStr (SProd _)  HeadStr      = HeadStr
    lubStr (SProd s1) (SProd s2)
        | length s1 == length s2   = mkSProd (zipWith lubMaybeStr s1 s2)
        | otherwise                = HeadStr
    lubStr (SProd _) (SCall _)     = HeadStr
    lubStr HeadStr   _             = HeadStr

    bothMaybeStr :: MaybeStr -> MaybeStr -> MaybeStr
    bothMaybeStr Lazy     s           = s
    bothMaybeStr s        Lazy        = s
    bothMaybeStr (Str s1) (Str s2) = Str (s1 `bothStr` s2)

    bothStr :: StrDmd -> StrDmd -> StrDmd
    bothStr HyperStr _             = HyperStr
    bothStr HeadStr s              = s
    bothStr (SCall _)  HyperStr    = HyperStr
    bothStr (SCall s1) HeadStr     = SCall s1
    bothStr (SCall s1) (SCall s2)  = SCall (s1 `bothStr` s2)
    bothStr (SCall _)  (SProd _)   = HyperStr  -- Weird

    bothStr (SProd _)  HyperStr    = HyperStr
    bothStr (SProd s1) HeadStr     = SProd s1
    bothStr (SProd s1) (SProd s2)
        | length s1 == length s2   = mkSProd (zipWith bothMaybeStr s1 s2)
        | otherwise                = HyperStr  -- Weird
    bothStr (SProd _) (SCall _)    = HyperStr

    -- utility functions to deal with memory leaks
    seqStrDmd :: StrDmd -> ()
    seqStrDmd (SProd ds)   = seqStrDmdList ds
    seqStrDmd (SCall s)     = s `seq` ()
    seqStrDmd _            = ()

    seqStrDmdList :: [MaybeStr] -> ()
    seqStrDmdList [] = ()
    seqStrDmdList (d:ds) = seqMaybeStr d `seq` seqStrDmdList ds

    seqMaybeStr :: MaybeStr -> ()
    seqMaybeStr Lazy    = ()
    seqMaybeStr (Str s) = seqStrDmd s

    -- Splitting polymorphic demands
    splitStrProdDmd :: Int -> StrDmd -> Maybe [MaybeStr]
    splitStrProdDmd n HyperStr   = Just (replicate n strBot)
    splitStrProdDmd n HeadStr    = Just (replicate n strTop)
    splitStrProdDmd n (SProd ds) = ASSERT( ds `lengthIs` n) Just ds
    splitStrProdDmd _ (SCall {}) = Nothing
          -- This can happen when the programmer uses unsafeCoerce,
          -- and we don't then want to crash the compiler (Trac #9208)

\end{code} %************************************************************************ %* * \subsection{Absence domain} %* * %************************************************************************ Used / \ UCall UProd \ / UHead | Abs \begin{code}

    -- Domain for genuine usage
    data UseDmd
      = UCall Count UseDmd   -- Call demand for absence
                             -- Used only for values of function type

      | UProd [MaybeUsed]     -- Product
                             -- Used only for values of product type
                             -- See Note [Don't optimise UProd(Used) to Used]
                             -- [Invariant] Not all components are Abs
                             --             (in that case, use UHead)

      | UHead                -- May be used; but its sub-components are
                             -- definitely *not* used.  Roughly U(AAA)
                             -- Eg the usage of x in x `seq` e
                             -- A polymorphic demand: used for values of all types,
                             --                       including a type variable
                             -- Since (UCall _ Abs) is ill-typed, UHead doesn't
                             -- make sense for lambdas

      | Used                 -- May be used; and its sub-components may be used
                             -- Top of the lattice
      deriving ( Eq, Show )

    -- Extended usage demand for absence and counting
    data MaybeUsed
      = Abs                  -- Definitely unused
                             -- Bottom of the lattice

      | Use Count UseDmd     -- May be used with some cardinality
      deriving ( Eq, Show )

    -- Abstract counting of usages
    data Count = One | Many
      deriving ( Eq, Show )

    -- Pretty-printing
    instance Outputable MaybeUsed where
      ppr Abs           = char 'A'
      ppr (Use Many a)   = ppr a
      ppr (Use One  a)   = char '1' <> char '*' <> ppr a

    instance Outputable UseDmd where
      ppr Used           = char 'U'
      ppr (UCall c a)    = char 'C' <> ppr c <> parens (ppr a)
      ppr UHead          = char 'H'
      ppr (UProd as)     = char 'U' <> parens (hcat (punctuate (char ',') (map ppr as)))

    instance Outputable Count where
      ppr One  = char '1'
      ppr Many = text ""

    -- Well-formedness preserving constructors for the Absence domain
    countOnce, countMany :: Count
    countOnce = One
    countMany = Many

    useBot, useTop :: MaybeUsed
    useBot     = Abs
    useTop     = Use Many Used

    mkUCall :: Count -> UseDmd -> UseDmd
    --mkUCall c Used = Used c
    mkUCall c a  = UCall c a

    mkUProd :: [MaybeUsed] -> UseDmd
    mkUProd ux
      | all (== Abs) ux    = UHead
      | otherwise          = UProd ux

    lubCount :: Count -> Count -> Count
    lubCount _ Many = Many
    lubCount Many _ = Many
    lubCount x _    = x

    lubMaybeUsed :: MaybeUsed -> MaybeUsed -> MaybeUsed
    lubMaybeUsed Abs x                   = x
    lubMaybeUsed x Abs                   = x
    lubMaybeUsed (Use c1 a1) (Use c2 a2) = Use (lubCount c1 c2) (lubUse a1 a2)

    lubUse :: UseDmd -> UseDmd -> UseDmd
    lubUse UHead       u               = u
    lubUse (UCall c u) UHead           = UCall c u
    lubUse (UCall c1 u1) (UCall c2 u2) = UCall (lubCount c1 c2) (lubUse u1 u2)
    lubUse (UCall _ _) _               = Used
    lubUse (UProd ux) UHead            = UProd ux
    lubUse (UProd ux1) (UProd ux2)
         | length ux1 == length ux2    = UProd $ zipWith lubMaybeUsed ux1 ux2
         | otherwise                   = Used
    lubUse (UProd {}) (UCall {})       = Used
    -- lubUse (UProd {}) Used             = Used
    lubUse (UProd ux) Used             = UProd (map (`lubMaybeUsed` useTop) ux)
    lubUse Used       (UProd ux)       = UProd (map (`lubMaybeUsed` useTop) ux)
    lubUse Used _                      = Used  -- Note [Used should win]

    -- `both` is different from `lub` in its treatment of counting; if
    -- `both` is computed for two used, the result always has
    --  cardinality `Many` (except for the inner demands of UCall demand -- [TODO] explain).
    --  Also,  x `bothUse` x /= x (for anything but Abs).

    bothMaybeUsed :: MaybeUsed -> MaybeUsed -> MaybeUsed
    bothMaybeUsed Abs x                   = x
    bothMaybeUsed x Abs                   = x
    bothMaybeUsed (Use _ a1) (Use _ a2)   = Use Many (bothUse a1 a2)

    bothUse :: UseDmd -> UseDmd -> UseDmd
    bothUse UHead       u               = u
    bothUse (UCall c u) UHead           = UCall c u

    -- Exciting special treatment of inner demand for call demands:
    --    use `lubUse` instead of `bothUse`!
    bothUse (UCall _ u1) (UCall _ u2)   = UCall Many (u1 `lubUse` u2)

    bothUse (UCall {}) _                = Used
    bothUse (UProd ux) UHead            = UProd ux
    bothUse (UProd ux1) (UProd ux2)
          | length ux1 == length ux2    = UProd $ zipWith bothMaybeUsed ux1 ux2
          | otherwise                   = Used
    bothUse (UProd {}) (UCall {})       = Used
    -- bothUse (UProd {}) Used             = Used  -- Note [Used should win]
    bothUse Used (UProd ux)             = UProd (map (`bothMaybeUsed` useTop) ux)
    bothUse (UProd ux) Used             = UProd (map (`bothMaybeUsed` useTop) ux)
    bothUse Used _                      = Used  -- Note [Used should win]

    peelUseCall :: UseDmd -> Maybe (Count, UseDmd)
    peelUseCall (UCall c u)   = Just (c,u)
    peelUseCall _             = Nothing

