[Bug c++/54498] New: incorrect code generation from g++ -O on x86_64

[Bug c++/54498] New: incorrect code generation from g++ -O on x86_64

[stevenj at alum dot mit.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

             Bug #: 54498
           Summary: incorrect code generation from g++ -O on x86_64
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: stevenj@alum.mit.edu


Created attachment 28136
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28136
preprocessed source exhibiting the problem

The attached preprocessed source (bug.ii) illustrates an apparent incorrect
code generation when it is compiled with g++ -O version 4.7.1 on x86_64 (Debian
GNU/Linux).

The program executes two iterations of a loop, calling a function that returns
two slightly different complex numbers in the two iterations.  After the second
iteration, it prints the absolute value of the difference.  The correct output
(when compiled without optimization) is:

   ft (it = 0) = -491.697+887.05i
   ft (it = 1) = -491.692+887.026i
   abs(ft - prev_ft) = 0.0245153

(0.0245153 is the correct absolute difference of the two previous numbers.) 
When compiled with -O, it produces:

   ft (it = 0) = -491.697+887.05i
   ft (it = 1) = -491.692+887.026i
   abs(ft - prev_ft) = 491.692

Note that the first two numbers are the same, but the absolute value of the
difference is wrong.

The problem disappears if I use g++ 4.4.5, or if I make minor changes to the
code; I've tried to boil it down to the minimal code that exhibits the problem.

Steven

PS. The preprocessed source is rather long only because it #includes <stdio.h>
and <complex>; the program source at the end is quite short.

PPS. Some of my g++ -v output follows, indicating the g++ configuration options
etcetera:

Target: x86_64-unknown-linux-gnu
Configured with: ../configure
--prefix=/home/stevenj/downloads/gcc/gcc-4.7.1/OBJ/../local --disable-multilib
--enable-languages=c,c++
GNU C++ (GCC) version 4.7.1 (x86_64-unknown-linux-gnu)
    compiled by GNU C version 4.7.1, GMP version 4.3.2, MPFR version 3.0.0-p3,
MPC version 0.8.2
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

[Bug tree-optimization/54498] [4.7/4.8 Regression] incorrect code generation from g++ -O

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |wrong-code
   Last reconfirmed|                            |2012-09-05
          Component|c++                         |tree-optimization
     Ever Confirmed|0                           |1
            Summary|incorrect code generation   |[4.7/4.8 Regression]
                   |from g++ -O on x86_64       |incorrect code generation
                   |                            |from g++ -O
   Target Milestone|---                         |4.7.2

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-09-05 22:21:58 UTC ---
Confirmed, here is the source unincluded:
#include <complex>
#include <cstdio>
using namespace std;

class bar_src {
 public:
  bar_src() : next(0) {}
  virtual ~bar_src() { delete next; }

  bar_src *next;
};

class foo_src : public bar_src {
 public:
  foo_src(double f, double fwidth, double s = 5.0);
  virtual ~foo_src() {}

 private:
  double freq, width, peak_time, cutoff;
};


foo_src::foo_src(double f, double fwidth, double s) {
  freq = f; width = 1/fwidth; cutoff = s*width; peak_time = cutoff;
}

complex<double> do_ft2(int i) __attribute__ ((noinline));

complex<double> do_ft2(int i) {
  return i == 0 ? complex<double>(-491.697,887.05) :
complex<double>(-491.692,887.026);
}

void foo(void) {
  complex<double> prev_ft = 0.0, ft = 0.0;
  for (int i=0; i < 2; i++) {
    prev_ft = ft;
    {
      foo_src src(1.0, 1.0 / 20);
      ft = do_ft2(i);
      printf("ft (it = %d) = %g%+gi\n", i, real(ft), imag(ft));
    }
    if (i > 0)
      printf("abs(ft - prev_ft) = %g\n",
      abs(ft - prev_ft));
  }
}

int main(void) {
  foo();
  return 0;
}

--- CUT ---
The problem comes from FRE, I think due to how it handles look though copies. 
It happens on more than just x86_64.

[Bug tree-optimization/54498] [4.7/4.8 Regression] incorrect code generation from g++ -O

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

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

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

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-06 09:58:26 UTC ---
Let me see.  We seem to pick up

<bb 2>:
  REALPART_EXPR <prev_ft._M_value> = 0.0;

when optimizing

 <bb 7>:
   D.25613_38 = ft._M_value;
   __r$_M_value = D.25613_38;
-  D.25615_39 = MEM[(const double &)&prev_ft];
+  D.25615_39 = 0.0;

possibly not realizing that

<bb 3>:
  prev_ft = ft;

clobbers it.

