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GeezerVison is showing github.com — gh-105059: Remove anonymous union from PyObject by eduardo-elizondo · Pull Request #105275 · python/cpython

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@eduardo-elizondo

@eduardo-elizondo eduardo-elizondo commented Jun 4, 2023 •
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Currently, the use of an anonymous union causes warnings in C++ extensions. This updates the implementation of PyObject to not rely on anonymous unions and remove these warnings.

Current Issue that goes away after this patch:

#include <Python.h>
int main (void) { return 0; }

-> gcc -I/path/to/include/python3.13 -std=c99 -pedantic repro.c
In file included from repro.c:1:
In file included from /Users/eelizondo/dev/cpython_no_union/build_install/include/python3.13/Python.h:44:
/Users/eelizondo/dev/cpython_no_union/build_install/include/python3.13/object.h:168:5: warning: anonymous unions are a C11 extension [-Wc11-extensions]
    union {

Note: Currently, this is being fixed through the use of a memcpy but a couple of more options will be tried as well. I will also follow-up with benchmark numbers to verify we are not regressing.

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

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@eduardo-elizondo
Remove anonymous union from PyObject
79851a5
@eduardo-elizondo
eduardo-elizondo requested a review from rhettinger as a code owner June 4, 2023 02:35
@bedevere-bot bedevere-bot added the awaiting review label Jun 4, 2023
@bedevere-bot bedevere-bot mentioned this pull request Jun 4, 2023
object.h uses an anonymous union in a struct (older C incompatible) #105059
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sunmy2019
sunmy2019 reviewed Jun 4, 2023
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Comment thread Include/object.h
return;
}
op->ob_refcnt_split[PY_BIG_ENDIAN] = new_refcnt;
memcpy(&op->ob_refcnt, &new_refcnt, sizeof(new_refcnt));

@sunmy2019 sunmy2019 Jun 4, 2023

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Isn't this require the little-endian?

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@vstinner vstinner Jun 4, 2023

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Maybe move the union here, use memcpy() and only use the bits that you need in the union?

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@vstinner vstinner Jun 14, 2023

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The problem of the current implementation (without this change) is the usage of anonymous union in PyObject: it is not compatible with (strict) C99. A named union is fine with C89 and C99. It's more about finding the right syntax to get the most efficient code.

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@rhettinger
rhettinger removed their request for review June 4, 2023 04:59
vstinner
vstinner reviewed Jun 4, 2023
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Comment thread Include/Python.h

#include <assert.h> // assert()
#include <wchar.h> // wchar_t
#include <string.h> // memcpy()

@vstinner vstinner Jun 4, 2023

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It makes me sad to use memcpy() in the limited C API which should attempt to hide as many implementation details as possible. I don't get how the immortal object change interacts with the stable ABI. I would prefer to convert Py_INCREF/Py_DECREF to opaque function calls. It would avoid any C and C++ compatibility issue. At least, it would make the ABI a little bit more "forward compatible".

But that can be done later, since the priority is to unblock the next Python 3.12 beta release.

By the way, the nogil fork has also a completely different implementation which is also ABI incompatible unless these functions are implemented as opaque function calls.

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@gpshead
gpshead marked this pull request as draft June 4, 2023 19:21
@bedevere-bot bedevere-bot removed the awaiting review label Jun 4, 2023
@gpshead gpshead added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Jun 4, 2023
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bedevere-bot commented Jun 4, 2023

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🤖 New build scheduled with the buildbot fleet by @gpshead for commit 79851a5 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.
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@bedevere-bot bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Jun 4, 2023
@vstinner vstinner mentioned this pull request Jun 6, 2023
PEP 683 (Immortal Objects): Implement Py_INCREF() as function call in limited C API 3.12 #105387
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@vstinner vstinner mentioned this pull request Jun 14, 2023
[DRAFT] gh-105059: In C++, Py_INCREF() uses a function call #105764
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@eduardo-elizondo

eduardo-elizondo commented Jun 14, 2023

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I'll be closing since there's better discussions on this in other places
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@eduardo-elizondo eduardo-elizondo closed this Jun 14, 2023
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@vstinner vstinner vstinner left review comments
@sunmy2019 sunmy2019 sunmy2019 left review comments
@rhettinger rhettinger Awaiting requested review from rhettinger rhettinger will be requested when the pull request is marked ready for review rhettinger is a code owner
@ZeroIntensity ZeroIntensity Awaiting requested review from ZeroIntensity ZeroIntensity will be requested when the pull request is marked ready for review ZeroIntensity is a code owner

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@eduardo-elizondo @bedevere-bot @vstinner @sunmy2019 @gpshead
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