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/a8a969ae7a05e408b29961d0a2ea621a16... 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/e0c1767d0ea8d12e0a4badf43682a08784e379...
[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