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Implement new `clz` inline primop

    • Closed Issue created by Herbert Valerio Riedel @hvr

    From a draft for #9281 (closed), I had to use a C FFI and using a gcc/clang __builtin to get access to the CLZ instruction:

    -- | 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#
    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#).

    Edited
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