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[C API] Support the limited C API in debug mode (Py_INCREF and Py_DECREF) #87854

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vstinner opened this issue Apr 1, 2021 · 12 comments
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[C API] Support the limited C API in debug mode (Py_INCREF and Py_DECREF) #87854

vstinner opened this issue Apr 1, 2021 · 12 comments

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@vstinner
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@vstinner vstinner commented Apr 1, 2021

BPO 43688
Nosy @vstinner, @encukou
PRs
  • #25131
  • #25133
  • #25134
  • #25135
  • Files
  • bench_limited.py
  • bench_testcapi.py
  • bench.patch
  • 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 2021-05-26.22:20:07.057>
    created_at = <Date 2021-04-01.06:47:31.015>
    labels = ['expert-C-API', '3.10']
    title = '[C API] Support the limited C API in debug mode (Py_INCREF and Py_DECREF)'
    updated_at = <Date 2021-05-26.22:20:07.056>
    user = 'https://github.com/vstinner'

    bugs.python.org fields:

    activity = <Date 2021-05-26.22:20:07.056>
    actor = 'vstinner'
    assignee = 'none'
    closed = True
    closed_date = <Date 2021-05-26.22:20:07.057>
    closer = 'vstinner'
    components = ['C API']
    creation = <Date 2021-04-01.06:47:31.015>
    creator = 'vstinner'
    dependencies = []
    files = ['49925', '49926', '49927']
    hgrepos = []
    issue_num = 43688
    keywords = ['patch']
    message_count = 12.0
    messages = ['389956', '389960', '389962', '389971', '389982', '389983', '390002', '390057', '390062', '390063', '390067', '394478']
    nosy_count = 2.0
    nosy_names = ['vstinner', 'petr.viktorin']
    pr_nums = ['25131', '25133', '25134', '25135']
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = None
    url = 'https://bugs.python.org/issue43688'
    versions = ['Python 3.10']

    @vstinner
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    @vstinner vstinner commented Apr 1, 2021

    Currently, setup.py doesn't build xxlimited and xxlimited_35 extension modules with the limited C API if Python is built in debug mode. I only found two functions affected by Py_DEBUG macro in the limited C API: Py_INCREF() and Py_DECREF().

    Example:
    ---

    #if defined(Py_DEBUG) && !defined(Py_REF_DEBUG)
    #define Py_REF_DEBUG
    #endif
    
    static inline void _Py_INCREF(PyObject *op)
    {
    #ifdef Py_REF_DEBUG
        _Py_RefTotal++;
    #endif
        op->ob_refcnt++;
    }
    #define Py_INCREF(op) _Py_INCREF(_PyObject_CAST(op))

    If Py_DEBUG is defined (Python built in debug mode), Py_INCREF() increments the private _Py_RefTotal variable.

    The limited C API is supposed to provide a stable ABI, but Py_INCREF() leaks _Py_RefTotal implementation if Python is built in debug mode.

    I propose to modify Py_INCREF() and Py_DECREF() of the limited C API to always declare them as opaque function calls. The regular (non limited) C API will continue to use the same static inline functions.

    See also #25115 and bpo-41111 "[C API] Convert a few stdlib extensions to the limited C API (PEP-384)".

    Note: Py_XINCREF() and Py_XDECREF() should be fine.

    @vstinner vstinner changed the title [C API] Fix Py_INCREF and Py_DECREF in the limited C API for Python built in debug mode [C API] Support the limited C API in debug build (Py_INCREF and Py_DECREF) Apr 1, 2021
    @vstinner vstinner changed the title [C API] Fix Py_INCREF and Py_DECREF in the limited C API for Python built in debug mode [C API] Support the limited C API in debug build (Py_INCREF and Py_DECREF) Apr 1, 2021
    @vstinner vstinner changed the title [C API] Support the limited C API in debug build (Py_INCREF and Py_DECREF) [C API] Support the limited C API in debug mode (Py_INCREF and Py_DECREF) Apr 1, 2021
    @vstinner vstinner changed the title [C API] Support the limited C API in debug build (Py_INCREF and Py_DECREF) [C API] Support the limited C API in debug mode (Py_INCREF and Py_DECREF) Apr 1, 2021
    @vstinner
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    @vstinner vstinner commented Apr 1, 2021

    One nice advantage of this change is that it becomes possible to check for reference leaks in C extensions written with the limited C API!

