[Bug c/53198] New: [4.6 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[Bug c/53198] New: [4.6 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[vapier at gentoo dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

             Bug #: 53198
           Summary: [4.6 Regression] gcc wrongly emits "array subscript is
                    above array bounds" for simple arrays
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vapier@gentoo.org
                CC: toolchain@gentoo.org
            Target: x86_64-linux-gnu


simple code that processes arrays based on NULL terminator rather than
ARRAY_SIZE

$ cat test.c
void *devices[] = { 0, };
int main()
{
        int i;
        for (i = 0; devices[i]; i++)
                ;
        return 0;
}

$ gcc -Warray-bounds -O2 test.c 
test.c: In function 'main':
test.c:5:21: warning: array subscript is above array bounds [-Warray-bounds]

looks like it started failing with 4.6.0 ... older versions are fine.

[Bug c/53198] [4.6 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-05-02 19:06:15 UTC ---
gcc version 4.8.0 20120430 (experimental) [trunk revision 186957] (GCC) is also
fine.

[Bug tree-optimization/53198] [4.6/4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-05-03
          Component|c                           |tree-optimization
            Summary|[4.6 Regression] gcc        |[4.6/4.7 Regression] gcc
                   |wrongly emits "array        |wrongly emits "array
                   |subscript is above array    |subscript is above array
                   |bounds" for simple arrays   |bounds" for simple arrays
     Ever Confirmed|0                           |1

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-05-03 10:46:30 UTC ---
Confirmed.  This is because of loop header copying doing

<bb 2>:
  D.1711_8 = devices[0];
  if (D.1711_8 != 0B)
    goto <bb 3>;
  else
    goto <bb 5>;

<bb 3>:
  i_7 = 0;

<bb 4>:
  # i_9 = PHI <0(3), i_4(4)>
  i_4 = i_9 + 1;
  D.1711_3 = devices[i_4];
  if (D.1711_3 != 0B)
    goto <bb 4>;
  else
    goto <bb 5>;

thus VRP sees that the access in the loop uses indices of [1, INF] which
is out-of bounds.  In 4.8 we recognize that the loop can only run once
(because of the initializer size) and optimize the variable array access
away.

[Bug tree-optimization/53198] [4.6/4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
   Target Milestone|---                         |4.6.4

[Bug tree-optimization/53198] [4.6/4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[Petr.Salinger at seznam dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

--- Comment #3 from Petr.Salinger at seznam dot cz 2012-07-16 07:28:26 UTC ---
Created attachment 27801
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27801
another testcase - from xorg mouse driver

[Bug tree-optimization/53198] [4.6/4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |4.8.0

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-10-23 22:33:15 UTC ---
(In reply to comment #3)
> Created attachment 27801 [details]
> another testcase - from xorg mouse driver

This warns with GCC 4.8 r192379:

pr53198.c: In function ‘CheckProtocol’:
pr53198.c:35:30: warning: array subscript is above array bounds
[-Warray-bounds]
     for (i = 0; internalNames[i]; i++)
                              ^

It doesn't warn with gcc 4.5.1

[Bug tree-optimization/53198] [4.6/4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |4.8.0
      Known to fail|4.7.0, 4.8.0                |4.7.2

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-18 15:16:16 UTC ---
(In reply to comment #4)
> (In reply to comment #3)
> > Created attachment 27801 [details]
> > another testcase - from xorg mouse driver
> 
> This warns with GCC 4.8 r192379:
> 
> pr53198.c: In function ‘CheckProtocol’:
> pr53198.c:35:30: warning: array subscript is above array bounds
> [-Warray-bounds]
>      for (i = 0; internalNames[i]; i++)
>                               ^
> 
> It doesn't warn with gcc 4.5.1

Doesn't warn any longer on trunk.

[Bug tree-optimization/53198] [4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.6.4                       |4.7.4

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-12 15:15:57 UTC ---
GCC 4.6.4 has been released and the branch has been closed.

[Bug tree-optimization/53198] [4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[Emmanuel.Thome at inria dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

Emmanuel Thomé <Emmanuel.Thome at inria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Emmanuel.Thome at inria dot fr

--- Comment #7 from Emmanuel Thomé <Emmanuel.Thome at inria dot fr> ---

Hi,

Seems to me that the bug is still present in 4.7.3 (stock version,
x86_64-unknown-linux-gnu)

int foo(unsigned long t)        /* implicit promise that t != 0 */
{
    unsigned long a[2] = {0, t};
    int i;
    for(i = 1 ; !a[i] ; i++);
    return a[i];
}

int bar(unsigned long t)        /* implicit promise that t != 0 */
{
    unsigned long a[2] = {0, t};
    int i;
    for(i = 0 ; !a[++i] ; );
    return a[i];
}

$ gcc -c -W -Wall -O2 b.c
b.c: In function ‘foo’:
b.c:5:19: warning: array subscript is above array bounds [-Warray-bounds]


Interestingly, the two functions are equivalent as far as I can tell (sorry for
headaches induced by contrived control), and behave differently with 4.7.3.
Both with respect to the fact that only one triggers the warning, and also that
assembly code generated differs slightly.

4.8.1 behaves fine, and optimizes everything away as expected.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX509

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

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

--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
Sorry for the delay.

[Bug tree-optimization/53198] [4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

--- Comment #8 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Nobody pretended it's fixed in 4.7.x.

[Bug tree-optimization/53198] [4.7 Regression] gcc wrongly emits "array subscript is above array bounds" for simple arrays

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53198

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|4.7.4                       |4.8.0
      Known to fail|                            |4.7.4

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed in 4.8.0.