{-# LANGUAGE BangPatterns, DeriveDataTypeable #-} module Control.Concurrent.Speculation ( -- * Speculative application spec , spec' , specBy , specBy' , specOn , specOn' -- * Speculative application with transactional rollback , specSTM , specSTM' , specOnSTM , specOnSTM' , specBySTM , specBySTM' -- * Determining if a closure is evaluated , unsafeGetTagBits , unsafeIsEvaluated ) where import Control.Concurrent.STM import Control.Exception (Exception, throw, fromException) import Control.Parallel (par) import Data.Typeable (Typeable) import Data.Function (on) import Data.Bits ((.&.)) import Foreign (sizeOf) import Unsafe.Coerce (unsafeCoerce) -- * Basic speculation -- | @'spec' g f a@ evaluates @f g@ while forcing @a@, if @g == a@ then @f g@ is returned. Otherwise @f a@ is evaluated. -- -- Furthermore, if the argument has already been evaluated, we avoid sparking the parallel computation at all. -- -- If a good guess at the value of @a@ is available, this is one way to induce parallelism in an otherwise sequential task. -- -- However, if the guess isn\'t available more cheaply than the actual answer, then this saves no work and if the guess is -- wrong, you risk evaluating the function twice. -- -- > spec a f a = f $! a -- -- The best-case timeline looks like: -- -- > [---- f g ----] -- > [----- a -----] -- > [-- spec g f a --] -- -- The worst-case timeline looks like: -- -- > [---- f g ----] -- > [----- a -----] -- > [---- f a ----] -- > [------- spec g f a -----------] -- -- Compare these to the timeline of @f $! a@: -- -- > [---- a -----] -- > [---- f a ----] spec :: Eq a => a -> (a -> b) -> a -> b spec = specBy (==) {-# INLINE spec #-} -- | Unlike 'spec', this version does not check to see if the argument has already been evaluated. This can save -- a small amount of work when you know the argument will always require computation. spec' :: Eq a => a -> (a -> b) -> a -> b spec' = specBy' (==) {-# INLINE spec' #-} -- | 'spec' with a user defined comparison function specBy :: (a -> a -> Bool) -> a -> (a -> b) -> a -> b specBy cmp g f a | unsafeIsEvaluated a = f a | otherwise = specBy' cmp g f a {-# INLINE specBy #-} -- | 'spec'' with a user defined comparison function specBy' :: (a -> a -> Bool) -> a -> (a -> b) -> a -> b specBy' cmp guess f a = speculation `par` if cmp guess a then speculation else f a where speculation = f guess {-# INLINE specBy' #-} -- | 'spec' comparing by projection onto another type specOn :: Eq c => (a -> c) -> a -> (a -> b) -> a -> b specOn = specBy . on (==) {-# INLINE specOn #-} -- | 'spec'' comparing by projection onto another type specOn' :: Eq c => (a -> c) -> a -> (a -> b) -> a -> b specOn' = specBy' . on (==) {-# INLINE specOn' #-} -- * STM-based speculation -- | @'specSTM' g f a@ evaluates @f g@ while forcing @a@, if @g == a@ then @f g@ is returned. Otherwise the side-effects -- of the current STM transaction are rolled back and @f a@ is evaluated. -- -- If the argument @a@ is already evaluated, we don\'t bother to perform @f g@ at all. -- -- If a good guess at the value of @a@ is available, this is one way to induce parallelism in an otherwise sequential task. -- -- However, if the guess isn\'t available more cheaply than the actual answer then this saves no work, and if the guess is -- wrong, you risk evaluating the function twice. -- -- > specSTM a f a = f $! a -- -- The best-case timeline looks like: -- -- > [------ f g ------] -- > [------- a -------] -- > [--- specSTM g f a ---] -- -- The worst-case timeline looks like: -- -- > [------ f g ------] -- > [------- a -------] -- > [-- rollback --] -- > [------ f a ------] -- > [------------------ spec g f a ------------------------] -- -- Compare these to the timeline of @f $! a@: -- -- > [------- a -------] -- > [------ f a ------] specSTM :: Eq a => STM a -> (a -> STM b) -> a -> STM b specSTM = specBySTM (==) {-# INLINE specSTM #-} -- | Unlike 'specSTM', 'specSTM'' doesn't check if the argument has already been evaluated. specSTM' :: Eq a => STM a -> (a -> STM b) -> a -> STM b specSTM' = specBySTM' (==) {-# INLINE specSTM' #-} -- | 'specSTM' using a user defined comparison function specBySTM :: (a -> a -> Bool) -> STM a -> (a -> STM b) -> a -> STM b specBySTM cmp g f a | unsafeIsEvaluated a = f a | otherwise = specBySTM' cmp g f a {-# INLINE specBySTM #-} -- | 'specSTM'' using a user defined comparison function specBySTM' :: (a -> a -> Bool) -> STM a -> (a -> STM b) -> a -> STM b specBySTM' cmp g f a = a `par` let try = do g' <- g result <- f g' if cmp g' a then return result else throw Speculation in try `catchSTM` \e -> case fromException e of Just Speculation -> f a -- rerun with alternative input _ -> throw e -- this is a bigger problem {-# INLINE specBySTM' #-} -- | @'specBySTM' . 'on' (==)@ specOnSTM :: Eq c => (a -> c) -> STM a -> (a -> STM b) -> a -> STM b specOnSTM = specBySTM . on (==) {-# INLINE specOnSTM #-} -- | @'specBySTM'' . 'on' (==)@ specOnSTM' :: Eq c => (a -> c) -> STM a -> (a -> STM b) -> a -> STM b specOnSTM' = specBySTM' . on (==) {-# INLINE specOnSTM' #-} data Speculation = Speculation deriving (Show,Eq,Typeable) instance Exception Speculation -- | Used to inspect tag bits data Box a = Box a -- | Inspect the dynamic pointer tagging bits of a closure. This is an impure function that relies on GHC internals and may falsely return 0, but never give the wrong tag number if it returns a non-0 value. unsafeGetTagBits :: a -> Int unsafeGetTagBits a = unsafeCoerce (Box a) .&. (sizeOf (undefined :: Int) - 1) {-# INLINE unsafeGetTagBits #-} -- | Returns a guess as to whether or not a value has been evaluated. This is an impure function that relies on GHC internals and will return false negatives, but no false positives. This is unsafe as the value of this function will vary (from False to True) over the course of otherwise pure invocations! unsafeIsEvaluated :: a -> Bool unsafeIsEvaluated a = unsafeGetTagBits a /= 0 {-# INLINE unsafeIsEvaluated #-}