Re: RFC: [PATCH] Add warn_if_not_aligned attribute

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

On Tue, Jun 6, 2017 at 9:10 AM, Martin Sebor <msebor@gmail.com> wrote:
> On 06/06/2017 10:07 AM, Martin Sebor wrote:
>>
>> On 06/05/2017 11:45 AM, H.J. Lu wrote:
>>>
>>> On Mon, Jun 5, 2017 at 8:11 AM, Joseph Myers <joseph@codesourcery.com>
>>> wrote:
>>>>
>>>> The new attribute needs documentation.  Should the test be in
>>>> c-c++-common
>>>
>>>
>>> This feature does support C++.  But C++ compiler issues a slightly
>>> different warning at a different location.
>>>
>>>> or does this feature not support C++?
>>>>
>>>
>>> Here is the updated patch with documentation and a C++ test.  This
>>> patch caused a few testsuite failures:
>>>
>>> FAIL: gcc.dg/compat/struct-align-1 c_compat_x_tst.o compile
>>>
>>>
>>> /export/gnu/import/git/sources/gcc/gcc/testsuite/gcc.dg/compat//struct-align-1.h:169:1:
>>>
>>> warning: alignment 1 of 'struct B2_m_inner_p_outer' is less than 16
>>>
>>> FAIL: g++.dg/torture/pr80334.C   -O0  (test for excess errors)
>>>
>>>
>>> /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/torture/pr80334.C:4:8:
>>>
>>> warning: alignment 1 of 'B' is less than 16
>>>
>>
>> Users often want the ability to control a warning, even when it
>> certainly indicates a bug.  I would suggest to add an option to
>> make it possible for this warning as well.
>>
>> Btw., a bug related to some of those this warning is meant to
>> detect is assigning the address of an underaligned object to
>> a pointer of a natively aligned type.  Clang has an option
>> to detect this problem: -Waddress-of-packed-member.  It might
>> make a nice follow-on enhancement to add support for the same
>> thing.  I mention this because I think it would make sense to
>> consider this when choosing the name of the GCC option (i.e.,
>> rather than having two distinct but closely related warnings,
>> have one that detects both of these alignment type of bugs.
>
>
> A bug that has some additional context on this is pr 51628.
> A possible name for the new option suggested there is -Wpacked.
>
> Martin

Isn't -Waddress-of-packed-member a subset of or the same as
-Wpacked?

-- 
H.J.