[Bug c/115027] New: Missing warning: unused struct's with self-referential initialisers

[Bug c/115027] New: Missing warning: unused struct's with self-referential initialisers

[dave at treblig dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115027

            Bug ID: 115027
           Summary: Missing warning: unused struct's with self-referential
                    initialisers
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dave at treblig dot org
  Target Milestone: ---

It would be nice to generate a warning in the following case, which I tripped
over in the Linux kernel:

#include <stdio.h>

struct foo {
  struct foo *a;
};

static struct foo f = { &f };
int main()
{
  printf("Hello\n");
}

Here 'f' is unused outside of it's initialiser, pointing to itself.
So technically used, but practically not.

In the Linux kernel, this corresponds to it's LIST_HEAD :
(from include/linux/list.h)

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
        struct list_head name = LIST_HEAD_INIT(name)

and I've just gone through and posted patches to remove a handful of
LIST_HEADs, some of which had been unused for many years.
It would be nice if the compiler told people instead.

[Bug c/115027] Missing warning: unused struct's with self-referential initialisers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115027

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-05-10
                 CC|                            |hubicka at gcc dot gnu.org
             Blocks|                            |89180
           Keywords|                            |diagnostic

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this is more an IPA issue but of course in theory the C fronted could
know it's parsing the initializer of 'f' when setting TREE_USED of 'f'.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89180
[Bug 89180] [meta-bug] bogus/missing -Wunused warnings