Use `countTrailingZeros` to speed up iterating over bitmaps?!

[sjakobi]

unordered-containers/Data/HashMap/Internal.hs

Lines 1615 to 1643 in 7237826

	-- | Strict in the result of @f@.
	unionArrayBy :: (a -> a -> a) -> Bitmap -> Bitmap -> A.Array a -> A.Array a
	-> A.Array a
	unionArrayBy f b1 b2 ary1 ary2 = A.run $ do
	let b' = b1 .|. b2
	mary <- A.new_ (popCount b')
	-- iterate over nonzero bits of b1 .|. b2
	-- it would be nice if we could shift m by more than 1 each time
	let ba = b1 .&. b2
	go !i !i1 !i2 !m
	| m > b' = return ()
	| b' .&. m == 0 = go i i1 i2 (m `unsafeShiftL` 1)
	| ba .&. m /= 0 = do
	x1 <- A.indexM ary1 i1
	x2 <- A.indexM ary2 i2
	A.write mary i $! f x1 x2
	go (i+1) (i1+1) (i2+1) (m `unsafeShiftL` 1)
	| b1 .&. m /= 0 = do
	A.write mary i =<< A.indexM ary1 i1
	go (i+1) (i1+1) i2 (m `unsafeShiftL` 1)
	| otherwise = do
	A.write mary i =<< A.indexM ary2 i2
	go (i+1) i1 (i2+1) (m `unsafeShiftL` 1)
	go 0 0 0 (b' .&. negate b') -- XXX: b' must be non-zero
	return mary
	-- TODO: For the case where b1 .&. b2 == b1, i.e. when one is a
	-- subset of the other, we could use a slightly simpler algorithm,
	-- where we copy one array, and then update.
	{-# INLINE unionArrayBy #-}

I don't quite understand this code yet, but the comment reminds me of this use of countTrailingZeros to find the next 1 bit:

elems ::
   ( BitOffset a
   , FiniteBits b
   , IndexableBits b
   , Eq b
   ) => BitSet b a -> [a]
elems (BitSet b) = go b
   where
      go !c
         | c == zeroBits = []
         | otherwise     = let e = countTrailingZeros c in fromBitOffset e : go (clearBit c e)