[Bug c/49705] New: -Wstrict-overflow should not diagnose unevaluated expressions

[Bug c/49705] New: -Wstrict-overflow should not diagnose unevaluated expressions

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

           Summary: -Wstrict-overflow should not diagnose unevaluated
                    expressions
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org
              Host: x86_64-unknown-linux-gnu
            Target: x86_64-unknown-linux-gnu
             Build: x86_64-unknown-linux-gnu


I ran into this problem when compiling a test version of Emacs.
Here's a stripped-down version of the problem:

   int
   check_image_width (int width)
   {
     return (1 || width <= width + 2);
   }

When compiled with "gcc -c -Wstrict-overflow -O2 t.c" the diagnostic is:

   t.c:4:3: warning: assuming signed overflow does not occur when assuming that
(X + c) >= X is always true [-Wstrict-overflow]

This diagnostic is wrong, as the expression in question (width <=
width + 2) is unreachable.  In the original source code the
expression is unreachable because the code is carefully checking for
integer overflow.

GCC should not diagnose potential integer overflow in unreachable
expressions, as that is a common idiom in code that is working correctly.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic
   Last reconfirmed|                            |2011.07.12 08:57:17
          Component|c                           |middle-end
                 CC|                            |iant at google dot com
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-12 08:57:17 UTC ---
Confirmed.  And very difficult to fix.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

Ian Lance Taylor <ian at airs dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ian at airs dot com

--- Comment #2 from Ian Lance Taylor <ian at airs dot com> 2011-07-12 14:25:13 UTC ---
Very difficult to fix in the general case but easy to fix in this particular
case.  When the warning is issued, c_inhibit_evaluation_warnings is 1.  All we
need to do is, when setting c_inhibit_evaluation_warnings non-zero, also call
fold_defer_overflow_warnings.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

--- Comment #3 from Ian Lance Taylor <ian at airs dot com> 2011-07-12 14:25:56 UTC ---
Created attachment 24744
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24744
Possible patch

Here is a possible patch to fix this test case.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

--- Comment #4 from eggert at gnu dot org 2011-07-12 16:30:44 UTC ---
Created attachment 24747
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24747
longer, more-realistic test case

Thanks for looking into the problem.  If it helps, attached is a longer test
case that more-closely corresponds to the original problem that I had
with the Emacs source code.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

--- Comment #5 from Ian Lance Taylor <ian at airs dot com> 2011-07-12 16:42:07 UTC ---
My proposed patch also fixes the larger test case.  Without the patch I see
this:

strict-overflow-bug.c: In function ‘check_image_width’:
strict-overflow-bug.c:14:4: warning: assuming signed overflow does not occur
when assuming that (X + c) >= X is always true [-Wstrict-overflow]

With the patch I see no warning.  This is compiling with -O2 -Wstrict-overflow.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

Ian Lance Taylor <ian at airs dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |ian at airs dot com
                   |gnu.org                     |

--- Comment #6 from Ian Lance Taylor <ian at airs dot com> 2011-07-12 22:47:33 UTC ---
Patch tested and sent for approval.

http://gcc.gnu.org/ml/gcc-patches/2011-07/msg00971.html

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[ian at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

--- Comment #7 from ian at gcc dot gnu.org <ian at gcc dot gnu.org> 2011-07-21 21:30:26 UTC ---
Author: ian
Date: Thu Jul 21 21:30:24 2011
New Revision: 176591

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=176591
Log:
gcc/c-family:
    PR middle-end/49705
    * c-common.c (c_disable_warnings): New static function.
    (c_enable_warnings): New static function.
    (c_fully_fold_internal): Change local unused_p to bool.  Call
    c_disable_warnings and c_enable_warnings rather than change
    c_inhibit_evaluation_warnings.
gcc/testsuite:
    PR middle-end/49705
    * gcc.dg/pr49705.c: New test.

Added:
    trunk/gcc/testsuite/gcc.dg/pr49705.c
Modified:
    trunk/gcc/c-family/ChangeLog
    trunk/gcc/c-family/c-common.c
    trunk/gcc/testsuite/ChangeLog

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

Ian Lance Taylor <ian at airs dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW

--- Comment #8 from Ian Lance Taylor <ian at airs dot com> 2011-07-21 21:32:01 UTC ---
Fixed in mainline.

[Bug middle-end/49705] -Wstrict-overflow should not diagnose unevaluated expressions

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49705

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |manu at gcc dot gnu.org
         Resolution|                            |FIXED

--- Comment #9 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2011-09-21 15:33:24 UTC ---
(In reply to comment #8)
> Fixed in mainline.

So closing.