\end{code} Note [Don't optimise UProd(Used) to Used] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ These two UseDmds: UProd [Used, Used] and Used are semantically equivalent, but we do not turn the former into the latter, for a regrettable-subtle reason. Suppose we did. then f (x,y) = (y,x) would get StrDmd = Str = SProd [Lazy, Lazy] UseDmd = Used = UProd [Used, Used] But with the joint demand of doesn't convey any clue that there is a product involved, and so the worthSplittingFun will not fire. (We'd need to use the type as well to make it fire.) Moreover, consider g h p@(_,_) = h p This too would get , but this time there really isn't any point in w/w since the components of the pair are not used at all. So the solution is: don't aggressively collapse UProd [Used,Used] to Used; intead leave it as-is. In effect we are using the UseDmd to do a little bit of boxity analysis. Not very nice. Note [Used should win] ~~~~~~~~~~~~~~~~~~~~~~ Both in lubUse and bothUse we want (Used `both` UProd us) to be Used. Why? Because Used carries the implication the whole thing is used, box and all, so we don't want to w/w it. If we use it both boxed and unboxed, then we are definitely using the box, and so we are quite likely to pay a reboxing cost. So we make Used win here. Example is in the Buffer argument of GHC.IO.Handle.Internals.writeCharBuffer Baseline: (A) Not making Used win (UProd wins) Compare with: (B) making Used win for lub and both Min -0.3% -5.6% -10.7% -11.0% -33.3% Max +0.3% +45.6% +11.5% +11.5% +6.9% Geometric Mean -0.0% +0.5% +0.3% +0.2% -0.8% Baseline: (B) Making Used win for both lub and both Compare with: (C) making Used win for both, but UProd win for lub Min -0.1% -0.3% -7.9% -8.0% -6.5% Max +0.1% +1.0% +21.0% +21.0% +0.5% Geometric Mean +0.0% +0.0% -0.0% -0.1% -0.1% \begin{code}

    -- If a demand is used multiple times (i.e. reused), than any use-once
    -- mentioned there, that is not protected by a UCall, can happen many times.
    markReusedDmd :: MaybeUsed -> MaybeUsed
    markReusedDmd Abs         = Abs
    markReusedDmd (Use _ a)   = Use Many (markReused a)

    markReused :: UseDmd -> UseDmd
    markReused (UCall _ u)      = UCall Many u   -- No need to recurse here
    markReused (UProd ux)       = UProd (map markReusedDmd ux)
    markReused u                = u

    isUsedMU :: MaybeUsed -> Bool
    -- True <=> markReusedDmd d = d
    isUsedMU Abs          = True
    isUsedMU (Use One _)  = False
    isUsedMU (Use Many u) = isUsedU u

    isUsedU :: UseDmd -> Bool
    -- True <=> markReused d = d
    isUsedU Used           = True
    isUsedU UHead          = True
    isUsedU (UProd us)     = all isUsedMU us
    isUsedU (UCall One _)  = False
    isUsedU (UCall Many _) = True  -- No need to recurse

    -- Squashing usage demand demands
    seqUseDmd :: UseDmd -> ()
    seqUseDmd (UProd ds)   = seqMaybeUsedList ds
    seqUseDmd (UCall c d)  = c `seq` seqUseDmd d
    seqUseDmd _            = ()

    seqMaybeUsedList :: [MaybeUsed] -> ()
    seqMaybeUsedList []     = ()
    seqMaybeUsedList (d:ds) = seqMaybeUsed d `seq` seqMaybeUsedList ds

    seqMaybeUsed :: MaybeUsed -> ()
    seqMaybeUsed (Use c u)  = c `seq` seqUseDmd u
    seqMaybeUsed _          = ()

    -- Splitting polymorphic Maybe-Used demands
    splitUseProdDmd :: Int -> UseDmd -> Maybe [MaybeUsed]
    splitUseProdDmd n Used          = Just (replicate n useTop)
    splitUseProdDmd n UHead         = Just (replicate n Abs)
    splitUseProdDmd n (UProd ds)    = ASSERT2( ds `lengthIs` n, text "splitUseProdDmd" $$ ppr n $$ ppr ds )
                                      Just ds
    splitUseProdDmd _ (UCall _ _)   = Nothing

\end{code} %************************************************************************ %* * \subsection{Joint domain for Strictness and Absence} %* * %************************************************************************ \begin{code}

    data JointDmd = JD { strd :: MaybeStr, absd :: MaybeUsed }
      deriving ( Eq, Show )

    -- Pretty-printing
    instance Outputable JointDmd where
      ppr (JD {strd = s, absd = a}) = angleBrackets (ppr s <> char ',' <> ppr a)

    -- Well-formedness preserving constructors for the joint domain
    mkJointDmd :: MaybeStr -> MaybeUsed -> JointDmd
    mkJointDmd s a = JD { strd = s, absd = a }

    mkJointDmds :: [MaybeStr] -> [MaybeUsed] -> [JointDmd]
    mkJointDmds ss as = zipWithEqual "mkJointDmds" mkJointDmd ss as

    absDmd :: JointDmd
    absDmd = mkJointDmd Lazy Abs

    apply1Dmd, apply2Dmd :: Demand
    -- C1(U), C1(C1(U)) respectively
    apply1Dmd = JD { strd = Lazy, absd = Use Many (UCall One Used) }
    apply2Dmd = JD { strd = Lazy, absd = Use Many (UCall One (UCall One Used)) }

    topDmd :: JointDmd
    topDmd = mkJointDmd Lazy useTop

    seqDmd :: JointDmd
    seqDmd = mkJointDmd (Str HeadStr) (Use One UHead)

    botDmd :: JointDmd
    botDmd = mkJointDmd strBot useBot

    lubDmd :: JointDmd -> JointDmd -> JointDmd
    lubDmd (JD {strd = s1, absd = a1})
           (JD {strd = s2, absd = a2}) = mkJointDmd (s1 `lubMaybeStr` s2) (a1 `lubMaybeUsed` a2)

    bothDmd :: JointDmd -> JointDmd -> JointDmd
    bothDmd (JD {strd = s1, absd = a1})
            (JD {strd = s2, absd = a2}) = mkJointDmd (s1 `bothMaybeStr` s2) (a1 `bothMaybeUsed` a2)

    isTopDmd :: JointDmd -> Bool
    isTopDmd (JD {strd = Lazy, absd = Use Many Used}) = True
    isTopDmd _                                        = False

    isBotDmd :: JointDmd -> Bool
    isBotDmd (JD {strd = Str HyperStr, absd = Abs}) = True
    isBotDmd _                                      = False

    isAbsDmd :: JointDmd -> Bool
    isAbsDmd (JD {absd = Abs})  = True   -- The strictness part can be HyperStr
    isAbsDmd _                  = False  -- for a bottom demand

    isSeqDmd :: JointDmd -> Bool
    isSeqDmd (JD {strd=Str HeadStr, absd=Use _ UHead}) = True
    isSeqDmd _                                         = False

    -- More utility functions for strictness
    seqDemand :: JointDmd -> ()
    seqDemand (JD {strd = x, absd = y}) = seqMaybeStr x `seq` seqMaybeUsed y `seq` ()

    seqDemandList :: [JointDmd] -> ()
    seqDemandList [] = ()
    seqDemandList (d:ds) = seqDemand d `seq` seqDemandList ds

    isStrictDmd :: Demand -> Bool
    -- See Note [Strict demands]
    isStrictDmd (JD {absd = Abs})  = False
    isStrictDmd (JD {strd = Lazy}) = False
    isStrictDmd _                  = True

    isWeakDmd :: Demand -> Bool
    isWeakDmd (JD {strd = s, absd = a}) = isLazy s && isUsedMU a

    cleanUseDmd_maybe :: JointDmd -> Maybe UseDmd
    cleanUseDmd_maybe (JD { absd = Use _ ud }) = Just ud
    cleanUseDmd_maybe _                        = Nothing

    splitFVs :: Bool   -- Thunk
             -> DmdEnv -> (DmdEnv, DmdEnv)
    splitFVs is_thunk rhs_fvs
      | is_thunk  = foldUFM_Directly add (emptyVarEnv, emptyVarEnv) rhs_fvs
      | otherwise = partitionVarEnv isWeakDmd rhs_fvs
      where
        add uniq dmd@(JD { strd = s, absd = u }) (lazy_fv, sig_fv)
          | Lazy <- s = (addToUFM_Directly lazy_fv uniq dmd, sig_fv)
          | otherwise = ( addToUFM_Directly lazy_fv uniq (JD { strd = Lazy, absd = u })
                        , addToUFM_Directly sig_fv  uniq (JD { strd = s,    absd = Abs }) )

