[GHC] #9340: Implement new `clz` inline primop

[GHC]

#9340: Implement new `clz` inline primop
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.1
       Component:  Compiler          |                 Version:  7.8.2
  (CodeGen)                          |  Differential Revisions:
        Keywords:  primop clz        |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |              Difficulty:  Unknown
       Test Case:                    |              Blocked By:
        Blocking:                    |         Related Tickets:
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 From a draft for #9281, I had to use a C FFI and using a gcc/clang
 `__builtin` to get access to the
 [http://en.wikipedia.org/wiki/Find_first_set CLZ instruction]:

 {{{#!hs
 -- | Compute base-2 log of 'Word#'
 ​--
 ​-- This is internally implemented as count-leading-zeros machine
 instruction.
 ​foreign import ccall unsafe "integer_gmp_word_log2"
   wordLog2# :: Word# -> Int#
 }}}

 {{{#!c
 HsWord
 integer_gmp_word_log2(HsWord x)
 ​{
         ​#if (SIZEOF_HSWORD) == (SIZEOF_INT)
         ​ return x ? (WORD_SIZE_IN_BITS-1) - __builtin_clz(x) : -1;
         ​#elif (SIZEOF_HSWORD) == (SIZEOF_LONG)
         ​ return x ? (WORD_SIZE_IN_BITS-1) - __builtin_clzl(x) : -1;
         ​#elif (SIZEOF_HSWORD) == (SIZEOF_LONG_LONG)
         ​ return x ? (WORD_SIZE_IN_BITS-1) - __builtin_clzll(x) : -1;
         ​#else
         ​# error unsupported SIZEOF_HSWORD
         ​#endif
 ​}
 }}}

 Since  a `clz`-like operation should be available on most cpus GHC should
 expose that as a primop ({{{clz# :: Word# -> Int#}}}).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9340>
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