openpower/sv/cookbook/pospopcnt.mdwn

   1 # Positional popcount SVP64
   2
   3 * <https://bugs.libre-soc.org/show_bug.cgi?id=672>
   4 * <https://github.com/clausecker/pospop/blob/master/countsse2_amd64.s>
   5
   6 Positional popcount in optimised assembler is typically done on SIMD ISAs in
   7 around 500 lines.  Power ISA thanks to `bpermd` can be much more efficient:
   8 with SVP64 even more so.  The reference implementation showing the concept
   9 is below.
  10
  11 ```
  12 // Copyright (c) 2020 Robert Clausecker <fuz@fuz.su>
  13 // count8 reference implementation for tests.  Do not alter.
  14 func count8safe(counts *[8]int, buf []uint8) {
  15         for i := range buf {
  16                 for j := 0; j < 8; j++ {
  17                         counts[j] += int(buf[i] >> j & 1)
  18                 }
  19         }
  20 }
  21 ```
  22 Array popcount is just standard popcount function ([[!wikipedia Hamming weight]]) on an array of values whereas positional popcount adds up the totals of each bit set to 1 in each bit-position, of an array of input values.
  23
  24 <img src="/openpower/sv/cookbook/popcount.svg " alt="pospopcnt" width="70%" />
  25
  26 [[!tag svp64_cookbook ]]