CTZW - RISC-V instruction reference

ctzwCount trailing zeros word (32-bit) RV64 only
Sets t1 to the number of 0 bits below the least significant 1 bit of the low 32 bits of t2, or 32 when they are zero (RV64 only).
RISC-V manual: ctzw 
Forms  ctzw ,   
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; ctzw writes 0 to t2.
ctzw t2, t0