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

[Herbert Valerio Riedel]

This proposal passed with six +1s (other than myself), specifically by
 
 - David Feuer
 - Johan Tibell
 - Milan Straka
 - Edward Kmett
 - Carter Schonwald
 - Vincent Hanquez

There were no -1s (I'm aware of), and no concerns were raised.


The proposed implementation has already been pushed as per

   http://git.haskell.org/ghc.git/commitdiff/a8a969ae7a05e408b29961d0a2ea621a16d73d3e


On the whole, this was quite a bor^H^H^Hefficient proposal discussion :-)


Cheers,
  hvr

On 2014-08-15 at 10:22:38 +0200, Herbert Valerio Riedel 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.

-- 
"Elegance is not optional" -- Richard O'Keefe