[Bug c/21664] array-of-empty-structure extension not properly defined

[Bug c/21664] array-of-empty-structure extension not properly defined

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2005-10-25 19:47 -------
Confirmed.


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |minor
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-10-25 19:47:54
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21664

[Bug c/21664] array-of-empty-structure extension not properly defined

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21664

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Testcase in #c0 is now rejected.

[Bug c/21664] array-of-empty-structure extension not properly defined

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21664

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
...so marking as "fixed".

[Bug c/21664] New: array-of-empty-structure extension not properly defined

[rguenth at gcc dot gnu dot org]

For

struct {} a[4]; int main() { return &a[2] - &a[1]; }

the C frontend emits

  return 0 /[ex] 0;

which will, at -O0 fault at runtime, at -O not due to RTL CSE.

Is this even well-defined?  Should the C-frontend rather emit

 return 0;

?  Of course this contradicts the C standard, but that does not
allow empty structures anyway.

-- 
           Summary: array-of-empty-structure extension not properly defined
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21664

[Bug c/21664] array-of-empty-structure extension not properly defined

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2005-05-19 19:45 -------
Subject: Re:  New: array-of-empty-structure extension not properly defined

"rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| For
| 
| struct {} a[4]; int main() { return &a[2] - &a[1]; }
| 
| the C frontend emits
| 
|   return 0 /[ex] 0;
| 
| which will, at -O0 fault at runtime, at -O not due to RTL CSE.
| 
| Is this even well-defined?

No, as you noted below.

|  Should the C-frontend rather emit
| 
|  return 0;
| 
| ?  Of course this contradicts the C standard, but that does not
| allow empty structures anyway.

If you go back to the GCC mailing list (circa 2001, 2002), you'll see
heavy and hot debates about what the answer should be.
For that matter, the C++ standards allow empty structures but do not
allow "return 0".  I suppose the real question is what use do you have
for an array of empty structures.

-- Gaby


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21664

[Bug c/21664] array-of-empty-structure extension not properly defined

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-05-19 20:20 -------
None.  But I have to deal with the possibilities of these during folding
of &a[2] == &a[1].  At least until the middle-end makes a promise that
they cannot exist (which they do), or the C frontend rejects this.  It's
really a question for advice on details of our own extension to the C language.

Richard.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21664