Re: [Boost-bugs] [Boost C++ Libraries] #12401: UB when sizeof(std::size_t) in floor_log2

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #12401: UB when sizeof(std::size_t) in floor_log2
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-08-19 18:50:26

#12401: UB when sizeof(std::size_t) in floor_log2
-------------------------------+------------------------
  Reporter:  viboes            |      Owner:  igaztanaga
      Type:  Bugs              |     Status:  closed
 Milestone:  To Be Determined  |  Component:  intrusive
   Version:  Boost 1.61.0      |   Severity:  Problem
Resolution:  invalid           |   Keywords:
-------------------------------+------------------------
Changes (by igaztanaga):

 * status:  new => closed
 * resolution:   => invalid

Comment:

 That function is only called if sizeof(std::size_t)*CHAR_BIT is 64:

 {{{

    inline std::size_t floor_log2 (std::size_t x)
    {
       const std::size_t Bits = sizeof(std::size_t)*CHAR_BIT;
       return floor_log2(x, integral_constant<std::size_t, Bits>());
    }
 }}}

 The 32 bit version uses only 16 bit shift:

 {{{
    inline std::size_t floor_log2 (std::size_t v,
 integral_constant<std::size_t, 32>)
    {
       static const int MultiplyDeBruijnBitPosition[32] =
       {
          0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30,
          8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31
       };

       v |= v >> 1;
       v |= v >> 2;
       v |= v >> 4;
       v |= v >> 8;
       v |= v >> 16;

       return MultiplyDeBruijnBitPosition[(std::size_t)(v * 0x07C4ACDDU) >>
 27];
    }

 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12401#comment:1>
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