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bpo-40052: Fix alignment issue in PyVectorcall_Function() by cryptomilk · Pull Request #19133 · python/cpython · GitHub

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bpo-40052: Fix alignment issue in PyVectorcall_Function() #19133

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bpo-40052: Fix alignment issue in PyVectorcall_Function() #19133

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9 changes: 6 additions & 3 deletions Include/cpython/abstract.h
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Original file line number Diff line number Diff line change
Expand Up @@ -67,7 +67,10 @@ PyVectorcall_Function(PyObject *callable)
{
PyTypeObject *tp;
Py_ssize_t offset;
vectorcallfunc *ptr;
union {
char *data;
vectorcallfunc *ptr;
} vc;

assert(callable != NULL);
tp = Py_TYPE(callable);
Expand All @@ -77,8 +80,8 @@ PyVectorcall_Function(PyObject *callable)
assert(PyCallable_Check(callable));
offset = tp->tp_vectorcall_offset;
assert(offset > 0);
ptr = (vectorcallfunc *)(((char *)callable) + offset);
return *ptr;
vc.data = (char *)callable + offset;
return *vc.ptr;

pitrou Apr 18, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I don't think this is better. The correct, idiomatic solution is something like:

    vectorcallfunc *ptr;
    memcpy(ptr, (char *) callable + offset, sizeof(ptr));
    return *ptr;

pitrou Apr 18, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

(also, note that any decent compiler will optimize the memcpy call away)
}

/* Call the callable object 'callable' with the "vectorcall" calling
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/C API/2020-03-24-09-27-10.bpo-40052.27P2KG.rst
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@@ -0,0 +1 @@
Fix an alignment build warning/error in function ``PyVectorcall_Function()`` publicly exposed by ``abstract.h``.
7 changes: 6 additions & 1 deletion Objects/call.c
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Original file line number Diff line number Diff line change
Expand Up @@ -205,6 +205,10 @@ PyObject *
PyVectorcall_Call(PyObject *callable, PyObject *tuple, PyObject *kwargs)
{
PyThreadState *tstate = _PyThreadState_GET();
union {
char *data;
vectorcallfunc *ptr;
} vc;

/* get vectorcallfunc as in PyVectorcall_Function, but without
* the Py_TPFLAGS_HAVE_VECTORCALL check */
Expand All @@ -215,7 +219,8 @@ PyVectorcall_Call(PyObject *callable, PyObject *tuple, PyObject *kwargs)
Py_TYPE(callable)->tp_name);
return NULL;
}
vectorcallfunc func = *(vectorcallfunc *)(((char *)callable) + offset);
vc.data = (char *)callable + offset;
vectorcallfunc func = *vc.ptr;

pitrou Apr 18, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Same comment here.
if (func == NULL) {
_PyErr_Format(tstate, PyExc_TypeError,
"'%.200s' object does not support vectorcall",
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