From: lkcl <lkcl@web>
Date: Thu, 30 Nov 2023 15:25:26 +0000 (+0000)
Subject: (no commit message)
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=a803c8b6d3f864293e9dac770ec913b16c10232e;p=libreriscv.git

---

diff --git a/openpower/sv/cookbook/pospopcnt.mdwn b/openpower/sv/cookbook/pospopcnt.mdwn
index 53767c25e..bb6181aae 100644
--- a/openpower/sv/cookbook/pospopcnt.mdwn
+++ b/openpower/sv/cookbook/pospopcnt.mdwn
@@ -19,6 +19,27 @@ func count8safe(counts *[8]int, buf []uint8) {
 	}
 }
 ```
+
+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=CTR and CR0 (Rc=1)
+# 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%" />