The proper fix for this in the code would likely break ABI compatibility (ie: not possible in python 2.7 or any other stable release).
Clang's UBSAN (undefined behavior sanitizer) has been flagging this one for a long time. In Python 3 a double is used instead of long double since 2012 as I did some digging at the time: https://github.com/python/cpython/commit/e348c8d154cf6342c79d627ebfe89dfe9de23817 -gps On Tue, Jan 30, 2018 at 10:59 AM Florian Weimer <f...@deneb.enyo.de> wrote: > I hope this is the right list for this kind of question. We recently > tried to build Python 2.6 with GCC 8, and ran into this issue: > > <https://bugzilla.redhat.com/show_bug.cgi?id=1540316> > > Also quoting for context: > > | PyInstance_NewRaw contains this code: > | > | inst = PyObject_GC_New(PyInstanceObject, &PyInstance_Type); > | if (inst == NULL) { > | Py_DECREF(dict); > | return NULL; > | } > | inst->in_weakreflist = NULL; > | Py_INCREF(klass); > | inst->in_class = (PyClassObject *)klass; > | inst->in_dict = dict; > | _PyObject_GC_TRACK(inst); > | > | _PyObject_GC_TRACK expands to: > | > | #define _PyObject_GC_TRACK(o) do { \ > | PyGC_Head *g = _Py_AS_GC(o); \ > | if (g->gc.gc_refs != _PyGC_REFS_UNTRACKED) \ > | Py_FatalError("GC object already tracked"); \ > | … > | > | Via: > | > | #define _Py_AS_GC(o) ((PyGC_Head *)(o)-1) > | > | We get to this: > | > | /* GC information is stored BEFORE the object structure. */ > | typedef union _gc_head { > | struct { > | union _gc_head *gc_next; > | union _gc_head *gc_prev; > | Py_ssize_t gc_refs; > | } gc; > | long double dummy; /* force worst-case alignment */ > | } PyGC_Head; > | > | PyGC_Head has 16-byte alignment. The net result is that > | > | _PyObject_GC_TRACK(inst); > | > | promises to the compiler that inst is properly aligned for the > | PyGC_Head type, but it is not: PyObject_GC_New returns a pointer which > | is only 8-byte-aligned. > | > | Objects/obmalloc.c contains this: > | > | /* > | * Alignment of addresses returned to the user. 8-bytes alignment works > | * on most current architectures (with 32-bit or 64-bit address busses). > | * The alignment value is also used for grouping small requests in size > | * classes spaced ALIGNMENT bytes apart. > | * > | * You shouldn't change this unless you know what you are doing. > | */ > | #define ALIGNMENT 8 /* must be 2^N */ > | #define ALIGNMENT_SHIFT 3 > | #define ALIGNMENT_MASK (ALIGNMENT - 1) > | > | So either the allocator alignment needs to be increased, or the > | PyGC_Head alignment needs to be decreased. > > Is this a known issue? As far as I can see, it has not been fixed on > the 2.7 branch. > > (Store merging is a relatively new GCC feature. Among other things, > this means that on x86-64, for sufficiently aligned pointers, vector > instructions are used to update multiple struct fields at once. These > vector instructions can trigger alignment traps, similar to what > happens on some other architectures for scalars.) > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > https://mail.python.org/mailman/options/python-dev/greg%40krypto.org >
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