GHC/Float/ConversionUtils.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE CPP, MagicHash, UnboxedTuples, NoImplicitPrelude #-}
    {-# OPTIONS_GHC -O2 #-}
    {-# OPTIONS_HADDOCK not-home #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Float.ConversionUtils
    -- Copyright   :  (c) Daniel Fischer 2010
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- Utilities for conversion between Double/Float and Rational
    --
    -----------------------------------------------------------------------------

    #include "MachDeps.h"

    module GHC.Float.ConversionUtils ( elimZerosInteger, elimZerosInt# ) where

    import GHC.Base
    import GHC.Num.Integer
    #if WORD_SIZE_IN_BITS < 64
    import GHC.IntWord64
    #endif

    default ()

    #if WORD_SIZE_IN_BITS < 64

    #define TO64    integerToInt64#

    -- Double mantissae have 53 bits, too much for Int#
    elim64# :: Int64# -> Int# -> (# Integer, Int# #)
    elim64# n e =
        case zeroCount (int64ToInt# n) of
          t | isTrue# (e <=# t) -> (# integerFromInt64# (uncheckedIShiftRA64# n e), 0# #)
            | isTrue# (t <# 8#) -> (# integerFromInt64# (uncheckedIShiftRA64# n t), e -# t #)
            | otherwise         -> elim64# (uncheckedIShiftRA64# n 8#) (e -# 8#)

    #else

    #define TO64    integerToInt#

    -- Double mantissae fit it Int#
    elim64# :: Int# -> Int# -> (# Integer, Int# #)
    elim64# = elimZerosInt#

    #endif

    {-# INLINE elimZerosInteger #-}
    elimZerosInteger :: Integer -> Int# -> (# Integer, Int# #)
    elimZerosInteger m e = elim64# (TO64 m) e

    elimZerosInt# :: Int# -> Int# -> (# Integer, Int# #)
    elimZerosInt# n e =
        case zeroCount n of
          t | isTrue# (e <=# t) -> (# IS (uncheckedIShiftRA# n e), 0# #)
            | isTrue# (t <# 8#) -> (# IS (uncheckedIShiftRA# n t), e -# t #)
            | otherwise         -> elimZerosInt# (uncheckedIShiftRA# n 8#) (e -# 8#)

    -- | Number of trailing zero bits in a byte
    zeroCount :: Int# -> Int#
    zeroCount i = int8ToInt# (indexInt8OffAddr# arr (word2Int# (narrow8Word# (int2Word# i)))) -- index must be in [0,255]
      where
        arr = "\8\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\5\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\6\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\5\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\7\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\5\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\6\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\5\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0\4\0\1\0\2\0\1\0\3\0\1\0\2\0\1\0"#