[Bug c/105598] New: Flag -O2 causes code to misbehave

[Bug c/105598] New: Flag -O2 causes code to misbehave

[greenfoo at u92 dot eu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105598

            Bug ID: 105598
           Summary: Flag -O2 causes code to misbehave
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: greenfoo at u92 dot eu
  Target Milestone: ---

Created attachment 52974
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52974&action=edit
Small example that triggers the issue when using -O2 in gcc 11.x

I found a situation where the "-O2" flag was causing trouble at least in
x86_64.

The attached file "example.c" code is expected to print "4" when compiled and
executed, but it would print "2" when compiled with "-02".

This happens with gcc 11.1, 11.2 and 11.3, but works correctly with gcc 10.3
and 12.1:

> $ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp gcc:10.3 gcc -o example example.c && ./example
> 4 
> $ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp gcc:11.1 gcc -o example example.c && ./example
> 2
> $ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp gcc:11.2 gcc -o example example.c && ./example
> 2
> $ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp gcc:11.3 gcc -o example example.c && ./example
> 2
> $ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp gcc:12.1 gcc -o example example.c && ./example
> 4

I don't know if this deserves further investigation. If not, please feel free
to discard this bug report.

Thanks!

[Bug c/105598] [11 Regression] Flag -O2 causes code to misbehave

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105598

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.4
           Keywords|                            |wrong-code
            Summary|Flag -O2 causes code to     |[11 Regression] Flag -O2
                   |misbehave                   |causes code to misbehave
             Status|UNCONFIRMED                 |NEW
           Priority|P3                          |P2
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-05-13
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |jakub at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This went away with r12-3918-g5b8b1522e04adc20980f396571be1929a32d148a
on the trunk and started r11-3772-gc8fd2be174a15ff154a3102decb0e643f67cad02
typedef struct { unsigned int num; } info_t;
typedef struct { unsigned int flag, type; } block_t;
info_t info;
block_t blocks[] = { {2,0}, {3,0}, {1,0}, {1,0} };

static block_t *
f (info_t *i, block_t *b) {
  while (1) {
    unsigned int is_last = b->flag & 0x01;
    i->num++;
    if (b->flag & 0x02) {
      if (b->type != 0x1) b->type = b->type;
      b = f (i, b+1);
    }
    if (is_last)
      break;
    b++;
  }
  return b;
}

int
main () {
  f(&info, &blocks[0]);
  if (info.num != 4)
    __builtin_abort ();
}

[Bug tree-optimization/105598] [11 Regression] Flag -O2 causes code to misbehave

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105598

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
r12-3918-g5b8b1522e04adc20980f396571be1929a32d148a is an optimization
improvement.  When backporting the change the failure goes away but it
re-appears with -fdisable-tree-fre1 and can be mitigated with -fno-ipa-cp.
-fdisable-tree-fre1 doesn't re-introduce the issue on trunk.

Can you bisect what fixed this with -fdisable-tree-fre1?

[Bug tree-optimization/105598] [11 Regression] Flag -O2 causes code to misbehave

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105598

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
With -O2 -fdisable-tree-fre1 it started with the same revision, but went away
with r12-5104-gb406bb901f7e98f7c6d66a240c2eb64b7cd7e615 instead.

[Bug tree-optimization/105598] [11 Regression] Flag -O2 causes code to misbehave

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105598

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

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

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
I'm going to backport this rev.

[Bug tree-optimization/105598] [11 Regression] Flag -O2 causes code to misbehave

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105598

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
      Known to fail|                            |11.3.0
      Known to work|                            |11.3.1, 12.1.0
             Status|ASSIGNED                    |RESOLVED

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