[Bug tree-optimization/97135] New: [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[Bug tree-optimization/97135] New: [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[acoplan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

            Bug ID: 97135
           Summary: [11 Regression] Wrong code at -Os since
                    r11-408-g84935c98221
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

For the following testcase:

long c;
long *d = &c;
int a, e, f;
int main(void) {
  for (; f <= 5; f++) {
    e = 0;
    for (; e <= 5; e++) {
      a = 1;
      for (; a <= 5; a++);
      *d = 0;
      if (f)
        break;
    }
  }
  return a;
}

Compiling with -O{0,1,2,3} (for both gcc and clang), as expected, the program
returns 6.

Compiling with -Os, the program returns 1 (since
r11-408-g84935c9822183ce403bb361c5f405711b9a808c6).

I've reproduced this on both x86 and aarch64. Interestingly, compiling with -Os
-fno-strict-aliasing, we no longer see the wrong code bug (the program returns
6). However, I believe this program is free of strict aliasing violations.

To reproduce:

$ bin/gcc wrong_code.c
$ ./a.out || echo $?
6
$ bin/gcc wrong_code.c -Os
$ ./a.out || echo $?
1
$ bin/gcc wrong_code.c -Os -fno-strict-aliasing
$ ./a.out || echo $?
6

[Bug tree-optimization/97135] [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-09-21
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Slightly adjusted testcase for the testsuite (gcc.c-torture/execute/ ?):

long long e, *d = &e;
int a, b, c;

int
main ()
{
  for (; c <= 5; c++)
    for (b = 0; b <= 5; b++)
      {
        for (a = 1; a <= 5; a++)
          ;
        *d = 0;
        if (c)
          break;
      }
  if (a != 6)
    __builtin_abort ();
  return 0;
}

[Bug tree-optimization/97135] [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org
             Status|NEW                         |ASSIGNED
   Target Milestone|---                         |11.0

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I'll have a look.

[Bug tree-optimization/97135] [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's lim4 that does the bogus transform.  It seems to be confused by the dead
store in BB3:

  <bb 3> [local count: 118111600]:
  a = 1;
  a = 6;
  *d.2_3 = 0;
  ivtmp.15_9 = ivtmp.15_24 + 1;
  if (c.7_7 != 0)
    goto <bb 9>; [5.50%]
  else
    goto <bb 10>; [94.50%]

and re-materializes the a = 1 store but not the a = 6 one.

[Bug tree-optimization/97135] [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

--- Comment #4 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:0df746afc50a47d1eb53a401e017c4373cf05641

commit r11-3321-g0df746afc50a47d1eb53a401e017c4373cf05641
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Sep 21 14:04:25 2020 +0200

    tree-optimization/97135 - fix dependence check in store-motion

    The following fixes a dependence check where in the particular place
    we cannot ignore self-dependences.

    2020-09-21  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/97135
            * tree-ssa-loop-im.c (sm_seq_push_down): Do not ignore
            self-dependences.

            * gcc.dg/torture/pr97135.c: New testcase.

[Bug tree-optimization/97135] [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

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

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

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed.

[Bug tree-optimization/97135] [11 Regression] Wrong code at -Os since r11-408-g84935c98221

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97135

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |stefansf at linux dot ibm.com

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
*** Bug 97152 has been marked as a duplicate of this bug. ***