[Bug c/52904] New: -Wstrict-overflow false alarm with bounded loop

[Bug c/52904] New: -Wstrict-overflow false alarm with bounded loop

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

             Bug #: 52904
           Summary: -Wstrict-overflow false alarm with bounded loop
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org


Created attachment 27113
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27113
simplified version of Emacs code, illustrating the bug

I ran into this problem when trying to build GNU Emacs with -Wstrict-overflow.

Compiling the attached program 'v.i' with the following command:

gcc -c -Wstrict-overflow -O2 v.i

generates the diagnostic:

v.i: In function 'wait_reading_process_output':
v.i:14:6: error: assuming signed overflow does not occur when simplifying
conditional to constant [-Werror=strict-overflow]

The diagnostic is obviously incorrect, since the variable 'nfds' cannot
possibly exceed 1024.

I will also attach the output of "gcc -v -c -Wstrict-overflow -O2 v.i".

[Bug c/52904] -Wstrict-overflow false alarm with bounded loop

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

--- Comment #1 from eggert at gnu dot org 2012-04-08 08:59:43 UTC ---
Created attachment 27114
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27114
gcc -v output, for test case

[Bug c/52904] -Wstrict-overflow false alarm with bounded loop

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

eggert at gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #27113|0                           |1
        is obsolete|                            |

--- Comment #2 from eggert at gnu dot org 2012-04-08 09:02:22 UTC ---
Created attachment 27115
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27115
simplified version of Emacs code, illustrating the bug

Re-uploading the test case, marked as "text/plain" this time, so it's easier to
download.

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
          Component|c                           |tree-optimization

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-04-08 10:05:26 UTC ---
(In reply to comment #2)
> Created attachment 27115 [details]
> simplified version of Emacs code, illustrating the bug
> 
> Re-uploading the test case, marked as "text/plain" this time, so it's easier to
> download.

You did not have to re-upload the testcase, you can change the MIME type of the
atttachment after the fact.

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-04-10
                 CC|                            |rguenth at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-04-10 12:16:54 UTC ---
It's the

  if (nfds < 0)

conditional that we warn on.  We cannot currently derive an upper bound
for nfds based on the number of iterations of the loop as it is not
an induction variable we can analyze.

int
wait_reading_process_output (void)
{
  int nfds = 0;
  int channel;
  for (channel = 0; channel < 1024; ++channel)
    {
      if (foo (channel))
    nfds++;
    }
  if (nfds < 0)
    return 1;

This is in some way a missed optimization in value-range analysis as well.

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[Laurent.Rineau__gcc at normalesup dot org]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset="UTF-8", Size: 4794 bytes --]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

Laurent Rineau <Laurent.Rineau__gcc at normalesup dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Laurent.Rineau__gcc@normale
                   |                            |sup.org

--- Comment #5 from Laurent Rineau <Laurent.Rineau__gcc at normalesup dot org> ---
With the following gcc version:
  gcc (GCC) 4.8.2 20131017 (Red Hat 4.8.2-1)

I have a similar result:

$ gcc -c -Wstrict-overflow -O2 v.i
v.i: In function ‘wait_reading_process_output’:
v.i:14:6: warning: assuming signed overflow does not occur when simplifying
conditional to constant [-Wstrict-overflow]
   if (nfds < 0)
      ^

That diagnostic seems right, according to the documentation of
-Wstrict-overflow.
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To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/59484] execute_command_line doesn't play with environment variables
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY484

--- Comment #2 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Thu, Dec 12, 2013 at 02:27:19PM +0000, kargl at gcc dot gnu.org wrote:
> --- Comment #1 from kargl at gcc dot gnu.org ---
> (In reply to Reinhold Straub from comment #0)
> > Please compile:
> >
> > program test
> >   implicit none
> >
> >   character(len\x11) :: cmdmsg,str
> >   integer :: exitstat,cmdstat,status,length
> >   logical :: wait
> >   wait   = .true.
> >   cmdmsg = "ok"
> >
> >   call execute_command_line("export newvar='some text'")
> >   print *,"result 1:", exitstat, cmdstat, cmdmsg
> >   call get_environment_variable("newvar",str,length,status)
> >   print *,"1:     ",str,length,status
> >   print *,"expected:","some text",9,0; print *,""
> >
>
> Looks like an invalid program to me.  exitstat, cmdstat, and cmdmsg
> are undefined in the 1st print statement.

Actually, cmdmsg is defined.  exitstat and cmdstat are not defined.

> execute_command_line(command) executes the 'command' in a child
> process.  You've managed to export newvar in the child's environment.
> By the time you use get_environment_variable, the child process has
> completed and more importantly it has not changed its parent
> environment.
>
> If you want to change the parent's environment, you'll most likely
> need to use ISO C interop and the setenv function from the C lib.

If you want to change the environment of the parent you can do

program foo

   use iso_c_binding

   implicit none

   integer status
   character(len ) var

   interface
      function putenv(str) bind(c,name="putenv")
         use iso_c_binding
         integer(c_int) putenv
         character(len=1,kind=c_char) str
      end function putenv
   end interface

   status = putenv('OHMYúrmer tan' // C_NULL_CHAR)
   print '(A,I0)', 'status = ', status

   call get_environment_variable('OHMY', var)
   print '(A)', trim(var)

end program foo

gfortran -o z foo.f90
./z
status = 0
Farmer tan

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[Laurent.Rineau__gcc at normalesup dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

--- Comment #7 from Laurent Rineau <Laurent.Rineau__gcc at normalesup dot org> ---
In the test case, nfds cannot overflow, because of two reasons:
  - nfds is only incremented from 0, and -fstrict-overflow allows gcc to
suppose it will not overflow,
  - the number of iterations of the loop cannot allow nfds to overflow, even
without -fstrict-overflow.

