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Use countTrailingZeros for indexOfTheOnlyBit #379

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Merged
merged 2 commits into from
Jul 24, 2019
Merged

Use countTrailingZeros for indexOfTheOnlyBit #379

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25aefe9
Use countTrailingZeros for indexOfTheOnlyBit
Jan 3, 2017
2cd5bb2
Use clz/ctz for {lowest,highest}BitSet
Jan 3, 2017
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22 changes: 16 additions & 6 deletions containers/src/Data/IntSet/Internal.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -1414,6 +1414,16 @@ foldr'Bits :: Int -> (Int -> a -> a) -> a -> Nat -> a
{-# INLINE foldr'Bits #-}

#if defined(__GLASGOW_HASKELL__) && (WORD_SIZE_IN_BITS==32 || WORD_SIZE_IN_BITS==64)
indexOfTheOnlyBit :: Nat -> Int
{-# INLINE indexOfTheOnlyBit #-}
#if MIN_VERSION_base(4,8,0) && (WORD_SIZE_IN_BITS==64)
indexOfTheOnlyBit bitmask = countTrailingZeros bitmask

lowestBitSet x = countTrailingZeros x

highestBitSet x = WORD_SIZE_IN_BITS - 1 - countLeadingZeros x

#else
{----------------------------------------------------------------------
For lowestBitSet we use wordsize-dependant implementation based on
multiplication and DeBrujn indeces, which was proposed by Edward Kmett
Expand All @@ -1427,8 +1437,6 @@ foldr'Bits :: Int -> (Int -> a -> a) -> a -> Nat -> a
before changing this code.
----------------------------------------------------------------------}

indexOfTheOnlyBit :: Nat -> Int
{-# INLINE indexOfTheOnlyBit #-}
indexOfTheOnlyBit bitmask =
I# (lsbArray `indexInt8OffAddr#` unboxInt (intFromNat ((bitmask * magic) `shiftRL` offset)))
where unboxInt (I# i) = i
Expand All @@ -1448,6 +1456,12 @@ indexOfTheOnlyBit bitmask =
-- is 48B on 32-bit and 56B on 64-bit architectures -- so the 32B and 64B array
-- is actually improvement on 32-bit and only a 8B size increase on 64-bit.

lowestBitSet x = indexOfTheOnlyBit (lowestBitMask x)

highestBitSet x = indexOfTheOnlyBit (highestBitMask x)

#endif

lowestBitMask :: Nat -> Nat
lowestBitMask x = x .&. negate x
{-# INLINE lowestBitMask #-}
Expand All @@ -1469,10 +1483,6 @@ revNat x1 = case ((x1 `shiftRL` 1) .&. 0x5555555555555555) .|. ((x1 .&. 0x555555
x6 -> ( x6 `shiftRL` 32 ) .|. ( x6 `shiftLL` 32);
#endif

lowestBitSet x = indexOfTheOnlyBit (lowestBitMask x)

highestBitSet x = indexOfTheOnlyBit (highestBitMask x)

foldlBits prefix f z bitmap = go bitmap z
where go 0 acc = acc
go bm acc = go (bm `xor` bitmask) ((f acc) $! (prefix+bi))
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