Nested if/else gets phantom else trace again (3.10)

[nedbat]

BPO	44840
Nosy	@nedbat, @markshannon, @skirpichev, @pablogsal
PRs	bpo-44840: Compiler: Move duplication of exit blocks with no line numbers to after CFG optimization. #27656 [3.10] bpo-44840: Compiler: Move duplication of exit blocks with no line numbers to after CFG optimization. (GH-27656) #27673

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-08-09.11:45:36.803>
created_at = <Date 2021-08-05.13:11:40.354>
labels = ['interpreter-core', 'type-bug', '3.10']
title = 'Nested if/else gets phantom else trace again (3.10)'
updated_at = <Date 2021-08-09.11:45:36.803>
user = 'https://github.com/nedbat'

bugs.python.org fields:

activity = <Date 2021-08-09.11:45:36.803>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-09.11:45:36.803>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2021-08-05.13:11:40.354>
creator = 'nedbat'
dependencies = []
files = []
hgrepos = []
issue_num = 44840
keywords = ['patch', '3.10regression']
message_count = 4.0
messages = ['399003', '399251', '399253', '399255']
nosy_count = 4.0
nosy_names = ['nedbat', 'Mark.Shannon', 'Sergey.Kirpichev', 'pablogsal']
pr_nums = ['27656', '27673']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44840'
versions = ['Python 3.10']

[nedbat]

Note: this is very similar to https://bugs.python.org/issue42810
This was originally reported against coverage.py: nedbat/coveragepy#1205

---8<-------------

import linecache, sys

def trace(frame, event, arg):
    # The weird globals here is to avoid a NameError on shutdown...
    if frame.f_code.co_filename == globals().get("__file__"):
        lineno = frame.f_lineno
        print("{} {}: {}".format(event[:4], lineno, linecache.getline(__file__, lineno).rstrip()))
    return trace

print(sys.version)
sys.settrace(trace)

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

A = B = True
C = False

func()

---

This produces this trace output:

3.10.0rc1 (default, Aug 3 2021, 15:03:55) [Clang 12.0.0 (clang-1200.0.32.29)]
call 13: def func():
line 14: if A:
line 15: if B:
line 16: if C:
line 21: elif E and F:
retu 21: elif E and F:

The elif on line 21 is not executed, and should not be traced.

Also, if I change line 21 to `elif E:`, then the trace changes to:

3.10.0rc1 (default, Aug 3 2021, 15:03:55) [Clang 12.0.0 (clang-1200.0.32.29)]
call 13: def func():
line 14: if A:
line 15: if B:
line 16: if C:
line 22: return True
retu 22: return True

[markshannon]

New changeset b854557 by Mark Shannon in branch 'main':
bpo-44840: Compiler: Move duplication of exit blocks with no line numbers to after CFG optimization. (GH-27656)
b854557

[pablogsal]

New changeset 762ef85 by Mark Shannon in branch '3.10':
bpo-44840: Compiler: Move duplication of exit blocks with no line numbers to after CFG optimization. (GH-27656) (bpo-27673)
762ef85

[nedbat]

This fix looks good, thanks.