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Compiler hangs during jump elimination #89931

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brandtbucher opened this issue Nov 9, 2021 · 6 comments
Closed

Compiler hangs during jump elimination #89931

brandtbucher opened this issue Nov 9, 2021 · 6 comments
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@brandtbucher
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3.11 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@brandtbucher
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BPO 45773
Nosy @markshannon, @pablogsal, @miss-islington, @brandtbucher
PRs
  • bpo-45773: Stop "optimizing" certain jump patterns #29505
  • [3.10] bpo-45773: Stop "optimizing" certain jump patterns (GH-29505) #29526
  • bpo-45773: Remove invalid peephole optimizations #31066
  • [3.10] bpo-45773: Remove invalid peephole optimizations (GH-31066) #31101
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    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 = 'https://github.com/brandtbucher'
closed_at = <Date 2022-02-03.16:04:53.162>
created_at = <Date 2021-11-09.19:56:22.487>
labels = ['interpreter-core', 'type-bug', '3.11']
title = 'Compiler hangs during jump elimination'
updated_at = <Date 2022-02-03.16:04:53.162>
user = 'https://github.com/brandtbucher'

    bugs.python.org fields:

    activity = <Date 2022-02-03.16:04:53.162>
actor = 'brandtbucher'
assignee = 'brandtbucher'
closed = True
closed_date = <Date 2022-02-03.16:04:53.162>
closer = 'brandtbucher'
components = ['Interpreter Core']
creation = <Date 2021-11-09.19:56:22.487>
creator = 'brandtbucher'
dependencies = []
files = []
hgrepos = []
issue_num = 45773
keywords = ['patch']
message_count = 6.0
messages = ['406046', '406174', '406179', '412318', '412433', '412447']
nosy_count = 4.0
nosy_names = ['Mark.Shannon', 'pablogsal', 'miss-islington', 'brandtbucher']
pr_nums = ['29505', '29526', '31066', '31101']
priority = None
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45773'
versions = ['Python 3.11']
    @brandtbucher
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    The following code hangs during compilation on 3.11 and 3.10:

    >> while True or spam: pass

    Our peepholer gets stuck in a loop, repeatedly "optimizing" instruction 4 (POP_JUMP_IF_TRUE -> JUMP_ABSOLUTE):

    1 0 JUMP_ABSOLUTE 0 (to 0)
    2 LOAD_NAME 0 (spam)
    4 POP_JUMP_IF_TRUE 0 (to 0)

    After optimizing jumps to jumps like these, we always back up and attempt to optimize the same instruction again (which makes it possible to optimize three or more chained jumps on the same line). The issue is that this particular optimization doesn't actually change the instruction, since both jumps target the same exact block. Since nothing changes, we loop forever.

    The fix is really simple: just skip the optimization if instr->i_target == target->i_target. We already do this for several other types of jumps; it just looks like POP_JUMP_IF_TRUE and POP_JUMP_IF_FALSE might have been missed.

    I'll have a PR up soon.

    @brandtbucher brandtbucher added type-crash A hard crash of the interpreter, possibly with a core dump 3.11 only security fixes labels Nov 9, 2021
    @brandtbucher brandtbucher self-assigned this Nov 9, 2021
    @brandtbucher brandtbucher added interpreter-core (Objects, Python, Grammar, and Parser dirs) type-crash A hard crash of the interpreter, possibly with a core dump 3.11 only security fixes labels Nov 9, 2021
    @brandtbucher brandtbucher self-assigned this Nov 9, 2021
    @brandtbucher brandtbucher added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Nov 9, 2021
    @brandtbucher
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    New changeset 27b69e6 by Brandt Bucher in branch 'main':
    bpo-45773: Stop "optimizing" certain jump patterns (GH-29505)
    27b69e6

    @brandtbucher
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    New changeset a89bbde by Brandt Bucher in branch '3.10':
    [3.10] bpo-45773: Stop "optimizing" certain jump patterns (GH-29526)
    a89bbde

    @brandtbucher
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    My fix for this seems to have erroneously added two invalid new jump threads:

    POP_JUMP_IF_FALSE(a) to JUMP_IF_FALSE_OR_POP(b) is folded into POP_JUMP_IF_FALSE(b)
    POP_JUMP_IF_TRUE(a) to JUMP_IF_TRUE_OR_POP(b) is folded into POP_JUMP_IF_TRUE(b)

    The good news is that I can't get the compiler to actually emit these particular jump sequences. It still needs to be fixed ASAP, though.

    @brandtbucher brandtbucher reopened this Feb 1, 2022
    @brandtbucher brandtbucher added type-bug An unexpected behavior, bug, or error release-blocker and removed type-crash A hard crash of the interpreter, possibly with a core dump labels Feb 1, 2022
    @brandtbucher brandtbucher reopened this Feb 1, 2022
    @brandtbucher brandtbucher added type-bug An unexpected behavior, bug, or error and removed type-crash A hard crash of the interpreter, possibly with a core dump labels Feb 1, 2022
    @markshannon
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    New changeset e0433c1 by Brandt Bucher in branch 'main':
    bpo-45773: Remove invalid peephole optimizations (GH-31066)
    e0433c1

    @miss-islington
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    New changeset ff6948b by Miss Islington (bot) in branch '3.10':
    bpo-45773: Remove invalid peephole optimizations (GH-31066)
    ff6948b

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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    @iritkatriel @markshannon @miss-islington @brandtbucher