# Positional popcount SVP64

* <https://bugs.libre-soc.org/show_bug.cgi?id=672>
* <https://github.com/clausecker/pospop/blob/master/countsse2_amd64.s>
* RISC-V Bitmanip Extension Document Version 0.94-draft Editor: Claire Wolf Symbiotic GmbH
<https://raw.githubusercontent.com/riscv/riscv-bitmanip/master/bitmanip-draft.pdf>

Positional popcount in optimised assembler is typically done on SIMD ISAs in
around 500 lines.  Power ISA thanks to `bpermd` can be much more efficient:
with SVP64 even more so.  The reference implementation showing the concept
is below.

```
// Copyright (c) 2020 Robert Clausecker <fuz@fuz.su>
// count8 reference implementation for tests.  Do not alter.
func count8safe(counts *[8]int, buf []uint8) {
	for i := range buf {
		for j := 0; j < 8; j++ {
			counts[j] += int(buf[i] >> j & 1)
		}
	}
}
```

A simple but still hardware-paralleliseable SVP64 assembler for
8-bit input values (`count8safe`) is as follows:

```
mtspr 9, 3"                # move r3 to CTR
# VL = MIN(CTR,MAXVL=8), Rc=1 (CR0 set if CTR ends)
setvl 3,0,8,0,1,1"         # set MVL=8, VL=MIN(MVL,CTR)
# load VL bytes (update r4 addr) but compressed (dw=8)
addi 6, 0, 0               # initialise all 64-bits of r6 to zero
sv.lbzu/pi/dw=8 *6, 1(4)   # should be /lf here as well
# gather performs the transpose (which gets us to positional..)
gbbd 8,6
# now those bits have been turned around, popcount and sum them
setvl 0,0,8,0,1,1          # set MVL=VL=8
sv.popcntd/sw=8 *24,*8     # do the (now transposed) popcount
sv.add *16,*16,*24         # and accumulate in results
# branch back if CTR still non-zero. works even though VL=8
sv.bc/all 16, *0, -0x28   # reduce CTR by VL and stop if -ve
```

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.

<img src="/openpower/sv/cookbook/popcount.svg " alt="pospopcnt" width="70%" />


[[!tag svp64_cookbook ]]