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GeezerVison is showing github.com — [DRAFT] gh-105059: In C++, Py_INCREF() uses a function call by vstinner · Pull Request #105764 · python/cpython

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@vstinner

@vstinner vstinner commented Jun 14, 2023 •
edited by bedevere-bot
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In C++, Py_INCREF() and Py_DECREF() are now implemented as function calls. Moreover, in C++, PyObject.ob_refcnt type is just Py_ssize_t: don't use an anonymous union to avoid compiler warnings (anonymous unions were added to C11).

  • Issue: object.h uses an anonymous union in a struct (older C incompatible) #105059

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@bedevere-bot bedevere-bot mentioned this pull request Jun 14, 2023
object.h uses an anonymous union in a struct (older C incompatible) #105059
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@vstinner
pythongh-105059: In C++, Py_INCREF() uses a function call
2d6d57a 
In C++, Py_INCREF() and Py_DECREF() are now implemented as function
calls. Moreover, in C++, PyObject.ob_refcnt type is just Py_ssize_t:
don't use an anonymous union to avoid compiler warnings (anonymous
unions were added to C11).
@vstinner
vstinner force-pushed the incref_cpp branch from 7cb779e to 2d6d57a Compare June 14, 2023 00:55
@vstinner

vstinner commented Jun 14, 2023

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In the issue #105387, I modified Py_INCREF() and Py_DECREF() to implement them as opaque function calls in the limited C API version 3.12 and newer (and when Python is built in debug mode): commit b542972

We can consider to do something similar in C++ and/or in C without anonymous union (ex: ISO C89). This PR implements this idea for C++: in C++, PyObject.ob_refcnt type is again the simple Py_ssize_t, and Py_INCREF() and Py_DECREF() are implemented as function calls.

Well, I wrote this PR to show that we now have this technical solution, but it has a cost on performance. I don't know if it's acceptable for Python 3.12. Alternatives are discussed in issue #105059 and PR #105275.

cc @eduardo-elizondo
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@vstinner

vstinner commented Jun 14, 2023

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C without anonymous union (ex: ISO C89)

C compiler configured for C99 in pedantic mode also emits a compiler warning (which may be treated as error) in Include/object.h because of the PyObject.ob_refcnt anonymous union: #105059 (comment)
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@vstinner

vstinner commented Jun 14, 2023

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Abandoned in favor of PR #105767. C++ is unaffected by the issue: #105059 (comment)
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@vstinner vstinner closed this Jun 14, 2023
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vstinner deleted the incref_cpp branch June 14, 2023 02:35
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@vstinner
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