execution does not run in the source line's order

[rivers horse]

hi,
     I  debug gdb using itself,  i find  a strange  issue  ：
              the programme  being  debugged  execute  not in line
order,for example,  it goes into  line 20,then it goes  into line 18 ,
    which are simply assignment operations,

    what is the reasin for this?

thanks!