[Bug tree-optimization/104521] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[Bug tree-optimization/104521] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104521

            Bug ID: 104521
           Summary: wrong code at -O2 and -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 recent regression and is very likely related to PR 104519.
But opposite to PR 104519, it doesn't reproduce at -Os, and instead reproduces
at -O2 and -O3. At -Os, there is a spurious UB warning though.

[611] % 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/12.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=/local/suz-local/software/local/gcc-trunk --enable-languages=c,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.1 20220213 (experimental) [master r12-7216-g7e204bd2f18] (GCC) 
[612] % 
[612] % gcctk -Os small.c; ./a.out
small.c: In function ‘main’:
small.c:6:30: warning: iteration 42 invokes undefined behavior
[-Waggressive-loop-optimizations]
    6 |       for (c = 0; c != -4; c -= 3) {
      |                              ^~
small.c:6:21: note: within this loop
    6 |       for (c = 0; c != -4; c -= 3) {
      |                   ~~^~~~~
[613] % 
[613] % gcctk -O2 small.c
[614] % timeout -s 9 5 ./a.out
Killed
[615] % 
[615] % cat small.c
char a, b, c;
int main() {
  unsigned char d = 1;
  while (1) {
    if (c >= a) {
      for (c = 0; c != -4; c -= 3) {
        while (!d)
          b = 0;
        continue;
      }
    }
    d = ~a;
    if (!d)
      continue;
    return 0;
  }
}

[Bug tree-optimization/104521] wrong code at -O2 and -O3 on x86_64-linux-gnu

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104521

--- Comment #1 from Zhendong Su <zhendong.su at inf dot ethz.ch> ---
>From Compiler Explorer: https://godbolt.org/z/Y37Wo9e8c

[Bug tree-optimization/104521] [12 Regression] wrong code at -O2 and -O3 with char as loop induction variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104521

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|wrong code at -O2 and -O3   |[12 Regression] wrong code
                   |on x86_64-linux-gnu         |at -O2 and -O3 with char as
                   |                            |loop induction variable
   Target Milestone|---                         |12.0
      Known to work|                            |11.2.0
      Known to fail|                            |12.0
           Keywords|                            |diagnostic, wrong-code

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect it was introduced by r12-6844-gf1af8528d3 .

[Bug tree-optimization/104521] [12 Regression] wrong code at -O2 and -O3 with char as loop induction variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104521

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 104519.

*** This bug has been marked as a duplicate of bug 104519 ***