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Counter for JUMP_BACKWARD is initialized to 17 instead of 0 #108488

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gvanrossum opened this issue Aug 25, 2023 · 0 comments · Fixed by #108591
Closed

Counter for JUMP_BACKWARD is initialized to 17 instead of 0 #108488

gvanrossum opened this issue Aug 25, 2023 · 0 comments · Fixed by #108591
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@gvanrossum @markshannon
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type-bug An unexpected behavior, bug, or error

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gvanrossum commented Aug 25, 2023 •
edited by bedevere-bot

Bug report

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Output from running 'python -VV' on the command line:

Python 3.13.0a0 (heads/main-dirty:5a25daa512, Aug 25 2023, 21:20:04) [Clang 14.0.3 (clang-1403.0.22.14.1)]

A clear and concise description of the bug:

From #108311 (comment):

FWIW I also discovered that the JUMP_BACKWARD counter is initialized to 17 (the constant computed by adaptive_counter_warmup()), like all other counters, by _PyCode_Quicken() in specialize.c. Arguably, since this counter is supposed to count upwards from zero (in steps of 16, i.e. 1 << OPTIMIZER_BITS_IN_COUNTER), it ought to be initialized to zero (as I had expected it would be). But this ought to be a separate issue and PR.

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@gvanrossum gvanrossum added the type-bug An unexpected behavior, bug, or error label Aug 25, 2023
@gvanrossum gvanrossum mentioned this issue Aug 25, 2023
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@bedevere-bot bedevere-bot mentioned this issue Aug 28, 2023
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gvanrossum added a commit that referenced this issue Aug 29, 2023
@gvanrossum
gh-108488: Initialize JUMP_BACKWARD cache to 0, not 17 (#108591)
59e4693 
This mis-initialization caused the executor optimization to kick in sooner than intended. It also set the lower 4 bits of the counter to `1` -- those bits are supposed to be reserved (the actual counter is in the upper 12 bits).
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gh-108488: Initialize JUMP_BACKWARD cache to 0, not 17 gvanrossum/cpython
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