Gcc warns that the test (nfds < 0) is useless, because of -fstrict-overflow.
The developer has two solutions:
  - remove that test,
  - or compile with -fno-strict-overflow.

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

--- Comment #8 from eggert at gnu dot org ---
On 12/12/2013 10:19 AM, Laurent.Rineau__gcc at normalesup dot org wrote:
> The developer has two solutions:
>   - remove that test,
>   - or compile with -fno-strict-overflow.

Sure, and because of this problem, GNU Emacs has chosen #2, that
is, Emacs doesn't use -Wstrict-overflow any more.  (Removing
the test would unnecessarily complicate Emacs, since the
test is needed on some platforms that are conditionally
compiled away in this stripped-down example.)

A goal of -Wstrict-overflow, at least at the 1 level, is to
not generate false alarms, so that it's generally useful in
real programs.  This bug report gives one example where the
goal is not being met.  Emacs currently has a half dozen or
so such examples of this (please see below for what the
current Emacs bzr trunk would output, if we enabled this
warning) and I thought that the GCC developers would find it
useful to see one of them.  If you're not interested, that's
OK too; Emacs will continue to not use -Wstrict-overflow.

In file included from dispnew.c:26:0:
dispnew.c: In function ‘update_window’:
lisp.h:749:30: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
   return num < lower ? lower : num <= upper ? num : upper;

dispnew.c: In function ‘update_frame_1’:
dispnew.c:4490:19: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
   pause_p = 0 < i && i < FRAME_LINES (f) - 1;
fileio.c: In function ‘Finsert_file_contents’:
fileio.c:3630:11: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
        if (nread < 0)
           ^
fileio.c:3632:16: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
        else if (nread > 0)
                ^
eval.c: In function ‘backtrace_eval_unrewind’:
eval.c:3496:3: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
   for (; distance > 0; distance--)
   ^
intervals.c: In function ‘offset_intervals’:
intervals.c:1364:6: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
   if (length == TOTAL_LENGTH (tree))
      ^
intervals.c:1379:9: warning: assuming signed overflow does not occur when
simplifying conditional to constant [-Wstrict-overflow]
   while (left_to_delete > 0)
         ^
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Subject: [Bug fortran/59440] [4.9 Regression] ICE in force_decl_die, at dwarf2out.c:20111 with -g
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY440

--- Comment #4 from Tobias Burnus <burnus at gcc dot gnu.org> ---
Author: burnus
Date: Thu Dec 12 19:41:11 2013
New Revision: 205939

URL: http://gcc.gnu.org/viewcvs?rev 5939&root=gcc&view=rev
Log:
2013-12-12  Tobias Burnus  <burnus@net-b.de>

        PR fortran/59440
        * trans-decl.c (generate_namelist_decl): Ensure debug DIE
        is created by setting DECL_IGNORED_P to 0.

2013-12-12  Tobias Burnus  <burnus@net-b.de>

        PR fortran/59440
        * gfortran.dg/namelist_83.f90: New.
        * gfortran.dg/namelist_83_2.f90: New.


Added:
    trunk/gcc/testsuite/gfortran.dg/namelist_83.f90
    trunk/gcc/testsuite/gfortran.dg/namelist_83_2.f90
Modified:
    trunk/gcc/fortran/ChangeLog
    trunk/gcc/fortran/trans-decl.c
    trunk/gcc/testsuite/ChangeLog

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[kugan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

kugan at gcc dot gnu.org changed:

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

--- Comment #9 from kugan at gcc dot gnu.org ---
Incorrect warning is gone from the latest trunk build. VRP is now able to
detect the correct range. This bug can now be closed.

----------------------------------------------------
Value ranges after VRP:

channel_8: [1, 1023]  EQUIVALENCES: { channel_10 } (1 elements)
channel_10: [1, 1024]
channel_15: [0, 1023]
.MEM_16: VARYING


Removing basic block 5
wait_reading_process_output ()
{
  int channel;
  int nfds;

  <bb 2>:

  <bb 3>:
  # channel_15 = PHI <channel_10(3), 0(2)>
  foo (channel_15);
  channel_10 = channel_15 + 1;
  if (channel_10 != 1024)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 4>:
  return 0;

}

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[kugan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

--- Comment #11 from kugan at gcc dot gnu.org ---
I agree. I will post a patch to add this test-case and let the maintainers
decide if this is necessary.

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[kugan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

--- Comment #12 from kugan at gcc dot gnu.org ---
Author: kugan
Date: Mon Aug 18 06:28:35 2014
New Revision: 214084

URL: https://gcc.gnu.org/viewcvs?rev=214084&root=gcc&view=rev
Log:
gcc/testsuite
2014-08-18  Kugan Vivekanandarajah  <kuganv@linaro.org>

    PR tree-optimization/52904
    * gcc.dg/pr52904.c: New test.


Added:
    trunk/gcc/testsuite/gcc.dg/pr52904.c
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug tree-optimization/52904] -Wstrict-overflow false alarm with bounded loop

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |8.0
             Status|REOPENED                    |RESOLVED
         Resolution|---                         |FIXED
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=31130

--- Comment #14 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The xfail was removed with r8-395 which removed -fno-strict-overflow usage.