simplCore/SimplMonad.hs

    {-
    (c) The AQUA Project, Glasgow University, 1993-1998

    \section[SimplMonad]{The simplifier Monad}
    -}

    module SimplMonad (
            -- The monad
            SimplM,
            initSmpl, traceSmpl,
            getSimplRules, getFamEnvs,

            -- Unique supply
            MonadUnique(..), newId,

            -- Counting
            SimplCount, tick, freeTick, checkedTick,
            getSimplCount, zeroSimplCount, pprSimplCount,
            plusSimplCount, isZeroSimplCount
        ) where

    import Id               ( Id, mkSysLocalOrCoVar )
    import Type             ( Type )
    import FamInstEnv       ( FamInstEnv )
    import CoreSyn          ( RuleEnv(..) )
    import UniqSupply
    import DynFlags
    import CoreMonad
    import Outputable
    import FastString
    import MonadUtils
    import ErrUtils
    import BasicTypes          ( IntWithInf, treatZeroAsInf, mkIntWithInf )
    import Control.Monad       ( when, liftM, ap )

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Monad plumbing}
    *                                                                      *
    ************************************************************************

    For the simplifier monad, we want to {\em thread} a unique supply and a counter.
    (Command-line switches move around through the explicitly-passed SimplEnv.)
    -}

    newtype SimplM result
      =  SM  { unSM :: SimplTopEnv  -- Envt that does not change much
                    -> UniqSupply   -- We thread the unique supply because
                                    -- constantly splitting it is rather expensive
                    -> SimplCount
                    -> IO (result, UniqSupply, SimplCount)}
      -- we only need IO here for dump output

    data SimplTopEnv
      = STE { st_flags     :: DynFlags
            , st_max_ticks :: IntWithInf  -- Max #ticks in this simplifier run
            , st_rules     :: RuleEnv
            , st_fams      :: (FamInstEnv, FamInstEnv) }

    initSmpl :: DynFlags -> RuleEnv -> (FamInstEnv, FamInstEnv)
             -> UniqSupply          -- No init count; set to 0
             -> Int                 -- Size of the bindings, used to limit
                                    -- the number of ticks we allow
             -> SimplM a
             -> IO (a, SimplCount)

    initSmpl dflags rules fam_envs us size m
      = do (result, _, count) <- unSM m env us (zeroSimplCount dflags)
           return (result, count)
      where
        env = STE { st_flags = dflags, st_rules = rules
                  , st_max_ticks = computeMaxTicks dflags size
                  , st_fams = fam_envs }

    computeMaxTicks :: DynFlags -> Int -> IntWithInf
    -- Compute the max simplifier ticks as
    --     (base-size + pgm-size) * magic-multiplier * tick-factor/100
    -- where
    --    magic-multiplier is a constant that gives reasonable results
    --    base-size is a constant to deal with size-zero programs
    computeMaxTicks dflags size
      = treatZeroAsInf $
        fromInteger ((toInteger (size + base_size)
                      * toInteger (tick_factor * magic_multiplier))
              `div` 100)
      where
        tick_factor      = simplTickFactor dflags
        base_size        = 100
        magic_multiplier = 40
            -- MAGIC NUMBER, multiplies the simplTickFactor
            -- We can afford to be generous; this is really
            -- just checking for loops, and shouldn't usually fire
            -- A figure of 20 was too small: see Trac #5539.

    {-# INLINE thenSmpl #-}
    {-# INLINE thenSmpl_ #-}
    {-# INLINE returnSmpl #-}

    instance Functor SimplM where
        fmap = liftM

    instance Applicative SimplM where
        pure  = returnSmpl
        (<*>) = ap
        (*>)  = thenSmpl_

    instance Monad SimplM where
       (>>)   = (*>)
       (>>=)  = thenSmpl

    returnSmpl :: a -> SimplM a
    returnSmpl e = SM (\_st_env us sc -> return (e, us, sc))

    thenSmpl  :: SimplM a -> (a -> SimplM b) -> SimplM b
    thenSmpl_ :: SimplM a -> SimplM b -> SimplM b

    thenSmpl m k
      = SM $ \st_env us0 sc0 -> do
          (m_result, us1, sc1) <- unSM m st_env us0 sc0
          unSM (k m_result) st_env us1 sc1

    thenSmpl_ m k
      = SM $ \st_env us0 sc0 -> do
          (_, us1, sc1) <- unSM m st_env us0 sc0
          unSM k st_env us1 sc1

