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

[vstinner]

BPO	43688
Nosy	@vstinner, @encukou
PRs	bpo-43688: Support the limited C API in debug mode #25131 bpo-43688: Support "make regen-limited-abi" in debug mode #25133 bpo-43688: Run make regen-limited-abi #25134 bpo-43688: Fix Py_LIMITED_API version of xxlimited #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]

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]

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]

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

[encukou]

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

[vstinner]

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

[vstinner]

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]

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]

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]

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.

[vstinner]

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

[vstinner]

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

[vstinner]

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