[Bug tree-optimization/95663] static_cast checks for null even when the pointer is dereferenced

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95663

--- Comment #22 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 13 Dec 2021, hubicka at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95663
> 
> --- Comment #20 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
> I really think with -fdelete-null-pointer-checks we should optimize away the
> pointer adjustment relying on the fact that program will segfault.
> 
> I was wondering, -fdelete-null-pointer-checks currently requires pointer to be
> precisely 0.  We are already iffy here since the access is at non-0 offset, but
> since infer_nonnull_range_by_dereference uses check_loadstore:
> 
> static bool
> check_loadstore (gimple *, tree op, tree, void *data)
> {
>   if (TREE_CODE (op) == MEM_REF || TREE_CODE (op) == TARGET_MEM_REF)
>     {
>       /* Some address spaces may legitimately dereference zero.  */
>       addr_space_t as = TYPE_ADDR_SPACE (TREE_TYPE (op));
>       if (targetm.addr_space.zero_address_valid (as))
>         return false;
> 
>       return operand_equal_p (TREE_OPERAND (op, 0), (tree)data, 0);
>     }
>   return false;
> }
> 
> which completely ignores MEM_REF_OFFSET we actually turn into trap accesses
> that are arbitrarily far from NULL.  We also ignore handled components so we

I think MEM_REF[(void *)0 + 4] is non-canonical (IIRC we "simplify" that 
to MEM_REF[(void *)4])