Re: [PATCH 1/2] Don't warn function alignment if warn_if_not_aligned_p is true

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Aug 25, 2017 at 12:24 PM, Martin Sebor <msebor@gmail.com> wrote:
> On 08/25/2017 11:31 AM, H.J. Lu wrote:
>>
>> On Fri, Aug 25, 2017 at 10:15 AM, Martin Sebor <msebor@gmail.com> wrote:
>>>
>>> On 08/21/2017 06:21 AM, H.J. Lu wrote:
>>>>
>>>>
>>>> When warn_if_not_aligned_p is true, a warning will be issued on function
>>>> declaration later.  There is no need to warn function alignment when
>>>> warn_if_not_aligned_p is true.
>>>>
>>>> OK for trunk?
>>>>
>>>> H.J.
>>>> --
>>>>         * c-attribs.c (common_handle_aligned_attribute): Don't warn
>>>>         function alignment if warn_if_not_aligned_p is true.
>>>> ---
>>>>  gcc/c-family/c-attribs.c | 5 ++++-
>>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
>>>> index 5f79468407f..78969532543 100644
>>>> --- a/gcc/c-family/c-attribs.c
>>>> +++ b/gcc/c-family/c-attribs.c
>>>> @@ -1754,9 +1754,12 @@ common_handle_aligned_attribute (tree *node, tree
>>>> args, int flags,
>>>>        This formally comes from the c++11 specification but we are
>>>>        doing it for the GNU attribute syntax as well.  */
>>>>      *no_add_attrs = true;
>>>> -  else if (TREE_CODE (decl) == FUNCTION_DECL
>>>> +  else if (!warn_if_not_aligned_p
>>>> +          && TREE_CODE (decl) == FUNCTION_DECL
>>>>            && DECL_ALIGN (decl) > (1U << i) * BITS_PER_UNIT)
>>>>      {
>>>> +      /* Don't warn function alignment here if warn_if_not_aligned_p is
>>>> +        true.  It will be warned later.  */
>>>>        if (DECL_USER_ALIGN (decl))
>>>>         error ("alignment for %q+D was previously specified as %d "
>>>>                "and may not be decreased", decl,
>>>
>>>
>>>
>>> Your comment refers to warning but the code here uses error().
>>> That raises two questions for me: a) will the later diagnostic
>>> really be a warning or an error, and if a warning, under what
>>> option will it be issued? and b) why is an error appropriate
>>> here when a warning is appropriate elsewhere (most other
>>> attribute conflicts are at present diagnosed with -Wattributes).
>>
>>
>> It can be changed to warning.
>
>
> Okay, that's goo to hear (it validates what I did in the patch
> I referenced).
>
> As for your patch, I don't quite see ho the block can ever be
> entered.  In fact, I had to make changes to the function in
> my patch below to let it detect and diagnose the condition
> (attempting to reduce the alignment of a function).  The
> details are in bug 81566.  Did I miss something?

Try this:

__attribute__((warn_if_not_aligned(8)))
void
foo2 (void)
{
}


-- 
H.J.