[Bug c++/109824] New: aligned attribute lost on first usage

[Bug c++/109824] New: aligned attribute lost on first usage

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109824

            Bug ID: 109824
           Summary: aligned attribute lost on first usage
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stsp at users dot sourceforge.net
  Target Milestone: ---

Created attachment 55063
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55063&action=edit
test case

struct a {
    short aa;
};
typedef struct a less_aligned_a __attribute__ ((aligned (1)));

static inline void foo(void)
{
#ifndef BUG
    struct a aa __attribute__((unused));
#endif
}

class t1 {
    less_aligned_a i;
public:
    less_aligned_a &get_ref() {
        return i;
    }
} __attribute__((packed));

t1 ap;
less_aligned_a* a = &ap.get_ref();

-----

$ g++ -Wall -c a.cpp 
a.cpp: In instantiation of ‘less_aligned_a& t1<T>::get_ref() [with T = int;
less_aligned_a = a]’:
a.cpp:23:32:   required from here
a.cpp:18:16: error: cannot bind packed field ‘((t1<int>*)this)->t1<int>::i’ to
‘less_aligned_a&’ {aka ‘a&’}
   18 |         return i;



But comment out the line 9 under "ifndef BUG"
and it compiles without an error.

[Bug c++/109824] aligned attribute lost on first usage

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109824

--- Comment #1 from Stas Sergeev <stsp at users dot sourceforge.net> ---
Sorry, copied the output from wrong place.
The real error msg looks like this:

$ g++ -Wall -c a.cpp 
a.cpp: In member function ‘less_aligned_a& t1::get_ref()’:
a.cpp:17:16: error: cannot bind packed field ‘((t1*)this)->t1::i’ to
‘less_aligned_a&’ {aka ‘a&’}
   17 |         return i;

[Bug c++/109824] aligned attribute lost on first usage

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109824

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=77388
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is by design.

*** This bug has been marked as a duplicate of bug 109654 ***

[Bug c++/109824] aligned attribute lost on first usage

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109824

Stas Sergeev <stsp at users dot sourceforge.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |---

--- Comment #3 from Stas Sergeev <stsp at users dot sourceforge.net> ---
Andrew, why not to read the bug description
at least, or try what was said in it?
It says:

But comment out the line 9 under "ifndef BUG"
and it compiles without an error.

This has nothing to do with the bug you
referred as a duplicate.
Please indicate that you tried that, only
then close.

[Bug c++/109824] aligned attribute lost when using it on a typedef of a struct if there was not an usage of the struct beforehand

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109824

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|aligned attribute lost on   |aligned attribute lost when
                   |first usage                 |using it on a typedef of a
                   |                            |struct if there was not an
                   |                            |usage of the struct
                   |                            |beforehand
           Keywords|                            |accepts-invalid, wrong-code

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Right, the error message is correct when using it before hand; the alignment
attribute is there.

In the case of the used struct not used before hand, the alignment is lost.

This is the opposite of what you wrote even. You thought it was being lost if
there was an use before hand, rather the alignment is correct then.

[Bug c++/109824] aligned attribute lost when using it on a typedef of a struct if there was not an usage of the struct beforehand

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109824

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-05-12

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a better testcase:
```
struct a {
    int aa;
};
typedef struct a less_aligned_a __attribute__ ((aligned (1)));
static_assert(alignof(less_aligned_a) == 1);
struct a aa __attribute__((unused));
static_assert(alignof(less_aligned_a) == 1);
```
Which shows the issue.
The first static_assert works but the second one does not.