[Bug tree-optimization/115857] New missed optimisation for hamming weight to simple popcnt

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115857

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1
          Component|c                           |tree-optimization
   Last reconfirmed|                            |2024-07-10

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We handle

   int popcount64c (uint64_t x)
   {
     x -= (x >> 1) & 0x5555555555555555ULL;
     x = (x & 0x3333333333333333ULL) + ((x >> 2) & 0x3333333333333333ULL);
     x = (x + (x >> 4)) & 0x0f0f0f0f0f0f0f0fULL;
     return (x * 0x0101010101010101ULL) >> 56;
   }

   int popcount32c (uint32_t x)
   {
     x -= (x >> 1) & 0x55555555;
     x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
     x = (x + (x >> 4)) & 0x0f0f0f0f;
     return (x * 0x01010101) >> 24;
   }

at the moment.  This is detected by a pattern in match.pd.  Doing a similar
variant for your version should be possible.