[Bug fortran/59561] New: [4.9 Regression] warning: iteration 4 invokes undefined behavior

[Bug fortran/59561] New: [4.9 Regression] warning: iteration 4 invokes undefined behavior

[janus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

            Bug ID: 59561
           Summary: [4.9 Regression] warning: iteration 4 invokes
                    undefined behavior
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janus at gcc dot gnu.org

Found when working on PR 55207 ...


With the following modification:

Index: gcc/testsuite/gfortran.dg/inline_sum_bounds_check_1.f90
===================================================================
--- gcc/testsuite/gfortran.dg/inline_sum_bounds_check_1.f90    (revision
206100)
+++ gcc/testsuite/gfortran.dg/inline_sum_bounds_check_1.f90    (working copy)
@@ -3,7 +3,8 @@

       integer, parameter :: nx = 3, ny = 4

-      integer :: i, j, too_big
+      integer :: i, j
+      integer, save :: too_big

       integer, parameter, dimension(nx,ny) :: p = &
         reshape((/ (i*i, i=1,size(p)) /), shape(p))


so that this test case becomes:


! { dg-do run }
! { dg-options "-fbounds-check" }

      integer, parameter :: nx = 3, ny = 4

      integer :: i, j
      integer, save :: too_big

      integer, parameter, dimension(nx,ny) :: p = &
        reshape((/ (i*i, i=1,size(p)) /), shape(p))

      integer, dimension(nx,ny) :: a

      integer, dimension(:), allocatable :: b

      allocate(b(nx))

      a = p
      too_big = ny + 1

      b = sum(a(:,1:too_big),2)
      end
! { dg-shouldfail "outside of expected range" }


trunk gives:

$ gfortran-4.9 inline_sum_bounds_check_1.f90 -O2
inline_sum_bounds_check_1.f90: In function ‘MAIN__’:
inline_sum_bounds_check_1.f90:21:0: warning: iteration 4 invokes undefined
behavior [-Waggressive-loop-optimizations]
       b = sum(a(:,1:too_big),2)
 ^
inline_sum_bounds_check_1.f90:21:0: note: containing loop
inline_sum_bounds_check_1.f90: In function ‘main’:
inline_sum_bounds_check_1.f90:21:0: warning: iteration 4 invokes undefined
behavior [-Waggressive-loop-optimizations]
       b = sum(a(:,1:too_big),2)
 ^
inline_sum_bounds_check_1.f90:21:0: note: containing loop


with -O2 and -O3, while 4.8 did not do that. Probably the warning is ok, since
the test case is supposed to trigger a runtime error ("Index '5' of dimension 2
of array 'a' outside of expected range (4:1)").

However, I see three possible problems:
1) The question is why it only happens with SAVE.
2) I don't understand why the warning has 'iteration 4'. Shouldn't iteration 5
be the problem?
3) The warning appears twice.
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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior
Date: Thu, 19 Dec 2013 14:03:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY561

Tobias Burnus <burnus at gcc dot gnu.org> changed:

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

--- Comment #1 from Tobias Burnus <burnus at gcc dot gnu.org> ---
Isn't that effectively a duplicate of the P1 regression PR57904?

[Bug fortran/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.9.0

[Bug fortran/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[janus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

--- Comment #2 from janus at gcc dot gnu.org ---
(In reply to Tobias Burnus from comment #1)
> Isn't that effectively a duplicate of the P1 regression PR57904?

Might well be, I'm not sure. However, the patch posted in PR 57904 comment 9
does not seem to fix the problem here ...

[Bug fortran/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[dominiq at lps dot ens.fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-12-20
     Ever confirmed|0                           |1

--- Comment #3 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> (In reply to Tobias Burnus from comment #1)
> > Isn't that effectively a duplicate of the P1 regression PR57904?
>
> Might well be, I'm not sure. However, the patch posted in PR 57904 comment 9 
> does not seem to fix the problem here ...

I don't think it is a duplicate and I think the warning is correct (on
x86_64-apple-darwin13 I get it with/without SAVE and with/without the patch in
PR 57904 comment 9).
Too bad that the warning is emitted twice and the iteration count starts at 0
and not at one as in first (IMHO #iterations=end-start+1, i.e., 5).

[Bug middle-end/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[dominiq at lps dot ens.fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |law at redhat dot com
          Component|fortran                     |middle-end

--- Comment #4 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
I think it is a middle-end issue.

[Bug middle-end/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

--- Comment #5 from Jeffrey A. Law <law at redhat dot com> ---
Not a duplicate of 57904 AFAICT.

[Bug middle-end/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to janus from comment #0)
> with -O2 and -O3, while 4.8 did not do that. Probably the warning is ok,
> since the test case is supposed to trigger a runtime error ("Index '5' of
> dimension 2 of array 'a' outside of expected range (4:1)").

Of course the warning is correct.
> 
> However, I see three possible problems:
> 1) The question is why it only happens with SAVE.

I get the warning also without SAVE.  Furthermore, the warning is not
guaranteed for all undefined behaviors in the loop of course, only if the loop
has known constant number of iterations, no early exits and is known to trigger
selected kinds of undefined behavior in some of the iterations.

> 2) I don't understand why the warning has 'iteration 4'. Shouldn't iteration
> 5 be the problem?

This is a middle-end warning, I don't see how the middle-end could find out
that the frontend wants to count the iterations differently.

> 3) The warning appears twice.

For that I have an untested fix:

2014-01-22  Jakub Jelinek  <jakub@redhat.com>

    PR middle-end/59561
    * cfgloopmanip.c (copy_loop_info): If
    loop->warned_aggressive_loop_optimizations, make sure
    the flag is set in target loop too.

--- gcc/cfgloopmanip.c.jj    2014-01-03 11:40:33.000000000 +0100
+++ gcc/cfgloopmanip.c    2014-01-22 10:21:54.057871045 +0100
@@ -1022,6 +1022,8 @@ copy_loop_info (struct loop *loop, struc
   target->any_estimate = loop->any_estimate;
   target->nb_iterations_estimate = loop->nb_iterations_estimate;
   target->estimate_state = loop->estimate_state;
+  target->warned_aggressive_loop_optimizations
+    |= loop->warned_aggressive_loop_optimizations;
 }

 /* Copies copy of LOOP as subloop of TARGET loop, placing newly

[Bug middle-end/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Author: jakub
Date: Fri Jan 24 23:17:25 2014
New Revision: 207065

URL: http://gcc.gnu.org/viewcvs?rev=207065&root=gcc&view=rev
Log:
    PR middle-end/59561
    * cfgloopmanip.c (copy_loop_info): If
    loop->warned_aggressive_loop_optimizations, make sure
    the flag is set in target loop too.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/cfgloopmanip.c

[Bug middle-end/59561] [4.9 Regression] warning: iteration 4 invokes undefined behavior

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59561

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

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

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed as far as we care.