Re: C++ PATCH to fix missing warning (PR c++/70194)

[Patrick Palka <patrick@parcs.ath.cx>]

On Thu, Mar 17, 2016 at 12:27 PM, Jeff Law <law@redhat.com> wrote:
> On 03/16/2016 06:43 PM, Martin Sebor wrote:
>>>
>>> @@ -3974,6 +3974,38 @@ build_vec_cmp (tree_code code, tree type,
>>>     return build3 (VEC_COND_EXPR, type, cmp, minus_one_vec, zero_vec);
>>>   }
>>>
>>> +/* Possibly warn about an address never being NULL.  */
>>> +
>>> +static void
>>> +warn_for_null_address (location_t location, tree op, tsubst_flags_t
>>> complain)
>>> +{
>>
>> ...
>>>
>>> +  if (TREE_CODE (cop) == ADDR_EXPR
>>> +      && decl_with_nonnull_addr_p (TREE_OPERAND (cop, 0))
>>> +      && !TREE_NO_WARNING (cop))
>>> +    warning_at (location, OPT_Waddress, "the address of %qD will never "
>>> +        "be NULL", TREE_OPERAND (cop, 0));
>>> +
>>> +  if (CONVERT_EXPR_P (op)
>>> +      && TREE_CODE (TREE_TYPE (TREE_OPERAND (op, 0))) == REFERENCE_TYPE)
>>> +    {
>>> +      tree inner_op = op;
>>> +      STRIP_NOPS (inner_op);
>>> +
>>> +      if (DECL_P (inner_op))
>>> +    warning_at (location, OPT_Waddress,
>>> +            "the compiler can assume that the address of "
>>> +            "%qD will never be NULL", inner_op);
>>
>>
>> Since I noted the subtle differences between the phrasing of
>> the various -Waddress warnings in the bug, I have to ask: what is
>> the significance of the difference between the two warnings here?
>>
>> Would it not be appropriate to issue the first warning in the latter
>> case?  Or perhaps even use the same text as is already used elsewhere:
>> "the address of %qD will always evaluate as ‘true’" (since it may not
>> be the macro NULL that's mentioned in the expression).
>
> They were added at different times AFAICT.  The former is fairly old
> (Douglas Gregor, 2008) at this point.  The latter was added by Patrick Palka
> for 65168 about a year ago.
>
> You could directly ask Patrick about motivations for a different message.

There is no plausible way for the address of a non-reference variable
to be NULL even in code with UB (aside from __attribute__ ((weak)) in
which case the warning is suppressed).  But the address of a reference
can easily seem to be NULL if one performs UB and assigns to it *(int
*)NULL or something like that.  I think that was my motivation, anyway
:)