\end{code} %************************************************************************ %* * \subsection{Clean demand for Strictness and Usage} %* * %************************************************************************ This domain differst from JointDemand in the sence that pure absence is taken away, i.e., we deal *only* with non-absent demands. Note [Strict demands] ~~~~~~~~~~~~~~~~~~~~~ isStrictDmd returns true only of demands that are both strict and used In particular, it is False for , which can and does arise in, say (Trac #7319) f x = raise# Then 'x' is not used, so f gets strictness -> . Now the w/w generates fx = let x = absentError "unused" in raise At this point we really don't want to convert to fx = case absentError "unused" of x -> raise Since the program is going to diverge, this swaps one error for another, but it's really a bad idea to *ever* evaluate an absent argument. In Trac #7319 we get T7319.exe: Oops! Entered absent arg w_s1Hd{v} [lid] [base:GHC.Base.String{tc 36u}] Note [Dealing with call demands] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Call demands are constructed and deconstructed coherently for strictness and absence. For instance, the strictness signature for the following function f :: (Int -> (Int, Int)) -> (Int, Bool) f g = (snd (g 3), True) should be: m \begin{code}

    data CleanDemand = CD { sd :: StrDmd, ud :: UseDmd }
      deriving ( Eq, Show )

    instance Outputable CleanDemand where
      ppr (CD {sd = s, ud = a}) = angleBrackets (ppr s <> comma <> ppr a)

    mkCleanDmd :: StrDmd -> UseDmd -> CleanDemand
    mkCleanDmd s a = CD { sd = s, ud = a }

    bothCleanDmd :: CleanDemand -> CleanDemand -> CleanDemand
    bothCleanDmd (CD { sd = s1, ud = a1}) (CD { sd = s2, ud = a2})
      = CD { sd = s1 `bothStr` s2, ud = a1 `bothUse` a2 }

    mkHeadStrict :: CleanDemand -> CleanDemand
    mkHeadStrict (CD { ud = a }) = mkCleanDmd HeadStr a

    oneifyDmd :: JointDmd -> JointDmd
    oneifyDmd (JD { strd = s, absd = Use _ a }) = JD { strd = s, absd = Use One a }
    oneifyDmd jd                                = jd

    mkOnceUsedDmd, mkManyUsedDmd :: CleanDemand -> JointDmd
    mkOnceUsedDmd (CD {sd = s,ud = a}) = mkJointDmd (Str s) (Use One a)
    mkManyUsedDmd (CD {sd = s,ud = a}) = mkJointDmd (Str s) (Use Many a)

    getUsage :: CleanDemand -> UseDmd
    getUsage = ud

    evalDmd :: JointDmd
    -- Evaluated strictly, and used arbitrarily deeply
    evalDmd = mkJointDmd (Str HeadStr) useTop

    mkProdDmd :: [JointDmd] -> CleanDemand
    mkProdDmd dx
      = mkCleanDmd sp up
      where
        sp = mkSProd $ map strd dx
        up = mkUProd $ map absd dx

    mkCallDmd :: CleanDemand -> CleanDemand
    mkCallDmd (CD {sd = d, ud = u})
      = mkCleanDmd (mkSCall d) (mkUCall One u)

    cleanEvalDmd :: CleanDemand
    cleanEvalDmd = mkCleanDmd HeadStr Used

    cleanEvalProdDmd :: Arity -> CleanDemand
    cleanEvalProdDmd n = mkCleanDmd HeadStr (UProd (replicate n useTop))

    isSingleUsed :: JointDmd -> Bool
    isSingleUsed (JD {absd=a}) = is_used_once a
      where
        is_used_once Abs         = True
        is_used_once (Use One _) = True
        is_used_once _           = False

\end{code} Note [Threshold demands] ~~~~~~~~~~~~~~~~~~~~~~~~ Threshold usage demand is generated to figure out if cardinality-instrumented demands of a binding's free variables should be unleashed. See also [Aggregated demand for cardinality]. Note [Replicating polymorphic demands] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Some demands can be considered as polymorphic. Generally, it is applicable to such beasts as tops, bottoms as well as Head-Used adn Head-stricts demands. For instance, S ~ S(L, ..., L) Also, when top or bottom is occurred as a result demand, it in fact can be expanded to saturate a callee's arity. \begin{code}

    splitProdDmd_maybe :: JointDmd -> Maybe [JointDmd]
    -- Split a product into its components, iff there is any
    -- useful information to be extracted thereby
    -- The demand is not necessarily strict!
    splitProdDmd_maybe (JD {strd = s, absd = u})
      = case (s,u) of
          (Str (SProd sx), Use _ u)          | Just ux <- splitUseProdDmd (length sx) u
                                             -> Just (mkJointDmds sx ux)
          (Str s,          Use _ (UProd ux)) | Just sx <- splitStrProdDmd (length ux) s
                                             -> Just (mkJointDmds sx ux)
          (Lazy,           Use _ (UProd ux)) -> Just (mkJointDmds (replicate (length ux) Lazy) ux)
          _ -> Nothing

\end{code} %************************************************************************ %* * Demand results %* * %************************************************************************ DmdResult: Dunno CPRResult / Diverges CPRResult: NoCPR / \ RetProd RetSum ConTag Product contructors return (Dunno (RetProd rs)) In a fixpoint iteration, start from Diverges We have lubs, but not glbs; but that is ok. \begin{code}

    ------------------------------------------------------------------------
    -- Constructed Product Result
    ------------------------------------------------------------------------

    data Termination r = Diverges    -- Definitely diverges
                       | Dunno r     -- Might diverge or converge
                   deriving( Eq, Show )

    type DmdResult = Termination CPRResult

    data CPRResult = NoCPR          -- Top of the lattice
                   | RetProd        -- Returns a constructor from a product type
                   | RetSum ConTag  -- Returns a constructor from a data type
                   deriving( Eq, Show )

    lubCPR :: CPRResult -> CPRResult -> CPRResult
    lubCPR (RetSum t1) (RetSum t2)
      | t1 == t2                       = RetSum t1
    lubCPR RetProd     RetProd     = RetProd
    lubCPR _ _                     = NoCPR

    lubDmdResult :: DmdResult -> DmdResult -> DmdResult
    lubDmdResult Diverges       r              = r
    lubDmdResult (Dunno c1)     Diverges       = Dunno c1
    lubDmdResult (Dunno c1)     (Dunno c2)     = Dunno (c1 `lubCPR` c2)
    -- This needs to commute with defaultDmd, i.e.
    -- defaultDmd (r1 `lubDmdResult` r2) = defaultDmd r1 `lubDmd` defaultDmd r2
    -- (See Note [Default demand on free variables] for why)

    bothDmdResult :: DmdResult -> Termination () -> DmdResult
    -- See Note [Asymmetry of 'both' for DmdType and DmdResult]
    bothDmdResult _              Diverges   = Diverges
    bothDmdResult r              _          = r
    -- This needs to commute with defaultDmd, i.e.
    -- defaultDmd (r1 `bothDmdResult` r2) = defaultDmd r1 `bothDmd` defaultDmd r2
    -- (See Note [Default demand on free variables] for why)

    instance Outputable DmdResult where
      ppr Diverges      = char 'b'
      ppr (Dunno c)     = ppr c

    instance Outputable CPRResult where
      ppr NoCPR        = empty
      ppr (RetSum n)   = char 'm' <> int n
      ppr RetProd      = char 'm'

    seqDmdResult :: DmdResult -> ()
    seqDmdResult Diverges = ()
    seqDmdResult (Dunno c)     = seqCPRResult c

    seqCPRResult :: CPRResult -> ()
    seqCPRResult NoCPR        = ()
    seqCPRResult (RetSum n)   = n `seq` ()
    seqCPRResult RetProd      = ()

