Consider this little session from the tip of the spear:

>>> sys.version
'3.10.0a6+ (heads/master:0ab152c6b5, Mar 15 2021, 17:24:38) [GCC 10.2.0]'
>>> def while2(a):
...     while a >= 0:
...         a -= 1
...     return a
...
>>> dis.dis(while2)
  2           0 LOAD_FAST                0 (a)
              2 LOAD_CONST               1 (0)
              4 COMPARE_OP               5 (>=)
              6 POP_JUMP_IF_FALSE       24

  3     >>    8 LOAD_FAST                0 (a)
             10 LOAD_CONST               2 (1)
             12 INPLACE_SUBTRACT
             14 STORE_FAST               0 (a)

  2          16 LOAD_FAST                0 (a)
             18 LOAD_CONST               1 (0)
             20 COMPARE_OP               5 (>=)
             22 POP_JUMP_IF_TRUE         8

  4     >>   24 LOAD_FAST                0 (a)
             26 RETURN_VALUE
>>> list(dis.findlinestarts(while2.__code__))
[(0, 2), (8, 3), (16, 2), (24, 4)]

I get that somewhere along the way the compiler has been optimized to duplicate the loop's test so as to eliminate some jumps (perhaps). It's not clear to me that the line number should be "2" in the duplicated test though. Shouldn't it be "3"?

I stumbled on this while trying to generate a line number table in my side project register VM. As I understand it, the line number delta in the output table is supposed to always be >= 0. In my code I'm using dis.findlinestarts() to determine the line numbers for each block. Perhaps I should be modifying its results. OTOH, maybe it's a bug. (If that's the consensus, I will create an issue.)

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