    -- TODO: this specializing is not allowed
    -- {-# SPECIALIZE mapM         :: (a -> SimplM b) -> [a] -> SimplM [b] #-}
    -- {-# SPECIALIZE mapAndUnzipM :: (a -> SimplM (b, c)) -> [a] -> SimplM ([b],[c]) #-}
    -- {-# SPECIALIZE mapAccumLM   :: (acc -> b -> SimplM (acc,c)) -> acc -> [b] -> SimplM (acc, [c]) #-}

    traceSmpl :: String -> SDoc -> SimplM ()
    traceSmpl herald doc
      = do { dflags <- getDynFlags
           ; when (dopt Opt_D_dump_simpl_trace dflags) $ liftIO $
             printOutputForUser dflags alwaysQualify $
             hang (text herald) 2 doc }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{The unique supply}
    *                                                                      *
    ************************************************************************
    -}

    instance MonadUnique SimplM where
        getUniqueSupplyM
           = SM (\_st_env us sc -> case splitUniqSupply us of
                                    (us1, us2) -> return (us1, us2, sc))

        getUniqueM
           = SM (\_st_env us sc -> case takeUniqFromSupply us of
                                    (u, us') -> return (u, us', sc))

        getUniquesM
            = SM (\_st_env us sc -> case splitUniqSupply us of
                                    (us1, us2) -> return (uniqsFromSupply us1, us2, sc))

    instance HasDynFlags SimplM where
        getDynFlags = SM (\st_env us sc -> return (st_flags st_env, us, sc))

    instance MonadIO SimplM where
        liftIO m = SM $ \_ us sc -> do
          x <- m
          return (x, us, sc)

    getSimplRules :: SimplM RuleEnv
    getSimplRules = SM (\st_env us sc -> return (st_rules st_env, us, sc))

    getFamEnvs :: SimplM (FamInstEnv, FamInstEnv)
    getFamEnvs = SM (\st_env us sc -> return (st_fams st_env, us, sc))

    newId :: FastString -> Type -> SimplM Id
    newId fs ty = do uniq <- getUniqueM
                     return (mkSysLocalOrCoVar fs uniq ty)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Counting up what we've done}
    *                                                                      *
    ************************************************************************
    -}

    getSimplCount :: SimplM SimplCount
    getSimplCount = SM (\_st_env us sc -> return (sc, us, sc))

    tick :: Tick -> SimplM ()
    tick t = SM (\st_env us sc -> let sc' = doSimplTick (st_flags st_env) t sc
                                  in sc' `seq` return ((), us, sc'))

    checkedTick :: Tick -> SimplM ()
    -- Try to take a tick, but fail if too many
    checkedTick t
      = SM (\st_env us sc -> if st_max_ticks st_env <= mkIntWithInf (simplCountN sc)
                             then pprPanic "Simplifier ticks exhausted" (msg sc)
                             else let sc' = doSimplTick (st_flags st_env) t sc
                                  in sc' `seq` return ((), us, sc'))
      where
        msg sc = vcat [ text "When trying" <+> ppr t
                      , text "To increase the limit, use -fsimpl-tick-factor=N (default 100)"
                      , text "If you need to do this, let GHC HQ know, and what factor you needed"
                      , pp_details sc
                      , pprSimplCount sc ]
        pp_details sc
          | hasDetailedCounts sc = empty
          | otherwise = text "To see detailed counts use -ddump-simpl-stats"

    freeTick :: Tick -> SimplM ()
    -- Record a tick, but don't add to the total tick count, which is
    -- used to decide when nothing further has happened
    freeTick t
       = SM (\_st_env us sc -> let sc' = doFreeSimplTick t sc
                               in sc' `seq` return ((), us, sc'))