    ------------------------------------------------------------------------
    -- Combined demand result                                             --
    ------------------------------------------------------------------------

    -- [cprRes] lets us switch off CPR analysis
    -- by making sure that everything uses TopRes
    topRes, botRes :: DmdResult
    topRes = Dunno NoCPR
    botRes = Diverges

    cprSumRes :: ConTag -> DmdResult
    cprSumRes tag | opt_CprOff = topRes
                  | otherwise  = Dunno $ RetSum tag

    cprProdRes :: [DmdType] -> DmdResult
    cprProdRes _arg_tys
      | opt_CprOff = topRes
      | otherwise  = Dunno $ RetProd

    vanillaCprProdRes :: Arity -> DmdResult
    vanillaCprProdRes _arity
      | opt_CprOff = topRes
      | otherwise  = Dunno $ RetProd

    isTopRes :: DmdResult -> Bool
    isTopRes (Dunno NoCPR) = True
    isTopRes _             = False

    isBotRes :: DmdResult -> Bool
    isBotRes Diverges = True
    isBotRes _        = False

    trimCPRInfo :: Bool -> Bool -> DmdResult -> DmdResult
    trimCPRInfo trim_all trim_sums res
      = trimR res
      where
        trimR (Dunno c)     = Dunno (trimC c)
        trimR Diverges      = Diverges

        trimC (RetSum n)   | trim_all || trim_sums = NoCPR
                           | otherwise             = RetSum n
        trimC RetProd      | trim_all  = NoCPR
                           | otherwise = RetProd
        trimC NoCPR = NoCPR

    returnsCPR_maybe :: DmdResult -> Maybe ConTag
    returnsCPR_maybe (Dunno c)     = retCPR_maybe c
    returnsCPR_maybe Diverges      = Nothing

    retCPR_maybe :: CPRResult -> Maybe ConTag
    retCPR_maybe (RetSum t)  = Just t
    retCPR_maybe RetProd     = Just fIRST_TAG
    retCPR_maybe NoCPR       = Nothing

    -- See Notes [Default demand on free variables]
    -- and [defaultDmd vs. resTypeArgDmd]
    defaultDmd :: Termination r -> JointDmd
    defaultDmd Diverges = botDmd
    defaultDmd _        = absDmd

    resTypeArgDmd :: DmdResult -> JointDmd
    -- TopRes and BotRes are polymorphic, so that
    --      BotRes === Bot -> BotRes === ...
    --      TopRes === Top -> TopRes === ...
    -- This function makes that concrete
    -- Also see Note [defaultDmd vs. resTypeArgDmd]
    resTypeArgDmd r | isBotRes r = botDmd
    resTypeArgDmd _              = topDmd

\end{code} Note [defaultDmd and resTypeArgDmd] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ These functions are similar: They express the demand on something not explicitly mentioned in the environment resp. the argument list. Yet they are different: * Variables not mentioned in the free variables environment are definitely unused, so we can use absDmd there. * Further arguments *can* be used, of course. Hence topDmd is used. Note [Worthy functions for Worker-Wrapper split] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For non-bottoming functions a worker-wrapper transformation takes into account several possibilities to decide if the function is worthy for splitting: 1. The result is of product type and the function is strict in some (or even all) of its arguments. The check that the argument is used is more of sanity nature, since strictness implies usage. Example: f :: (Int, Int) -> Int f p = (case p of (a,b) -> a) + 1 should be splitted to f :: (Int, Int) -> Int f p = case p of (a,b) -> $wf a $wf :: Int -> Int $wf a = a + 1 2. Sometimes it also makes sense to perform a WW split if the strictness analysis cannot say for sure if the function is strict in components of its argument. Then we reason according to the inferred usage information: if the function uses its product argument's components, the WW split can be beneficial. Example: g :: Bool -> (Int, Int) -> Int g c p = case p of (a,b) -> if c then a else b The function g is strict in is argument p and lazy in its components. However, both components are used in the RHS. The idea is since some of the components (both in this case) are used in the right-hand side, the product must presumable be taken apart. Therefore, the WW transform splits the function g to g :: Bool -> (Int, Int) -> Int g c p = case p of (a,b) -> $wg c a b $wg :: Bool -> Int -> Int -> Int $wg c a b = if c then a else b 3. If an argument is absent, it would be silly to pass it to a function, hence the worker with reduced arity is generated. Note [Worker-wrapper for bottoming functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We used not to split if the result is bottom. [Justification: there's no efficiency to be gained.] But it's sometimes bad not to make a wrapper. Consider fw = \x# -> let x = I# x# in case e of p1 -> error_fn x p2 -> error_fn x p3 -> the real stuff The re-boxing code won't go away unless error_fn gets a wrapper too. [We don't do reboxing now, but in general it's better to pass an unboxed thing to f, and have it reboxed in the error cases....] However we *don't* want to do this when the argument is not actually taken apart in the function at all. Otherwise we risk decomposing a masssive tuple which is barely used. Example: f :: ((Int,Int) -> String) -> (Int,Int) -> a f g pr = error (g pr) main = print (f fst (1, error "no")) Here, f does not take 'pr' apart, and it's stupid to do so. Imagine that it had millions of fields. This actually happened in GHC itself where the tuple was DynFlags %************************************************************************ %* * \subsection{Demand environments and types} %* * %************************************************************************ \begin{code}

    type Demand = JointDmd

    type DmdEnv = VarEnv Demand   -- See Note [Default demand on free variables]

    data DmdType = DmdType
                      DmdEnv        -- Demand on explicitly-mentioned
                                    --      free variables
                      [Demand]      -- Demand on arguments
                      DmdResult     -- See [Nature of result demand]

\end{code} Note [Nature of result demand] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A DmdResult contains information about termination (currently distinguishing definite divergence and no information; it is possible to include definite convergence here), and CPR information about the result. The semantics of this depends on whether we are looking at a DmdType, i.e. the demand put on by an expression _under a specific incoming demand_ on its environment, or at a StrictSig describing a demand transformer. For a * DmdType, the termination information is true given the demand it was generated with, while for * a StrictSig it is olds after applying enough arguments. The CPR information, though, is valid after the number of arguments mentioned in the type is given. Therefore, when forgetting the demand on arguments, as in dmdAnalRhs, this needs to be considere (via removeDmdTyArgs). Consider b2 x y = x `seq` y `seq` error (show x) this has a strictness signature of b meaning that "b2 `seq` ()" and "b2 1 `seq` ()" might well terminate, but for "b2 1 2 `seq` ()" we get definite divergence. For comparision, b1 x = x `seq` error (show x) has a strictness signature of b and "b1 1 `seq` ()" is known to terminate. Now consider a function h with signature "", and the expression e1 = h b1 now h puts a demand of onto its argument, and the demand transformer turns it into b Now the DmdResult "b" does apply to us, even though "b1 `seq` ()" does not diverge, and we do not anything being passed to b. Note [Asymmetry of 'both' for DmdType and DmdResult] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 'both' for DmdTypes is *assymetrical*, because there is only one result! For example, given (e1 e2), we get a DmdType dt1 for e1, use its arg demand to analyse e2 giving dt2, and then do (dt1 `bothType` dt2). Similarly with case e of { p -> rhs } we get dt_scrut from the scrutinee and dt_rhs from the RHS, and then compute (dt_rhs `bothType` dt_scrut). We 1. combine the information on the free variables, 2. take the demand on arguments from the first argument 3. combine the termination results, but 4. take CPR info from the first argument. 3 and 4 are implementd in bothDmdResult. \begin{code}

