bpo-44840: Compiler: Move duplication of exit blocks with no line numbers to after CFG optimization.

[markshannon]

Makes sure that all exits have line number.

I've tried to keep this change minimal, as it needs to be backported to 3.10 which is nearing release.
For the example code in https://bugs.python.org/issue44840,

def func():
    if A:
        if B:
            if C:
                if D:
                    return False
        else:
            return False
    elif E and F:
        return True

Compiles with main to:

  2           0 LOAD_GLOBAL              0 (A)
              2 POP_JUMP_IF_FALSE       13 (to 26)

  3           4 LOAD_GLOBAL              1 (B)
              6 POP_JUMP_IF_FALSE       11 (to 22)

  4           8 LOAD_GLOBAL              2 (C)
             10 POP_JUMP_IF_FALSE       10 (to 20)

  5          12 LOAD_GLOBAL              3 (D)
             14 POP_JUMP_IF_FALSE       10 (to 20)

  6          16 LOAD_CONST               1 (False)
             18 RETURN_VALUE
 
None    >>   20 JUMP_FORWARD            10 (to 42)

  8     >>   22 LOAD_CONST               1 (False)
             24 RETURN_VALUE

  9     >>   26 LOAD_GLOBAL              4 (E)
             28 POP_JUMP_IF_FALSE       23 (to 46)
             30 LOAD_GLOBAL              5 (F)
             32 POP_JUMP_IF_FALSE       19 (to 38)

 10          34 LOAD_CONST               2 (True)
             36 RETURN_VALUE

  9     >>   38 LOAD_CONST               0 (None)
             40 RETURN_VALUE
        >>   42 LOAD_CONST               0 (None)
             44 RETURN_VALUE
        >>   46 LOAD_CONST               0 (None)
             48 RETURN_VALUE

With this PR, the exit blocks are duplicated after optimization, and before line number propagation,
ensuring that all exit blocks have line numbers:

  2           0 LOAD_GLOBAL              0 (A)
              2 POP_JUMP_IF_FALSE       16 (to 32)

  3           4 LOAD_GLOBAL              1 (B)
              6 POP_JUMP_IF_FALSE       14 (to 28)

  4           8 LOAD_GLOBAL              2 (C)
             10 POP_JUMP_IF_FALSE       10 (to 20)

  5          12 LOAD_GLOBAL              3 (D)
             14 POP_JUMP_IF_FALSE       12 (to 24)

  6          16 LOAD_CONST               1 (False)
             18 RETURN_VALUE

  4     >>   20 LOAD_CONST               0 (None)
             22 RETURN_VALUE

  5     >>   24 LOAD_CONST               0 (None)
             26 RETURN_VALUE

  8     >>   28 LOAD_CONST               1 (False)
             30 RETURN_VALUE

  9     >>   32 LOAD_GLOBAL              4 (E)
             34 POP_JUMP_IF_FALSE       22 (to 44)
             36 LOAD_GLOBAL              5 (F)
             38 POP_JUMP_IF_FALSE       24 (to 48)

 10          40 LOAD_CONST               2 (True)
             42 RETURN_VALUE

  9     >>   44 LOAD_CONST               0 (None)
             46 RETURN_VALUE
        >>   48 LOAD_CONST               0 (None)
             50 RETURN_VALUE

https://bugs.python.org/issue44840

[markshannon]

This is part of PEP 626, so I'm skipping the NEWS.

[miss-islington]

Thanks @markshannon for the PR 🌮🎉.. I'm working now to backport this PR to: 3.10.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[miss-islington]

Sorry, @markshannon, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker b854557b49083d8625a433eb36aacb0c87d67c52 3.10

[bedevere-bot]

GH-27673 is a backport of this pull request to the 3.10 branch.