https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105078
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This changed with r12-2270-gdddb6ffdc5c25264dd75ad82dad8e48a0718d2d9
Before that commit it has been the forwprop2 pass that changed
_6 = &MEM[(struct QTypedArrayData *)header_8].D.2415;
_7 = _6 + 16;
to
_7 = &MEM <struct QArrayData> [(void *)header_8 + 16B];
and that was done before objsz2 pass, but now objsz is done before fwprop2
(intentionally).
The D.2415 there is the artificial base object.
I'd say the C++ derived class case is similar to:
struct S { int size; __PTRDIFF_TYPE__ offset; };
struct T { struct S base; };
__SIZE_TYPE__ foo (struct T *p) { return __builtin_object_size ((char *)
&p->base + sizeof (struct S), 1); }
__SIZE_TYPE__ bar (struct T *p) { return __builtin_object_size ((char *) p +
sizeof (struct S), 1); }
We've been since forever returning 0 from foo and all ones from bar, because in
the first case we are crossing subobject boundary.
I guess what would work is add char payload[]; to QArrayData and base the
pointer on address of that (+ offset - offsetof(QArrayData, payload)).
