[PROPOSAL] Add `FiniteBits(count{Leading,Trailing}Zeros)`

[Carter Schonwald]

+1


On Fri, Aug 15, 2014 at 11:26 AM, Edward Kmett <ekmett at gmail.com> wrote:

> A very enthusiastic +1 from me. I'll finally be able to drop custom
> foreign prims from my own code.
>
>
> On Fri, Aug 15, 2014 at 4:22 AM, Herbert Valerio Riedel <hvr at gnu.org>
> wrote:
>
>> Hello *,
>>
>> As GHC 7.10.1 will have support for new assembler-optimized CLZ & CTZ[1]
>> primops[2], it'd be useful to provide also a convenient high-level
>> interface to avoid having to work with -XMagicHash and unboxed values.
>>
>> To this end, I hereby propose to add two new methods to the 'FiniteBits'
>> class, specifically
>>
>>
>>   class Bits b => FiniteBits b where
>>     {- ... -}
>>
>>     countLeadingZeros :: b -> Int
>>     countLeadingZeros x = (w-1) - go (w-1)
>>       where
>>         go i | i < 0       = i -- no bit set
>>              | testBit x i = i
>>              | otherwise   = go (i-1)
>>
>>         w = finiteBitSize x
>>
>>     countTrailingZeros :: b -> Int
>>     countTrailingZeros x = go 0
>>       where
>>         go i | i >= w      = i
>>              | testBit x i = i
>>              | otherwise   = go (i+1)
>>
>>         w = finiteBitSize x
>>
>>
>> The full patch (including Haddock doc-strings) is available for code
>> review at
>>
>>   https://phabricator.haskell.org/D158
>>
>> I suggest to try to keep the discussion/voting/bikeshedding about the
>> proposal proper here (including any bike-shedding about naming). At same
>> time, I'd like to invite you to try out the Phab code-revision tool[3]
>> for pointing-out/discussing technical issues with the proposed patch.
>>
>>
>> Cheers,
>>   hvr
>>
>>  [1]: http://en.wikipedia.org/wiki/Find_first_set provides a good
>>       overview why CLZ/CTZ are desirable primitive operations.
>>
>>  [2]:
>> http://git.haskell.org/ghc.git/commit/e0c1767d0ea8d12e0a4badf43682a08784e379c6
>>
>>  [3]: Phab code-revisions allow you to directly annotate code-fragments.
>>       However, after having written inline annotations, you have to
>> actually
>>       submit those by submitting a non-inline (possibly empty) comment
>>       (see bottom of that page) to make them visible for everyone.
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>
>
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