    $ ./python -m test test_xxlimited -R 3:3
    0:00:00 load avg: 2.99 Run tests sequentially
    0:00:00 load avg: 2.99 [1/1] test_xxlimited
    beginning 6 repetitions
    123456
    ......

    == Tests result: SUCCESS ==

    1 test OK.

    Total duration: 367 ms
    Tests result: SUCCESS

    @vstinner
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    @vstinner vstinner commented Apr 1, 2021

    New changeset baf10da by Victor Stinner in branch 'master':
    bpo-43688: Run make regen-limited-abi (GH-25134)
    baf10da

    @encukou
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    @encukou encukou commented Apr 1, 2021

    If you do this, please check the performance impact. Py_INCREF/Py_DECREF are very common.

    @vstinner
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    @vstinner vstinner commented Apr 1, 2021

    New changeset 2ac0515 by Victor Stinner in branch 'master':
    bpo-43688: Fix Py_LIMITED_API version of xxlimited (GH-25135)
    2ac0515

    @vstinner
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    @vstinner vstinner commented Apr 1, 2021

    New changeset baf10da by Victor Stinner in branch 'master':
    bpo-43688: Run make regen-limited-abi (GH-25134)

    With this change, "Tests / Check if generated files are up to date" job started fails (on PR 25135): "Some symbols from the limited API are missing: PyType_HasFeature".

    I fixed this issue in bpo-43690 which removes PyType_HasFeature from Doc/data/stable_abi.dat.

    @vstinner
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    @vstinner vstinner commented Apr 1, 2021

    I wrote a microbenchmark on Py_INCREF()+Py_DECREF():

    $ python3 -m pyperf compare_to ref.json limited.json 
    Mean +- std dev: [ref] 3.45 ns +- 0.17 ns -> [limited] 6.03 ns +- 0.21 ns: 1.75x slower

    If a function is only made of Py_INCREF() and Py_DECREF(), it can be up to 1.8x slower in the worst case. But in practice, I don't think that functions are only made of Py_INCREF() and Py_DECREF(). They do a few other things.

    I'm not sure how to run a "macro benchmark" on my PR 25131, since I don't know any C extension doing anything useful. There is xxlimited, but it does almost nothing.

    What would be a fair benchmark for this change?

    --

    To run my microbenchmark:

    git apply bench.patch
    ./configure --with-lto --enable-optimizations
    make
    ./python -m venv env
    ./env/bin/python -m pip install pyperf
    ./env/bin/python ../bench_limited.py -o ../limited.json -v
    ./env/bin/python ../bench_testcapi.py -o ../ref.json -v

    @vstinner
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    @vstinner vstinner commented Apr 2, 2021

    I modified my PR 25131 to only implement Py_INCREF/Py_DECREF as opaque function calls under two conditions:

    • Python is built in debug mode
    • Py_LIMITED_API macro targets Python 3.10 or newer

    So this PR 25131 now only has an impact on performance if Python is built in debug mode. Performance on Python built in release mode is not affected.

    @vstinner
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    @vstinner vstinner commented Apr 2, 2021

    In release mode, I get the libraries:

    • xxlimited.cpython-310-x86_64-linux-gnu.so
    • xxlimited_35.cpython-310-x86_64-linux-gnu.so

    In debug mode, I get the libraries:

    • xxlimited.cpython-310d-x86_64-linux-gnu.so
    • xxlimited_35.cpython-310d-x86_64-linux-gnu.so

    It's still a different ABI: "cpython-310" vs "cpython-310d" ("D" for debug). So there is risk to be confused between the stable ABI in release mode and the stable ABI in debug mode :-) I don't how how to get the ".abi3.so" suffix.

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    @vstinner vstinner commented Apr 2, 2021

    New changeset 3359cab by Victor Stinner in branch 'master':
    bpo-43688: Support the limited C API in debug mode (GH-25131)
    3359cab

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    @vstinner vstinner commented Apr 2, 2021

    New changeset 9bb5658 by Victor Stinner in branch 'master':
    bpo-43688: Support "make regen-limited-abi" in debug mode (GH-25133)
    9bb5658

    @vstinner
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    @vstinner vstinner commented May 26, 2021

    Thanks for the help Petr. I close the issue, it's now fixed!

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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