[Bug c/31870] New: Failure to diagnose taking address of register variable

[Bug c/31870] New: Failure to diagnose taking address of register variable

[neil at gcc dot gnu dot org]

int d (void) { register int a[2]; return a, 1; }


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           Summary: Failure to diagnose taking address of register variable
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: neil at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870

[Bug c/31870] Failure to diagnose taking address of register variable

[bangerth at dealii dot org]

------- Comment #1 from bangerth at dealii dot org  2007-05-09 16:16 -------
Uh, can you elaborate? We get the warning you want if we have
  int d (void) { register int a[2]; return a; }
instead. In your case, i.e. "return a,1", we return 1, but we still
need evaluate the expression "a". I assume that you mean that this
implies that we have to do the array-to-pointer decay operation. However,
for that case, C99's clause 6.3.2.1/3 says that that's possible for 
register storage class arrays but that the result is undefined. The
prohibition in 6.5.3.2/1 against register objects only holds for the
application of the address-of operator "&". Both these clauses are
referenced also from footnote 100 on page 98.

W.


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bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at dealii dot org
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870

[Bug c/31870] Failure to diagnose taking address of register variable

[neil at daikokuya dot co dot uk]

------- Comment #2 from neil at daikokuya dot co dot uk  2007-05-09 23:39 -------
Subject: Re:  Failure to diagnose taking address of register variable

bangerth at dealii dot org wrote:-

> Uh, can you elaborate? We get the warning you want if we have
>   int d (void) { register int a[2]; return a; }
> instead. In your case, i.e. "return a,1", we return 1, but we still
> need evaluate the expression "a". I assume that you mean that this
> implies that we have to do the array-to-pointer decay operation. However,

Yes, it has to be done.

> for that case, C99's clause 6.3.2.1/3 says that that's possible for 
> register storage class arrays but that the result is undefined.

Right.  GCC seems to want to diagnose this undefined behaviour; it
does in all other circumstances.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870

[Bug c/31870] Failure to diagnose taking address of register variable

[bangerth at math dot tamu dot edu]

------- Comment #3 from bangerth at math dot tamu dot edu  2007-05-09 23:46 -------
Subject: Re:  Failure to diagnose taking address of register
 variable


> > for that case, C99's clause 6.3.2.1/3 says that that's possible for
> > register storage class arrays but that the result is undefined.
>
> Right.  GCC seems to want to diagnose this undefined behaviour; it
> does in all other circumstances.

It probably wouldn't hurt if it were diagnosed, but since the result of
the expression is thrown away anyway, there probably isn't much hurm done
in the current state either.

I agree that we might want to warn, but I wouldn't consider it very high
priority.

W.

-------------------------------------------------------------------------
Wolfgang Bangerth                email:            bangerth@math.tamu.edu
                                 www: http://www.math.tamu.edu/~bangerth/


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870

[Bug c/31870] Failure to diagnose taking address of register variable

[bangerth at dealii dot org]

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bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-05-09 23:47:26
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870

[Bug c/31870] Failure to diagnose taking address of register variable

[neil at gcc dot gnu dot org]

------- Comment #4 from neil at gcc dot gnu dot org  2007-05-10 00:00 -------
Agreed it's minor; I think I flagged the PR that way.

I'm not sure but I suspect it indicates that the pointer decay is not
happening.  If so and you were using GCC to do source code analysis, you would
have an incorrect type in the expression tree.  Which may not be a big deal.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870

[Bug c/31870] Failure to diagnose taking address of register variable

[bangerth at dealii dot org]

------- Comment #5 from bangerth at dealii dot org  2007-05-10 03:52 -------
(In reply to comment #4)
> I'm not sure but I suspect it indicates that the pointer decay is not
> happening.

Or that the warning is only emitted if something is actually done with the
result. I don't know, someone else may check this.

W.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31870