CPOPW - RISC-V instruction reference

cpopwCount set bits word (32-bit) RV64 only
Sets t1 to the number of 1 bits in the low 32 bits of t2 (RV64 only).
RISC-V manual: cpopw 
Forms  cpopw ,   
Operands t1 Any integer register, x0 to x31, or its named alias, such as zero, t0 or t6. 
t2 An integer register. It may be the same register as t1. 
This example runs on RV64.
 .text
li t0, 13
# t0=13; cpopw writes 3 to t2.
cpopw t2, t0