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GH-95245: Move weakreflist into the pre-header. #95657

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GH-95245: Move weakreflist into the pre-header. #95657

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Move weakreflist into the pre-header.
markshannon Jul 26, 2022
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markshannon Aug 4, 2022
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2 changes: 1 addition & 1 deletion Include/internal/pycore_object.h
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Original file line number Diff line number Diff line change
Expand Up @@ -263,7 +263,7 @@ extern int _Py_CheckSlotResult(

// Test if a type supports weak references
static inline int _PyType_SUPPORTS_WEAKREFS(PyTypeObject *type) {
return (type->tp_weaklistoffset > 0);
return (type->tp_weaklistoffset != 0);
}

extern PyObject* _PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems);
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-08-04-16-04-18.gh-issue-95245.N4gOUV.rst
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@@ -0,0 +1,3 @@
Reduces the size of a "simple" Python object from 8 to 6 words by moving the
weakreflist pointer into the pre-header directly before the object's
dict/values pointer.
34 changes: 23 additions & 11 deletions Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2483,8 +2483,9 @@ subtype_getweakref(PyObject *obj, void *context)
return NULL;
}
_PyObject_ASSERT((PyObject *)type,
type->tp_weaklistoffset > 0);
type->tp_weaklistoffset != 0);
_PyObject_ASSERT((PyObject *)type,
(type->tp_weaklistoffset == -4 * ((int)sizeof(PyObject *))) ||
((type->tp_weaklistoffset + sizeof(PyObject *))
<= (size_t)(type->tp_basicsize)));
weaklistptr = (PyObject **)((char *)obj + type->tp_weaklistoffset);
Expand Down Expand Up @@ -3078,19 +3079,28 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type)
}
}

if (ctx->add_dict && ctx->base->tp_itemsize) {
type->tp_dictoffset = -(long)sizeof(PyObject *);
slotoffset += sizeof(PyObject *);
if (ctx->add_dict) {
if (ctx->base->tp_itemsize) {
type->tp_dictoffset = -(long)sizeof(PyObject *);
slotoffset += sizeof(PyObject *);
}
else {
assert((type->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
type->tp_flags |= Py_TPFLAGS_MANAGED_DICT;
}
}

if (ctx->add_weak) {
assert(!ctx->base->tp_itemsize);
type->tp_weaklistoffset = slotoffset;
slotoffset += sizeof(PyObject *);
if (type->tp_flags & Py_TPFLAGS_MANAGED_DICT) {
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Could Py_TPFLAGS_MANAGED_DICT be set even if wasn't set on line 3089? In other words, could the flag have been set before type_new_descriptors() was called?

If so, that implies that Py_TPFLAGS_MANAGED_DICT also implies a managed weaklist, which I'm not sure is correct.

Otherwise the assert on line 3088 should probably move up to before line 3082.

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Further, if it's always either both-are-managed or neither then it would be helpful if the flag were more clear (e.g. Py_TPFLAGS_MANAGED_DICT_AND_WEAKLIST).

If either can be managed separately then there should probably be a separate Py_TPFLAGS_MANAGED_WEAKLIST flag.

That might be necessary anyway since Py_TPFLAGS_MANAGED_DICT is already part of 3.11.

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We want both to be managed, or neither, but our hand might be forced in the case of multiple inheritance.
One base might have a managed dict, and the other an explicit weakreflist.
I expect this case to be rare.

Currently Py_TPFLAGS_MANAGED_DICT is internal, it is set by the VM, but shouldn't be set by third-party code.

When we make it public, as we should do for 3.12, then it would make sense to add Py_TPFLAGS_MANAGED_WEAKLIST.

I'd like some sort of decision on #95589 first, though.

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Thinking about this a bit more, we can ensure both are explicit or both managed.
If one is explicit, the we make both explicit.

Moving from an explicitly declare offset to a managed one is unsafe, so we can't do that.
But we can do it the other way around.

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Here it only matters in the context of type_new_descriptors(). If we can guarantee the both-or-neither aspect in this "limited" context then that is sufficient for this PR (assuming we leave a comment to that effect).

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Regardless, my original question stands: could Py_TPFLAGS_MANAGED_DICT already be set when type_new_descriptors() is called? If so, the branch on 3095 is now problematic, no?

type->tp_weaklistoffset = -4 * sizeof(PyObject *);
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Is the pre-header layout documented, either in a comment, the docs, or an adjacent .txt file? Either way, I don't see the weaklist's place in the pre-header added to such documentation in this PR. That information would be super helpful, since I can only guess what -4 * sizeof(PyObject *) means. A short comment here pointing to that documentation would be nice too.

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Documenting the object layout is on the to-do list.
Early draft here: https://github.com/faster-cpython/cpython/blob/document-object-layout/Objects/object_layout.md

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What? No SVG?

}
else {
type->tp_weaklistoffset = slotoffset;
slotoffset += sizeof(PyObject *);
}
}
if (ctx->add_dict && ctx->base->tp_itemsize == 0) {
assert((type->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
type->tp_flags |= Py_TPFLAGS_MANAGED_DICT;
if (type->tp_flags & Py_TPFLAGS_MANAGED_DICT) {
Comment on lines -3092 to +3103
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Likewise about Py_TPFLAGS_MANAGED_DICT implying something it didn't before (but only if the flag could possibly be set before type_new_descriptors() gets called).

type->tp_dictoffset = -slotoffset - sizeof(PyObject *)*3;
}

Expand Down Expand Up @@ -5070,11 +5080,13 @@ compatible_for_assignment(PyTypeObject* oldto, PyTypeObject* newto, const char*
!same_slots_added(newbase, oldbase))) {
goto differs;
}
/* The above does not check for managed __dicts__ */
/* The above does not check for managed __dict__ or __weakref__ */
if ((oldto->tp_flags & Py_TPFLAGS_MANAGED_DICT) ==
((newto->tp_flags & Py_TPFLAGS_MANAGED_DICT)))
{
return 1;
if (oldto->tp_weaklistoffset == newto->tp_weaklistoffset) {
return 1;
}
}
differs:
PyErr_Format(PyExc_TypeError,
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