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, and it adds up the totals of each bit set to 1 in each bit-position,
  10 of an array of input alues.
  11
  12 ```
  13 // Copyright (c) 2020 Robert Clausecker <fuz@fuz.su>
  14 // count8 reference implementation for tests.  Do not alter.
  15 func count8safe(counts *[8]int, buf []uint8) {
  16         for i := range buf {
  17                 for j := 0; j < 8; j++ {
  18                         counts[j] += int(buf[i] >> j & 1)
  19                 }
  20         }
  21 }
  22 ```
  23
  24 <img src="/openpower/sv/cookbook/popcount.svg " alt="pospopcnt" width="100%" />
  25
  26 [[!tag svp64_cookbook ]]