[Bug tree-optimization/115347] New: wrong code at -O3 on x86_64-linux-gnu

[Bug tree-optimization/115347] New: wrong code at -O3 on x86_64-linux-gnu

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115347

            Bug ID: 115347
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
  Target Milestone: ---

It appears to be a regression from 11.*, affecting 12.* and later.

Compiler Explorer: https://godbolt.org/z/PY985hnff

[510] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/suz-local/software/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/15.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--enable-checking=yes --prefix=/local/suz-local/software/local/gcc-trunk
--enable-sanitizers --enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240604 (experimental) (GCC) 
[511] % 
[511] % gcctk -O2 small.c; ./a.out
[512] % gcctk -O3 small.c
[513] % ./a.out
Aborted
[514] % cat small.c
struct a {
  int b;
  int c;
} d, e[2];
int f, g, h;
int main() {
  for (; f < 1; f++) {
    for (h = 0; h < 2; h++) {
      d = e[f];
      g = e[1].c;
      e[f].c = 1;
    }
  }
  if (d.c != 1)
    __builtin_abort();
  return 0;
}

[Bug tree-optimization/115347] [12/13/14/15 Regression] wrong code at -O3 on x86_64-linux-gnu

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115347

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
            Summary|wrong code at -O3 on        |[12/13/14/15 Regression]
                   |x86_64-linux-gnu            |wrong code at -O3 on
                   |                            |x86_64-linux-gnu
   Target Milestone|---                         |12.4

[Bug tree-optimization/115347] [12/13/14/15 Regression] wrong code at -O3 on x86_64-linux-gnu

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115347

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-bisection
   Last reconfirmed|                            |2024-06-04
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug tree-optimization/115347] [12/13/14/15 Regression] wrong code at -O3 on x86_64-linux-gnu

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115347

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=112859
            Version|unknown                     |14.1.1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
it's loop distribution doing

t2.c:7:12: optimized: Loop nest 1 distributed: split to 2 loops and 0 library
calls.

We get

  for (; f < 1; f++) {
    for (h = 0; h < 2; h++) {
      d = e[f];
    }
  }
  for (; f < 1; f++) {
    for (h = 0; h < 2; h++) {
      g = e[1].c;
      e[f].c = 1;
    }
  }

I think this is similar to the other still open issue where zero-distance
inner loop dependences (&e[f].c doesnt't vary in the inner loop) cause
issues with the interpretation of classical dependence analysis.

I'm somewhat lost there.  PR112859.

[Bug tree-optimization/115347] [12/13/14/15 Regression] wrong code at -O3 on x86_64-linux-gnu since r12-2097

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115347

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[12/13/14/15 Regression]    |[12/13/14/15 Regression]
                   |wrong code at -O3 on        |wrong code at -O3 on
                   |x86_64-linux-gnu            |x86_64-linux-gnu since
                   |                            |r12-2097
           Priority|P3                          |P2
           Keywords|needs-bisection             |
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Started with r12-2097-g9f34b780b0461ec7b2b2defe96e44ab616ea2aa3

[Bug tree-optimization/115347] [12/13/14/15 Regression] wrong code at -O3 on x86_64-linux-gnu since r12-2097

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115347

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.4                        |12.5

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 12.4 is being released, retargeting bugs to GCC 12.5.