    -- Equality needed for fixpoints in DmdAnal
    instance Eq DmdType where
      (==) (DmdType fv1 ds1 res1)
           (DmdType fv2 ds2 res2) =  ufmToList fv1 == ufmToList fv2
                                  && ds1 == ds2 && res1 == res2

    lubDmdType :: DmdType -> DmdType -> DmdType
    lubDmdType d1 d2
      = DmdType lub_fv lub_ds lub_res
      where
        n = max (dmdTypeDepth d1) (dmdTypeDepth d2)
        (DmdType fv1 ds1 r1) = ensureArgs n d1
        (DmdType fv2 ds2 r2) = ensureArgs n d2

        lub_fv  = plusVarEnv_CD lubDmd fv1 (defaultDmd r1) fv2 (defaultDmd r2)
        lub_ds  = zipWithEqual "lubDmdType" lubDmd ds1 ds2
        lub_res = lubDmdResult r1 r2

\end{code} Note [The need for BothDmdArg] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Previously, the right argument to bothDmdType, as well as the return value of dmdAnalStar via postProcessDmdTypeM, was a DmdType. But bothDmdType only needs to know about the free variables and termination information, but nothing about the demand put on arguments, nor cpr information. So we make that explicit by only passing the relevant information. \begin{code}

    type BothDmdArg = (DmdEnv, Termination ())

    mkBothDmdArg :: DmdEnv -> BothDmdArg
    mkBothDmdArg env = (env, Dunno ())

    toBothDmdArg :: DmdType -> BothDmdArg
    toBothDmdArg (DmdType fv _ r) = (fv, go r)
      where
      go (Dunno {})     = Dunno ()
      go Diverges       = Diverges

    bothDmdType :: DmdType -> BothDmdArg -> DmdType
    bothDmdType (DmdType fv1 ds1 r1) (fv2, t2)
        -- See Note [Asymmetry of 'both' for DmdType and DmdResult]
        -- 'both' takes the argument/result info from its *first* arg,
        -- using its second arg just for its free-var info.
      = DmdType both_fv ds1 (r1 `bothDmdResult` t2)
      where both_fv = plusVarEnv_CD bothDmd fv1 (defaultDmd r1) fv2 (defaultDmd t2)

    instance Outputable DmdType where
      ppr (DmdType fv ds res)
        = hsep [text "DmdType",
                hcat (map ppr ds) <> ppr res,
                if null fv_elts then empty
                else braces (fsep (map pp_elt fv_elts))]
        where
          pp_elt (uniq, dmd) = ppr uniq <> text "->" <> ppr dmd
          fv_elts = ufmToList fv

    emptyDmdEnv :: VarEnv Demand
    emptyDmdEnv = emptyVarEnv

    -- nopDmdType is the demand of doing nothing
    -- (lazy, absent, no CPR information, no termination information).
    -- Note that it is ''not'' the top of the lattice (which would be "may use everything"),
    -- so it is (no longer) called topDmd
    nopDmdType, botDmdType :: DmdType
    nopDmdType = DmdType emptyDmdEnv [] topRes
    botDmdType = DmdType emptyDmdEnv [] botRes

    cprProdDmdType :: Arity -> DmdType
    cprProdDmdType _arity
      = DmdType emptyDmdEnv [] (Dunno RetProd)

    isNopDmdType :: DmdType -> Bool
    isNopDmdType (DmdType env [] res)
      | isTopRes res && isEmptyVarEnv env = True
    isNopDmdType _                        = False

    mkDmdType :: DmdEnv -> [Demand] -> DmdResult -> DmdType
    mkDmdType fv ds res = DmdType fv ds res

    dmdTypeDepth :: DmdType -> Arity
    dmdTypeDepth (DmdType _ ds _) = length ds

    -- Remove any demand on arguments. This is used in dmdAnalRhs on the body
    removeDmdTyArgs :: DmdType -> DmdType
    removeDmdTyArgs = ensureArgs 0

    -- This makes sure we can use the demand type with n arguments,
    -- It extends the argument list with the correct resTypeArgDmd
    -- It also adjusts the DmdResult: Divergence survives additional arguments,
    -- CPR information does not (and definite converge also would not).
    ensureArgs :: Arity -> DmdType -> DmdType
    ensureArgs n d | n == depth = d
                   | otherwise  = DmdType fv ds' r'
      where depth = dmdTypeDepth d
            DmdType fv ds r = d

            ds' = take n (ds ++ repeat (resTypeArgDmd r))
            r' | Diverges <- r = r
               | otherwise     = topRes
                    -- See [Nature of result demand]

    seqDmdType :: DmdType -> ()
    seqDmdType (DmdType _env ds res) =
      {- ??? env `seq` -} seqDemandList ds `seq` seqDmdResult res `seq` ()

    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 _ [] res_ty)       = (resTypeArgDmd res_ty, ty)

    -- When e is evaluated after executing an IO action, and d is e's demand, then
    -- what of this demand should we consider, given that the IO action can cleanly
    -- exit?
    -- * We have to kill all strictness demands (i.e. lub with a lazy demand)
    -- * We can keep demand information (i.e. lub with an absent deman)
    -- * We have to kill definite divergence
    -- * We can keep CPR information.
    -- See Note [IO hack in the demand analyser]
    deferAfterIO :: DmdType -> DmdType
    deferAfterIO d@(DmdType _ _ res) =
        case d `lubDmdType` nopDmdType of
            DmdType fv ds _ -> DmdType fv ds (defer_res res)
      where
      defer_res Diverges      = topRes
      defer_res r             = r

    strictenDmd :: JointDmd -> CleanDemand
    strictenDmd (JD {strd = s, absd = u})
      = CD { sd = poke_s s, ud = poke_u u }
      where
        poke_s Lazy      = HeadStr
        poke_s (Str s)   = s
        poke_u Abs       = UHead
        poke_u (Use _ u) = u

\end{code} Deferring and peeeling \begin{code}

    type DeferAndUse   -- Describes how to degrade a result type
       =( Bool        -- Lazify (defer) the type
        , Count)      -- Many => manify the type

    type DeferAndUseM = Maybe DeferAndUse
      -- Nothing <=> absent-ify the result type; it will never be used

    toCleanDmd :: Demand -> Type -> (CleanDemand, DeferAndUseM)
    toCleanDmd (JD { strd = s, absd = u }) expr_ty
      = case (s,u) of
          (Str s', Use c u') -> -- The normal case
                                (CD { sd = s',      ud = u' }, Just (False, c))

          (Lazy,   Use c u') -> -- See Note [Analyzing with lazy demand and lambdas]
                                (CD { sd = HeadStr, ud = u' }, Just (True,  c))

          (_,      Abs)  -- See Note [Analysing with absent demand]
             | isUnLiftedType expr_ty -> (CD { sd = HeadStr, ud = Used }, Just (False, One))
             | otherwise              -> (CD { sd = HeadStr, ud = Used }, Nothing)

    -- This is used in dmdAnalStar when post-processing
    -- a function's argument demand. So we only care about what
    -- does to free variables, and whether it terminates.
    -- see Note [The need for BothDmdArg]
    postProcessDmdTypeM :: DeferAndUseM -> DmdType -> BothDmdArg
    postProcessDmdTypeM Nothing   _  = (emptyDmdEnv, Dunno ())
      -- Incoming demand was Absent, so just discard all usage information
      -- We only processed the thing at all to analyse the body
      -- See Note [Always analyse in virgin pass]
    postProcessDmdTypeM (Just du) (DmdType fv _ res_ty)
        = (postProcessDmdEnv du fv, postProcessDmdResult du res_ty)

    postProcessDmdResult :: DeferAndUse -> DmdResult -> Termination ()
    postProcessDmdResult (True,_)  _          = Dunno ()
    postProcessDmdResult (False,_) (Dunno {}) = Dunno ()
    postProcessDmdResult (False,_) Diverges   = Diverges

    postProcessDmdEnv :: DeferAndUse -> DmdEnv -> DmdEnv
    postProcessDmdEnv (True,  Many) env = deferReuseEnv env
    postProcessDmdEnv (False, Many) env = reuseEnv env
    postProcessDmdEnv (True,  One)  env = deferEnv env
    postProcessDmdEnv (False, One)  env = env

