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gh-137681: Always initialize exception handler for new instruction #137655

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gh-137681: Always initialize exception handler for new instruction #137655

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DinoV
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@DinoV DinoV commented Aug 11, 2025 •
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basicblock_addop zero initializes some members but not all. If we remove some nops and then later re-add an instruction we can have previously initialized memory that can include an exception handler. If we compile something like:

def foo():
    try:
        [x for x in abc]
    except OSError:
        pass

    return```

We'll end up with different exception handling table because the `NOT_TAKEN` instruction is inserted after we do the exception handling analysis and gets a previously initialized exception handler.

<!-- gh-issue-number: gh-137681 -->
* Issue: gh-137681
<!-- /gh-issue-number -->

@DinoV DinoV force-pushed the flowgraph_init_except branch 2 times, most recently from 9e390cb to 3a3a975 Compare August 12, 2025 15:45
@DinoV DinoV added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Aug 12, 2025
@DinoV DinoV force-pushed the flowgraph_init_except branch from 3a3a975 to 17de2a3 Compare August 12, 2025 16:19
@DinoV DinoV changed the title Always initialize exception handler for new instruction gh-137681: Always initialize exception handler for new instruction Aug 12, 2025
@bedevere-app bedevere-app bot mentioned this pull request Aug 12, 2025
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@DinoV DinoV force-pushed the flowgraph_init_except branch from 17de2a3 to 36a5650 Compare August 12, 2025 16:23
@DinoV DinoV marked this pull request as ready for review August 12, 2025 17:07
@iritkatriel
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This change is fine, but I think basicblock_addop should get the target and except as args. There is a future bug here where setting the except matters, and someone will forget.

@DinoV
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DinoV commented Aug 12, 2025

This change is fine, but I think basicblock_addop should get the target and except as args. There is a future bug here where setting the except matters, and someone will forget.

Do you think we should back port this to 3.14? While the NOT_TAKEN case is pretty benign I'm a little worried there could be something inserted where it does matter.

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If we don't have a bug in 3.14 I think we can leave it alone. We're not likely to insert new bytecode transformations there.

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@markshannon markshannon Awaiting requested review from markshannon markshannon is a code owner

@iritkatriel iritkatriel Awaiting requested review from iritkatriel iritkatriel is a code owner

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@DinoV @iritkatriel