[Bug tree-optimization/54498] [4.7/4.8 Regression] incorrect code generation from g++ -O

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-06 10:21:36 UTC ---
Testing

Index: gcc/tree-ssa-alias.c
===================================================================
--- gcc/tree-ssa-alias.c        (revision 191016)
+++ gcc/tree-ssa-alias.c        (working copy)
@@ -2094,7 +2094,10 @@ walk_non_aliased_vuses (ao_ref *ref, tre
              /* Lookup succeeded.  */
              else if (res != NULL)
                break;
-             /* Translation succeeded, continue walking.  */
+             /* Translation succeeded, continue walking.  We have to
+                again visit cycles though.  */
+             if (visited)
+               bitmap_clear (visited);
            }
          vuse = gimple_vuse (def_stmt);
        }

[Bug tree-optimization/54498] [4.7/4.8 Regression] incorrect code generation from g++ -O

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #4 from H.J. Lu <hjl.tools at gmail dot com> 2012-09-06 12:21:36 UTC ---
It was caused by revision 177672:

http://gcc.gnu.org/ml/gcc-cvs/2011-08/msg00686.html

[Bug tree-optimization/54498] [4.6/4.7/4.8 Regression] incorrect code generation from g++ -O

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
   Target Milestone|4.7.2                       |4.6.4
            Summary|[4.7/4.8 Regression]        |[4.6/4.7/4.8 Regression]
                   |incorrect code generation   |incorrect code generation
                   |from g++ -O                 |from g++ -O

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-06 13:54:34 UTC ---
Latent in 4.6, too.

[Bug tree-optimization/54498] [4.6/4.7/4.8 Regression] incorrect code generation from g++ -O

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

--- Comment #6 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-06 14:47:50 UTC ---
Author: rguenth
Date: Thu Sep  6 14:47:42 2012
New Revision: 191030

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=191030
Log:
2012-09-06  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/54498
    * tree-ssa-alias.h (get_continuation_for_phi): Add flag to
    abort when reaching an already visited region.
    * tree-ssa-alias.c (maybe_skip_until): Likewise.  And do it.
    (get_continuation_for_phi_1): Likewise.
    (walk_non_aliased_vuses): When we translated the reference,
    abort when we re-visit a region.
    * tree-ssa-pre.c (translate_vuse_through_block): Adjust.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/tree-ssa-alias.c
    trunk/gcc/tree-ssa-alias.h
    trunk/gcc/tree-ssa-pre.c

[Bug tree-optimization/54498] [4.6/4.7/4.8 Regression] incorrect code generation from g++ -O

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-06 15:20:29 UTC ---
Author: rguenth
Date: Thu Sep  6 15:20:24 2012
New Revision: 191031

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=191031
Log:
2012-09-06  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/54498
    * tree-ssa-alias.h (get_continuation_for_phi): Add flag to
    abort when reaching an already visited region.
    * tree-ssa-alias.c (maybe_skip_until): Likewise.  And do it.
    (get_continuation_for_phi_1): Likewise.
    (walk_non_aliased_vuses): When we translated the reference,
    abort when we re-visit a region.
    * tree-ssa-pre.c (translate_vuse_through_block): Adjust.

Modified:
    branches/gcc-4_7-branch/gcc/ChangeLog
    branches/gcc-4_7-branch/gcc/tree-ssa-alias.c
    branches/gcc-4_7-branch/gcc/tree-ssa-alias.h
    branches/gcc-4_7-branch/gcc/tree-ssa-pre.c

[Bug tree-optimization/54498] [4.6 Regression] incorrect code generation from g++ -O

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |4.7.2
            Summary|[4.6/4.7/4.8 Regression]    |[4.6 Regression] incorrect
                   |incorrect code generation   |code generation from g++ -O
                   |from g++ -O                 |

--- Comment #8 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-07 08:19:52 UTC ---
Sofar fixed on trunk and 4.7 branch.

[Bug tree-optimization/54498] [4.6 Regression] incorrect code generation from g++ -O

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|                            |FIXED
   Target Milestone|4.6.4                       |4.7.2

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-12 16:30:08 UTC ---
The 4.6 branch has been closed, fixed in GCC 4.7.2.

[Bug tree-optimization/54498] [4.6 Regression] incorrect code generation from g++ -O

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54498

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:bff7c6f70166ee71ff6e42f58663dc255e4bc079

commit r13-6879-gbff7c6f70166ee71ff6e42f58663dc255e4bc079
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Mar 27 14:30:47 2023 +0200

    tree-optimization/54498 - testcase for the bug

    I realized I never added a testcase for the fix of this bug.  Now done
    after verifying it still fails when reverting the fix.

            PR tree-optimization/54498
            * g++.dg/torture/pr54498.C: New testcase.