    postProcessUnsat :: DeferAndUse -> DmdType -> DmdType
    postProcessUnsat (True,  Many) ty = deferReuse ty
    postProcessUnsat (False, Many) ty = reuseType ty
    postProcessUnsat (True,  One)  ty = deferType ty
    postProcessUnsat (False, One)  ty = ty

    deferType, reuseType, deferReuse :: DmdType -> DmdType
    deferType  (DmdType fv ds _)      = DmdType (deferEnv fv)      (map deferDmd ds)      topRes
    reuseType  (DmdType fv ds res_ty) = DmdType (reuseEnv fv)      (map reuseDmd ds)      res_ty
    deferReuse (DmdType fv ds _)      = DmdType (deferReuseEnv fv) (map deferReuseDmd ds) topRes

    deferEnv, reuseEnv, deferReuseEnv :: DmdEnv -> DmdEnv
    deferEnv      fv = mapVarEnv deferDmd fv
    reuseEnv      fv = mapVarEnv reuseDmd fv
    deferReuseEnv fv = mapVarEnv deferReuseDmd fv

    deferDmd, reuseDmd, deferReuseDmd :: JointDmd -> JointDmd
    deferDmd      (JD {strd=_, absd=a}) = mkJointDmd Lazy a
    reuseDmd      (JD {strd=d, absd=a}) = mkJointDmd d    (markReusedDmd a)
    deferReuseDmd (JD {strd=_, absd=a}) = mkJointDmd Lazy (markReusedDmd a)

    -- Peels one call level from the demand, and also returns
    -- whether it was unsaturated (separately for strictness and usage)
    peelCallDmd :: CleanDemand -> (CleanDemand, DeferAndUse)
    -- Exploiting the fact that
    -- on the strictness side      C(B) = B
    -- and on the usage side       C(U) = U
    peelCallDmd (CD {sd = s, ud = u})
      = case (s, u) of
          (SCall s', UCall c u') -> (CD { sd = s',       ud = u' },   (False, c))
          (SCall s', _)          -> (CD { sd = s',       ud = Used }, (False, Many))
          (HyperStr, UCall c u') -> (CD { sd = HyperStr, ud = u' },   (False, c))
          (HyperStr, _)          -> (CD { sd = HyperStr, ud = Used }, (False, Many))
          (_,        UCall c u') -> (CD { sd = HeadStr,  ud = u' },   (True,  c))
          (_,        _)          -> (CD { sd = HeadStr,  ud = Used }, (True,  Many))
           -- The _ cases for usage includes UHead which seems a bit wrong
           -- because the body isn't used at all!
           -- c.f. the Abs case in toCleanDmd

    -- Peels that multiple nestings of calls clean demand and also returns
    -- whether it was unsaturated (separately for strictness and usage
    -- see Note [Demands from unsaturated function calls]
    peelManyCalls :: Int -> CleanDemand -> DeferAndUse
    peelManyCalls n (CD { sd = str, ud = abs })
      = (go_str n str, go_abs n abs)
      where
        go_str :: Int -> StrDmd -> Bool  -- True <=> unsaturated, defer
        go_str 0 _          = False
        go_str _ HyperStr   = False -- == go_str (n-1) HyperStr, as HyperStr = Call(HyperStr)
        go_str n (SCall d') = go_str (n-1) d'
        go_str _ _          = True

        go_abs :: Int -> UseDmd -> Count -- Many <=> unsaturated, or at least
        go_abs 0 _              = One    --          one UCall Many in the demand
        go_abs n (UCall One d') = go_abs (n-1) d'
        go_abs _ _              = Many

\end{code} Note [Demands from unsaturated function calls] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider a demand transformer d1 -> d2 -> r for f. If a sufficiently detailed demand is fed into this transformer, e.g arising from "f x1 x2" in a strict, use-once context, then d1 and d2 is precisely the demand unleashed onto x1 and x2 (similar for the free variable environment) and furthermore the result information r is the one we want to use. An anonymous lambda is also an unsaturated function all (needs one argument, none given), so this applies to that case as well. But the demand fed into f might be less than . There are a few cases: * Not enough demand on the strictness side: - In that case, we need to zap all strictness in the demand on arguments and free variables. - Furthermore, we remove CPR information. It could be left, but given the incoming demand is not enough to evaluate so far we just do not bother. - And finally termination information: If r says that f diverges for sure, then this holds when the demand guarantees that two arguments are going to be passed. If the demand is lower, we may just as well converge. If we were tracking definite convegence, than that would still hold under a weaker demand than expected by the demand transformer. * Not enough demand from the usage side: The missing usage can be expanded using UCall Many, therefore this is subsumed by the third case: * At least one of the uses has a cardinality of Many. - Even if f puts a One demand on any of its argument or free variables, if we call f multiple times, we may evaluate this argument or free variable multiple times. So forget about any occurrence of "One" in the demand. In dmdTransformSig, we call peelManyCalls to find out if we are in any of these cases, and then call postProcessUnsat to reduce the demand appropriately. Similarly, dmdTransformDictSelSig and dmdAnal, when analyzing a Lambda, use peelCallDmd, which peels only one level, but also returns the demand put on the body of the function. \begin{code}

    peelFV :: DmdType -> Var -> (DmdType, Demand)
    peelFV (DmdType fv ds res) id = -- pprTrace "rfv" (ppr id <+> ppr dmd $$ ppr fv)
                                   (DmdType fv' ds res, dmd)
      where
      fv' = fv `delVarEnv` id
      -- See Note [Default demand on free variables]
      dmd  = lookupVarEnv fv id `orElse` defaultDmd res

    addDemand :: Demand -> DmdType -> DmdType
    addDemand dmd (DmdType fv ds res) = DmdType fv (dmd:ds) res

\end{code} Note [Default demand on free variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the variable is not mentioned in the environment of a demand type, its demand is taken to be a result demand of the type. For the stricness component, if the result demand is a Diverges, then we use HyperStr else we use Lazy For the usage component, we use Absent. So we use either absDmd or botDmd. Also note the equations for lubDmdResult (resp. bothDmdResult) noted there. Note [Always analyse in virgin pass] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tricky point: make sure that we analyse in the 'virgin' pass. Consider rec { f acc x True = f (...rec { g y = ...g... }...) f acc x False = acc } In the virgin pass for 'f' we'll give 'f' a very strict (bottom) type. That might mean that we analyse the sub-expression containing the E = "...rec g..." stuff in a bottom demand. Suppose we *didn't analyse* E, but just retuned botType. Then in the *next* (non-virgin) iteration for 'f', we might analyse E in a weaker demand, and that will trigger doing a fixpoint iteration for g. But *because it's not the virgin pass* we won't start g's iteration at bottom. Disaster. (This happened in $sfibToList' of nofib/spectral/fibheaps.) So in the virgin pass we make sure that we do analyse the expression at least once, to initialise its signatures. Note [Analyzing with lazy demand and lambdas] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The insight for analyzing lambdas follows from the fact that for strictness S = C(L). This polymorphic expansion is critical for cardinality analysis of the following example: {-# NOINLINE build #-} build g = (g (:) [], g (:) []) h c z = build (\x -> let z1 = z ++ z in if c then \y -> x (y ++ z1) else \y -> x (z1 ++ y)) One can see that `build` assigns to `g` demand . Therefore, when analyzing the lambda `(\x -> ...)`, we expect each lambda \y -> ... to be annotated as "one-shot" one. Therefore (\x -> \y -> x (y ++ z)) should be analyzed with a demand . This is achieved by, first, converting the lazy demand L into the strict S by the second clause of the analysis. Note [Analysing with absent demand] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we analyse an expression with demand . The "A" means "absent", so this expression will never be needed. What should happen? There are several wrinkles: * We *do* want to analyse the expression regardless. Reason: Note [Always analyse in virgin pass] But we can post-process the results to ignore all the usage demands coming back. This is done by postProcessDmdTypeM. * But in the case of an *unlifted type* we must be extra careful, because unlifted values are evaluated even if they are not used. Example (see Trac #9254): f :: (() -> (# Int#, () #)) -> () -- Strictness signature is -- -- I.e. calls k, but discards first component of result f k = case k () of (# _, r #) -> r g :: Int -> () g y = f (\n -> (# case y of I# y2 -> y2, n #)) Here f's strictness signature says (correctly) that it calls its argument function and ignores the first component of its result. This is correct in the sense that it'd be fine to (say) modify the function so that always returned 0# in the first component. But in function g, we *will* evaluate the 'case y of ...', because it has type Int#. So 'y' will be evaluated. So we must record this usage of 'y', else 'g' will say 'y' is absent, and will w/w so that 'y' is bound to an aBSENT_ERROR thunk. An alternative would be to replace the 'case y of ...' with (say) 0#, but I have not tried that. It's not a common situation, but it is not theoretical: unsafePerformIO's implementation is very very like 'f' above. %************************************************************************ %* * Demand signatures %* * %************************************************************************ In a let-bound Id we record its strictness info. In principle, this strictness info is a demand transformer, mapping a demand on the Id into a DmdType, which gives a) the free vars of the Id's value b) the Id's arguments c) an indication of the result of applying the Id to its arguments However, in fact we store in the Id an extremely emascuated demand transfomer, namely a single DmdType (Nevertheless we dignify StrictSig as a distinct type.) This DmdType gives the demands unleashed by the Id when it is applied to as many arguments as are given in by the arg demands in the DmdType. Also see Note [Nature of result demand] for the meaning of a DmdResult in a strictness signature. If an Id is applied to less arguments than its arity, it means that the demand on the function at a call site is weaker than the vanilla call demand, used for signature inference. Therefore we place a top demand on all arguments. Otherwise, the demand is specified by Id's signature. For example, the demand transformer described by the demand signature StrictSig (DmdType {x -> } m) says that when the function is applied to two arguments, it unleashes demand on the free var x, on the first arg, and on the second, then returning a constructor. If this same function is applied to one arg, all we can say is that it uses x with , and its arg with demand . \begin{code}

