user1494080 user1494080 - 29 days ago 10
C++ Question

Counting the number of leading zeros in a 128-bit integer

How can I count the number of leading zeros in a 128-bit integer (

uint128_t
) efficiently?

I know GCC's built-in functions:


  • __builtin_clz
    ,
    __builtin_clzl
    ,
    __builtin_clzll

  • __builtin_ffs
    ,
    __builtin_ffsl
    ,
    __builtin_ffsll



However, these functions only work with 32- and 64-bit integers.

I also found some SSE instructions:


  • __lzcnt16
    ,
    __lzcnt
    ,
    __lzcnt64



As you may guess, these only work with 16-, 32- and 64-bit integers.

Is there any similar, efficient built-in functionality for 128-bit integers?

Answer
inline int clz_u128 (uint128_t u) {
  uint64_t hi = u>>64;
  uint64_t lo = u;
  int retval[3]={
    __builtin_clzll(hi),
    __builtin_clzll(lo)+64,
    128
  };
  int idx = !hi + ((!lo)&(!hi));
  return retval[idx];
}

this is a branch free variant. Note that more work is done than in the branchy solution, and in practice the branching will probably be predictable.

It also relies on __builtin_clzll not crashing when fed 0: the docs say the result is undefined, but is it just unspecified or undefined?

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