Data/Text/Internal/Unsafe/Shift.hs

    {-# LANGUAGE MagicHash #-}

    -- |
    -- Module      : Data.Text.Internal.Unsafe.Shift
    -- Copyright   : (c) Bryan O'Sullivan 2009
    --
    -- License     : BSD-style
    -- Maintainer  : bos@serpentine.com
    -- Stability   : experimental
    -- Portability : GHC
    --
    -- /Warning/: this is an internal module, and does not have a stable
    -- API or name. Functions in this module may not check or enforce
    -- preconditions expected by public modules. Use at your own risk!
    --
    -- Fast, unchecked bit shifting functions.

    module Data.Text.Internal.Unsafe.Shift
        (
          UnsafeShift(..)
        ) where

    -- import qualified Data.Bits as Bits
    import GHC.Base
    import GHC.Word

    -- | This is a workaround for poor optimisation in GHC 6.8.2.  It
    -- fails to notice constant-width shifts, and adds a test and branch
    -- to every shift.  This imposes about a 10% performance hit.
    --
    -- These functions are undefined when the amount being shifted by is
    -- greater than the size in bits of a machine Int#.
    class UnsafeShift a where
        shiftL :: a -> Int -> a
        shiftR :: a -> Int -> a

    instance UnsafeShift Word16 where
        {-# INLINE shiftL #-}
        shiftL (W16# x#) (I# i#) = W16# (narrow16Word# (x# `uncheckedShiftL#` i#))

        {-# INLINE shiftR #-}
        shiftR (W16# x#) (I# i#) = W16# (x# `uncheckedShiftRL#` i#)

    instance UnsafeShift Word32 where
        {-# INLINE shiftL #-}
        shiftL (W32# x#) (I# i#) = W32# (narrow32Word# (x# `uncheckedShiftL#` i#))

        {-# INLINE shiftR #-}
        shiftR (W32# x#) (I# i#) = W32# (x# `uncheckedShiftRL#` i#)

    instance UnsafeShift Word64 where
        {-# INLINE shiftL #-}
        shiftL (W64# x#) (I# i#) = W64# (x# `uncheckedShiftL64#` i#)

        {-# INLINE shiftR #-}
        shiftR (W64# x#) (I# i#) = W64# (x# `uncheckedShiftRL64#` i#)

    instance UnsafeShift Int where
        {-# INLINE shiftL #-}
        shiftL (I# x#) (I# i#) = I# (x# `iShiftL#` i#)

        {-# INLINE shiftR #-}
        shiftR (I# x#) (I# i#) = I# (x# `iShiftRA#` i#)

    {-
    instance UnsafeShift Integer where
        {-# INLINE shiftL #-}
        shiftL = Bits.shiftL

        {-# INLINE shiftR #-}
        shiftR = Bits.shiftR
    -}