    newtype StrictSig = StrictSig DmdType
                      deriving( Eq )

    instance Outputable StrictSig where
       ppr (StrictSig ty) = ppr ty

    mkStrictSig :: DmdType -> StrictSig
    mkStrictSig dmd_ty = StrictSig dmd_ty

    mkClosedStrictSig :: [Demand] -> DmdResult -> StrictSig
    mkClosedStrictSig ds res = mkStrictSig (DmdType emptyDmdEnv ds res)

    splitStrictSig :: StrictSig -> ([Demand], DmdResult)
    splitStrictSig (StrictSig (DmdType _ dmds res)) = (dmds, res)

    increaseStrictSigArity :: Int -> StrictSig -> StrictSig
    -- Add extra arguments to a strictness signature
    increaseStrictSigArity arity_increase (StrictSig (DmdType env dmds res))
      = StrictSig (DmdType env (replicate arity_increase topDmd ++ dmds) res)

    isNopSig :: StrictSig -> Bool
    isNopSig (StrictSig ty) = isNopDmdType ty

    isBottomingSig :: StrictSig -> Bool
    isBottomingSig (StrictSig (DmdType _ _ res)) = isBotRes res

    nopSig, botSig :: StrictSig
    nopSig = StrictSig nopDmdType
    botSig = StrictSig botDmdType

    cprProdSig :: Arity -> StrictSig
    cprProdSig arity = StrictSig (cprProdDmdType arity)

    argsOneShots :: StrictSig -> Arity -> [[OneShotInfo]]
    argsOneShots (StrictSig (DmdType _ arg_ds _)) n_val_args
      = go arg_ds
      where
        good_one_shot
          | arg_ds `lengthExceeds` n_val_args = ProbOneShot
          | otherwise                         = OneShotLam

        go []               = []
        go (arg_d : arg_ds) = argOneShots good_one_shot arg_d `cons` go arg_ds

        cons [] [] = []
        cons a  as = a:as

    argOneShots :: OneShotInfo -> JointDmd -> [OneShotInfo]
    argOneShots one_shot_info (JD { absd = usg })
      = case usg of
          Use _ arg_usg -> go arg_usg
          _             -> []
      where
        go (UCall One  u) = one_shot_info : go u
        go (UCall Many u) = NoOneShotInfo : go u
        go _              = []

    dmdTransformSig :: StrictSig -> CleanDemand -> DmdType
    -- (dmdTransformSig fun_sig dmd) considers a call to a function whose
    -- signature is fun_sig, with demand dmd.  We return the demand
    -- that the function places on its context (eg its args)
    dmdTransformSig (StrictSig dmd_ty@(DmdType _ arg_ds _)) cd
      = postProcessUnsat (peelManyCalls (length arg_ds) cd) dmd_ty
        -- see Note [Demands from unsaturated function calls]

    dmdTransformDataConSig :: Arity -> StrictSig -> CleanDemand -> DmdType
    -- Same as dmdTransformSig but for a data constructor (worker),
    -- which has a special kind of demand transformer.
    -- If the constructor is saturated, we feed the demand on
    -- the result into the constructor arguments.
    dmdTransformDataConSig arity (StrictSig (DmdType _ _ con_res))
                                 (CD { sd = str, ud = abs })
      | Just str_dmds <- go_str arity str
      , Just abs_dmds <- go_abs arity abs
      = DmdType emptyDmdEnv (mkJointDmds str_dmds abs_dmds) con_res
                    -- Must remember whether it's a product, hence con_res, not TopRes

      | otherwise   -- Not saturated
      = nopDmdType
      where
        go_str 0 dmd        = splitStrProdDmd arity dmd
        go_str n (SCall s') = go_str (n-1) s'
        go_str n HyperStr   = go_str (n-1) HyperStr
        go_str _ _          = Nothing

        go_abs 0 dmd            = splitUseProdDmd arity dmd
        go_abs n (UCall One u') = go_abs (n-1) u'
        go_abs _ _              = Nothing

    dmdTransformDictSelSig :: StrictSig -> CleanDemand -> DmdType
    -- Like dmdTransformDataConSig, we have a special demand transformer
    -- for dictionary selectors.  If the selector is saturated (ie has one
    -- argument: the dictionary), we feed the demand on the result into
    -- the indicated dictionary component.
    dmdTransformDictSelSig (StrictSig (DmdType _ [dict_dmd] _)) cd
       | (cd',defer_use) <- peelCallDmd cd
       , Just jds <- splitProdDmd_maybe dict_dmd
       = postProcessUnsat defer_use $
         DmdType emptyDmdEnv [mkOnceUsedDmd $ mkProdDmd $ map (enhance cd') jds] topRes
       | otherwise
       = nopDmdType              -- See Note [Demand transformer for a dictionary selector]
      where
        enhance cd old | isAbsDmd old = old
                       | otherwise    = mkOnceUsedDmd cd  -- This is the one!

    dmdTransformDictSelSig _ _ = panic "dmdTransformDictSelSig: no args"

\end{code} Note [Demand transformer for a dictionary selector] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we evaluate (op dict-expr) under demand 'd', then we can push the demand 'd' into the appropriate field of the dictionary. What *is* the appropriate field? We just look at the strictness signature of the class op, which will be something like: U(AAASAAAAA). Then replace the 'S' by the demand 'd'. For single-method classes, which are represented by newtypes the signature of 'op' won't look like U(...), so the splitProdDmd_maybe will fail. That's fine: if we are doing strictness analysis we are also doing inling, so we'll have inlined 'op' into a cast. So we can bale out in a conservative way, returning nopDmdType. It is (just.. Trac #8329) possible to be running strictness analysis *without* having inlined class ops from single-method classes. Suppose you are using ghc --make; and the first module has a local -O0 flag. So you may load a class without interface pragmas, ie (currently) without an unfolding for the class ops. Now if a subsequent module in the --make sweep has a local -O flag you might do strictness analysis, but there is no inlining for the class op. This is weird, so I'm not worried about whether this optimises brilliantly; but it should not fall over. Note [Non-full application] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ If a function having bottom as its demand result is applied to a less number of arguments than its syntactic arity, we cannot say for sure that it is going to diverge. This is the reason why we use the function appIsBottom, which, given a strictness signature and a number of arguments, says conservatively if the function is going to diverge or not. \begin{code}

