[Bug tree-optimization/31602] New: Overflow warning causes GDB -Werror build failure

[Bug tree-optimization/31602] New: Overflow warning causes GDB -Werror build failure

[drow at gcc dot gnu dot org]

GCC HEAD now warns about this testcase for mips-linux, reduced from
gdb/value.c.  Compile with -O2 -Wall:

extern int foo();
int show_values (void)
{
  int i;
  static int num;

  if (num <= 0)
    num = 1;

  for (i = num; i < num + 10; i++)
    foo();

  return i;
}

overflow.c:10: warning: assuming signed overflow does not occur when assuming
that (X + c) >= X is always true

I think that the problem arises when we assume that the loop is executed at
least once; the initial condition is num < num + 10.  However the warning does
not arise if num is an argument to the function instead of a static variable.

Unfortunately the warning appears to be correct.  I say unfortunately because
there's no apparent way to avoid the warning - which sounds more like -Wextra
to -Wall.  Does this this example contradict the "easy to avoid" statement in
the -Wstrict-overflow=1 documentation?


-- 
           Summary: Overflow warning causes GDB -Werror build failure
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: drow at gcc dot gnu dot org
GCC target triplet: mips-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[drow at false dot org]

------- Comment #1 from drow at gcc dot gnu dot org  2007-04-17 13:26 -------
Subject: Re:   New: Overflow warning causes
        GDB -Werror build failure

On Tue, Apr 17, 2007 at 12:21:36PM -0000, drow at gcc dot gnu dot org wrote:
> GCC HEAD now warns about this testcase for mips-linux, reduced from
> gdb/value.c.

The warning also occurs once in top.c:

for (offset = num; offset < num + Hist_print && offset < hist_len; offset++)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[ian at airs dot com]

------- Comment #2 from ian at airs dot com  2007-04-18 15:14 -------
Yes, the warning is happening because gcc relies on undefined signed overflow
when assuming that it will execute the loop at least once.  Of course when
using -fwrapv the loop should not be executed at all when num > INT_MAX - 10.

I'll try to figure out what to do about this.


-- 

ian at airs dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|ian at gcc dot gnu dot org  |ian at airs dot com
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-04-18 15:14:27
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[ian at airs dot com]

------- Comment #3 from ian at airs dot com  2007-04-20 16:17 -------
Created an attachment (id=13394)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13394&action=view)
Proposed patch

This patch fixes the test case in the PR.  I am testing it.  It would be
interesting to hear whether it also fixes the actual code in gdb.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[drow at gcc dot gnu dot org]

------- Comment #4 from drow at gcc dot gnu dot org  2007-04-20 20:04 -------
Subject: Re:  Overflow warning causes GDB
        -Werror build failure

On Fri, Apr 20, 2007 at 03:17:19PM -0000, ian at airs dot com wrote:
> This patch fixes the test case in the PR.  I am testing it.  It would be
> interesting to hear whether it also fixes the actual code in gdb.

With this patch I can successfully build GDB for mips-linux - thanks!
(barring PR31605).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[ian at gcc dot gnu dot org]

------- Comment #5 from ian at gcc dot gnu dot org  2007-04-24 21:44 -------
Subject: Bug 31602

Author: ian
Date: Tue Apr 24 21:44:45 2007
New Revision: 124120

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=124120
Log:
./:
        PR tree-optimization/31602
        * tree-ssa-loop-ch.c (copy_loop_headers): Set TREE_NO_WARNING for
        conditionals in the copied loop header.
        * tree-cfg.c (fold_cond_expr_cond): Don't issue undefined overflow
        warnings if TREE_NO_WARNING is set.
        * doc/invoke.texi (Warning Options): Clarify that
        -Wstrict-overflow does not warn about loops.
testsuite/:
        PR tree-optimization/31602
        * gcc.dg/Wstrict-overflow-11.c: We no longer issue a warning.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/doc/invoke.texi
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/gcc.dg/Wstrict-overflow-11.c
    trunk/gcc/tree-cfg.c
    trunk/gcc/tree-ssa-loop-ch.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[ian at gcc dot gnu dot org]

------- Comment #6 from ian at gcc dot gnu dot org  2007-04-24 22:54 -------
Subject: Bug 31602

Author: ian
Date: Tue Apr 24 22:54:22 2007
New Revision: 124127

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=124127
Log:
        PR tree-optimization/31602
        * tree-ssa-loop-ch.c (copy_loop_headers): Set TREE_NO_WARNING for
        conditionals in the copied loop header.
        * tree-cfg.c (fold_cond_expr_cond): Don't issue undefined overflow
        warnings if TREE_NO_WARNING is set.
        * doc/invoke.texi (Warning Options): Clarify that
        -Wstrict-overflow does not warn about loops.

Modified:
    branches/gcc-4_2-branch/gcc/ChangeLog
    branches/gcc-4_2-branch/gcc/doc/invoke.texi
    branches/gcc-4_2-branch/gcc/tree-cfg.c
    branches/gcc-4_2-branch/gcc/tree-ssa-loop-ch.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602

[Bug tree-optimization/31602] Overflow warning causes GDB -Werror build failure

[ian at airs dot com]

------- Comment #7 from ian at airs dot com  2007-04-24 23:27 -------
Fixed on mainline and 4.2 branch.


-- 

ian at airs dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31602