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From: "soltys at ziu dot info" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/59485] New: may_alias attribute ignored in internal references while defining aggregate types
Date: Thu, 12 Dec 2013 14:54:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59485

            Bug ID: 59485
           Summary: may_alias attribute ignored in internal references
                    while defining aggregate types
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: soltys at ziu dot info

Note, I first asked about it on gcc-help, though got no responsens. This does
look like a bug - though obviously I'm not sure if it really should be
considered as such.

Anyway, consider following piece of code:

struct __attribute__((may_alias)) tag_si {
        int y;
};

struct __attribute__((may_alias)) tag_s {
        int x;
        struct tag_si *pi;
        struct tag_s *p;
};

int main(void)
{
        struct tag_s test = {0};
        struct tag_si **ppi;
        struct tag_s  **pp;

        ppi = &test.pi;  /* ok */
        pp = &test.p;    /* will generate warning */

        return 0;
}

The possible problem with the above example, is that pp assignment will cause
compiler to emit:

test.c: In function 'main':
test.c:18:12: warning: assignment from incompatible pointer type [enabled by
default]
         pp = &test.p;
             ^

At the same time analogous ppi assignment gives no issues. The only difference
between "tag_si" and "tag_s" is that the former is defined first outside and
then referenced in subsequent definition - while the latter is referenced
during its definition - and then it "forgets" about may_alias attribute (thus
pp = &test.p gives warrning).

I tried to workaround it with preceeding struct forward declaration, e.g.

struct __attribute__((may_alias)) tag_s; 
struct __attribute__((may_alias)) tag_s { ......

But this approach has no effect either.

The only workaround for that I found is extra typedef with may_alias and type
cast, e.g.:

typedef struct tag_s *tag_t_pa __attribute__((may_alias));
tag_t_pa *pp;
pp = (tag_t_pa *)&test.p;