    -- appIsBottom returns true if an application to n args would diverge
    appIsBottom :: StrictSig -> Int -> Bool
    appIsBottom (StrictSig (DmdType _ ds res)) n
                | isBotRes res                      = not $ lengthExceeds ds n
    appIsBottom _                                 _ = False

    seqStrictSig :: StrictSig -> ()
    seqStrictSig (StrictSig ty) = seqDmdType ty

    -- Used for printing top-level strictness pragmas in interface files
    pprIfaceStrictSig :: StrictSig -> SDoc
    pprIfaceStrictSig (StrictSig (DmdType _ dmds res))
      = hcat (map ppr dmds) <> ppr res

\end{code} Zap absence or one-shot information, under control of flags \begin{code}

    zapDemand :: DynFlags -> Demand -> Demand
    zapDemand dflags dmd
      | Just kfs <- killFlags dflags = zap_dmd kfs dmd
      | otherwise                    = dmd

    zapStrictSig :: DynFlags -> StrictSig -> StrictSig
    zapStrictSig dflags sig@(StrictSig (DmdType env ds r))
      | Just kfs <- killFlags dflags = StrictSig (DmdType env (map (zap_dmd kfs) ds) r)
      | otherwise                    = sig

    type KillFlags = (Bool, Bool)

    killFlags :: DynFlags -> Maybe KillFlags
    killFlags dflags
      | not kill_abs && not kill_one_shot = Nothing
      | otherwise                         = Just (kill_abs, kill_one_shot)
      where
        kill_abs      = gopt Opt_KillAbsence dflags
        kill_one_shot = gopt Opt_KillOneShot dflags

    zap_dmd :: KillFlags -> Demand -> Demand
    zap_dmd kfs (JD {strd = s, absd = u}) = JD {strd = s, absd = zap_musg kfs u}

    zap_musg :: KillFlags -> MaybeUsed -> MaybeUsed
    zap_musg (kill_abs, _) Abs
      | kill_abs  = useTop
      | otherwise = Abs
    zap_musg kfs (Use c u) = Use (zap_count kfs c) (zap_usg kfs u)

    zap_count :: KillFlags -> Count -> Count
    zap_count (_, kill_one_shot) c
      | kill_one_shot = Many
      | otherwise     = c

    zap_usg :: KillFlags -> UseDmd -> UseDmd
    zap_usg kfs (UCall c u) = UCall (zap_count kfs c) (zap_usg kfs u)
    zap_usg kfs (UProd us)  = UProd (map (zap_musg kfs) us)
    zap_usg _   u           = u

\end{code} \begin{code}

    -- If the argument is a used non-newtype dictionary, give it strict
    -- demand. Also split the product type & demand and recur in order to
    -- similarly strictify the argument's contained used non-newtype
    -- superclass dictionaries. We use the demand as our recursive measure
    -- to guarantee termination.
    strictifyDictDmd :: Type -> Demand -> Demand
    strictifyDictDmd ty dmd = case absd dmd of
      Use n _ |
        Just (tycon, _arg_tys, _data_con, inst_con_arg_tys)
          <- splitDataProductType_maybe ty,
        not (isNewTyCon tycon), isClassTyCon tycon -- is a non-newtype dictionary
        -> seqDmd `bothDmd` -- main idea: ensure it's strict
           case splitProdDmd_maybe dmd of
             -- superclass cycles should not be a problem, since the demand we are
             -- consuming would also have to be infinite in order for us to diverge
             Nothing -> dmd -- no components have interesting demand, so stop
                            -- looking for superclass dicts
             Just dmds
               | all (not . isAbsDmd) dmds -> evalDmd
                 -- abstract to strict w/ arbitrary component use, since this
                 -- smells like reboxing; results in CBV boxed
                 --
                 -- TODO revisit this if we ever do boxity analysis
               | otherwise -> case mkProdDmd $ zipWith strictifyDictDmd inst_con_arg_tys dmds of
                   CD {sd = s,ud = a} -> JD (Str s) (Use n a)
                 -- TODO could optimize with an aborting variant of zipWith since
                 -- the superclass dicts are always a prefix
      _ -> dmd -- unused or not a dictionary

\end{code} Note [HyperStr and Use demands] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The information "HyperStr" needs to be in the strictness signature, and not in the demand signature, because we still want to know about the demand on things. Consider f (x,y) True = error (show x) f (x,y) False = x+1 The signature of f should be m. If we were not distinguishing the uses on x and y in the True case, we could either not figure out how deeply we can unpack x, or that we do not have to pass y. %************************************************************************ %* * Serialisation %* * %************************************************************************ \begin{code}

    instance Binary StrDmd where
      put_ bh HyperStr     = do putByte bh 0
      put_ bh HeadStr      = do putByte bh 1
      put_ bh (SCall s)    = do putByte bh 2
                                put_ bh s
      put_ bh (SProd sx)   = do putByte bh 3
                                put_ bh sx
      get bh = do
             h <- getByte bh
             case h of
               0 -> do return HyperStr
               1 -> do return HeadStr
               2 -> do s  <- get bh
                       return (SCall s)
               _ -> do sx <- get bh
                       return (SProd sx)

    instance Binary MaybeStr where
        put_ bh Lazy         = do
                putByte bh 0
        put_ bh (Str s)    = do
                putByte bh 1
                put_ bh s

        get  bh = do
                h <- getByte bh
                case h of
                  0 -> return Lazy
                  _ -> do s  <- get bh
                          return $ Str s

    instance Binary Count where
        put_ bh One  = do putByte bh 0
        put_ bh Many = do putByte bh 1

        get  bh = do h <- getByte bh
                     case h of
                       0 -> return One
                       _ -> return Many

    instance Binary MaybeUsed where
        put_ bh Abs          = do
                putByte bh 0
        put_ bh (Use c u)    = do
                putByte bh 1
                put_ bh c
                put_ bh u

        get  bh = do
                h <- getByte bh
                case h of
                  0 -> return Abs
                  _ -> do c  <- get bh
                          u  <- get bh
                          return $ Use c u

    instance Binary UseDmd where
        put_ bh Used         = do
                putByte bh 0
        put_ bh UHead        = do
                putByte bh 1
        put_ bh (UCall c u)    = do
                putByte bh 2
                put_ bh c
                put_ bh u
        put_ bh (UProd ux)   = do
                putByte bh 3
                put_ bh ux

        get  bh = do
                h <- getByte bh
                case h of
                  0 -> return $ Used
                  1 -> return $ UHead
                  2 -> do c <- get bh
                          u <- get bh
                          return (UCall c u)
                  _ -> do ux <- get bh
                          return (UProd ux)

    instance Binary JointDmd where
        put_ bh (JD {strd = x, absd = y}) = do put_ bh x; put_ bh y
        get  bh = do
                  x <- get bh
                  y <- get bh
                  return $ mkJointDmd x y

    instance Binary StrictSig where
        put_ bh (StrictSig aa) = do
                put_ bh aa
        get bh = do
              aa <- get bh
              return (StrictSig aa)

    instance Binary DmdType where
      -- Ignore DmdEnv when spitting out the DmdType
      put_ bh (DmdType _ ds dr)
           = do put_ bh ds
                put_ bh dr
      get bh
          = do ds <- get bh
               dr <- get bh
               return (DmdType emptyDmdEnv ds dr)

    instance Binary DmdResult where
      put_ bh (Dunno c)     = do { putByte bh 0; put_ bh c }
      put_ bh Diverges      = putByte bh 2

      get bh = do { h <- getByte bh
                  ; case h of
                      0 -> do { c <- get bh; return (Dunno c) }
                      _ -> return Diverges }

    instance Binary CPRResult where
        put_ bh (RetSum n)   = do { putByte bh 0; put_ bh n }
        put_ bh RetProd      = putByte bh 1
        put_ bh NoCPR        = putByte bh 2

        get  bh = do
                h <- getByte bh
                case h of
                  0 -> do { n <- get bh; return (RetSum n) }
                  1 -> return RetProd
                  _ -> return NoCPR

\end{code}