[Bug tree-optimization/85244] [6/7/8 Regression] Bad optimisation with flexible array member (may be related to -ftree-dominator-opts)

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85244

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
That said, it is get_ref_base_and_extent that for the variable index returns
offset 96 bits and size == maxsize == 32, identical to the [0] case.

Perhaps we shouldn't trust:
638           /* If maxsize is unknown adjust it according to the size of the
639              base decl.  */
640           else if (!known_size_p (maxsize)
641                    && DECL_SIZE (exp)
642                    && poly_int_tree_p (DECL_SIZE (exp)))
643             maxsize = wi::to_poly_offset (DECL_SIZE (exp)) - bit_offset;
if exp is DECL_EXTERNAL and the struct/union ends with flexible array member
(or flexible array member-like array)?  Perhaps we'd need to remember from the
case where we set seen_variable_array_ref to true if it was
array_at_end_of_struct_p and if yes and DECL_EXTERNAL, ignore this?

Richard?