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

+1 On Fri, Aug 15, 2014 at 11:26 AM, Edward Kmett 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 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. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

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