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markshannon opened this issue Aug 8, 2024 · 1 comment
Open

Inconsistent locations for conditional branches in while statements #122821

markshannon opened this issue Aug 8, 2024 · 1 comment
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@markshannon
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3.13 bugs and security fixes 3.14 new features, bugs and security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@markshannon
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markshannon commented Aug 8, 2024 •
edited by bedevere-app bot
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Bug report

Bug description:

Reported here: #122762 (comment)

The locations for branches in while statements are not consistent. The first iteration shows different locations from the second and later iterations.

I've not check earlier versions, but this could be an issue for 3.12 as well.

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

No response

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@markshannon markshannon added type-bug An unexpected behavior, bug, or error interpreter-core (Objects, Python, Grammar, and Parser dirs) 3.13 bugs and security fixes 3.14 new features, bugs and security fixes labels Aug 8, 2024
@markshannon
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Here's my hypothesis:

This is an artifact of compiling while cond: body as:

start:
    if not cond: goto end
    body
    if cond goto start
end:

and the assembler converting condition backwards jumps from if cond goto target to

    if not cond goto end
    goto target
end:

So we end up with:

start:
    if not cond: goto end
    body
    if not cond goto end
    goto start
end:

When cond is True, the first branch jumps from cond to body, but the second branch jumps from cond to cond.

Compiling while cond: body as:

start:
    if not cond: goto end
    body
    goto start
end:

should fix this.

@markshannon markshannon mentioned this issue Aug 8, 2024
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@markshannon markshannon self-assigned this Aug 12, 2024
@bedevere-app bedevere-app bot mentioned this issue Aug 12, 2024
Merged
markshannon added a commit that referenced this issue Aug 13, 2024
@markshannon
GH-122821: Simplify compilation of while statements to ensure consist…
fe23f8e 
…ency of offsets for sys.monitoring (GH-122934)
blhsing pushed a commit to blhsing/cpython that referenced this issue Aug 22, 2024
@markshannon @blhsing
pythonGH-122821: Simplify compilation of while statements to ensure c…
6eb343a 
…onsistency of offsets for sys.monitoring (pythonGH-122934)
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3.13 bugs and security fixes 3.14 new features, bugs and security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error
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