CPOP - RISC-V instruction reference

cpopCount set bits 
Sets t1 to the number of 1 bits in t2.
RISC-V manual: cpop 
Forms  cpop ,   
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. 
 .text
li t0, 13
# t0=13; cpop writes 3 to t2.
cpop t2, t0