[Bug c/102711] New: Infinite loop with extern calls removed with -O2 or higher

[Bug c/102711] New: Infinite loop with extern calls removed with -O2 or higher

[andy.koppe at analog dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

            Bug ID: 102711
           Summary: Infinite loop with extern calls removed with -O2 or
                    higher
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andy.koppe at analog dot com
  Target Milestone: ---

Created attachment 51591
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51591&action=edit
Minimized example

Compiling the minimised example below (and attached) with -O2 or higher, if
FAIL is defined, the second infinite loop is removed. Code is produced for it
when FAIL is not defined.

Reproduced with gcc 10.3.1 for aarch64-none-elf, gcc 11.2.0 for
x86_64-pc-cygwin, and gcc 9.3.0 for x86_64-linux-gnu.

$ cat test.c
int bar(void);

void foo(void)
{
  if (!bar())
    for (;;);

  for (;;) {
    static int flag = 0;
    int ret = 0;
    bar();
    if (flag) {
      ret = bar();
      if (!ret)
        ret = bar();
    }
    flag = !bar();
#ifdef FAIL
    if (!ret)
#endif
      bar();
  }
}

$ aarch64-none-elf-gcc -O2 -S test.c -DFAIL

$ cat test.s
        .arch armv8-a
        .file   "test.c"
        .text
        .align  2
        .p2align 4,,11
        .global foo
        .type   foo, %function
foo:
        stp     x29, x30, [sp, -16]!
        mov     x29, sp
        bl      bar
.L2:
        b       .L2
        .size   foo, .-foo
        .ident  "GCC: (GNU Toolchain for the A-profile Architecture
10.3-2021.07 (arm-10.29)) 10.3.1 20210621"

[Bug tree-optimization/102711] [9/10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |9.5

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Looks to be fixed on the trunk.

[Bug tree-optimization/102711] [9/10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-10-12
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Just for refence this is the testcase that can be done at runtime:

void exit(int);
__attribute__((noipa))
int bar(void)
{
  static int t = 0;
  t++;
  if (t == 2) exit(0);
  return 1;
}
__attribute__((noipa))
void foo(void)
{
  if (!bar())
    for (;;);

  for (;;) {
    static int flag = 0;
    int ret = 0;
    bar();
    if (flag) {
      ret = bar();
      if (!ret)
        ret = bar();
    }
    flag = !bar();
    if (!ret)
      bar();
  }
}
int main(void)
{
  foo();
}

[Bug tree-optimization/102711] [9/10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
                 CC|                            |rguenth at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=99101
           Keywords|                            |needs-bisection

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Feels related to PR99101.  We fail to mark the first if (!bar()) condition
as useful.  On trunk we happen to mark it when visiting control dependences
of if (flag).

A related commit _might_ be g:58ad6b2802592f1448eed48e8ad4e0e87985cecb

Can somebody bisect whether that "fixed" the issue on trunk?  Note that
wasn't really a "fix" - but as PR99101 notes I have a hunch that the
control dependence code doesn't behave like we want in the context of
CD-DCE and infinite loops.

[Bug tree-optimization/102711] [9/10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
Fixed on master with r12-2591-g2e96b5f14e402569.
Note it started with r8-2318-g34e5c5116fa58f77.

[Bug tree-optimization/102711] [9/10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 13 Oct 2021, marxin at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711
> 
> Martin Liška <marxin at gcc dot gnu.org> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |marxin at gcc dot gnu.org
> 
> --- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
> Fixed on master with r12-2591-g2e96b5f14e402569.

OK, that likely made it latent only.

> Note it started with r8-2318-g34e5c5116fa58f77.

I see - probably makes sense since that subtly changes post-dominators
for backwards unreachable regions which reflects back on control
dependence.  But there's nothing "wrong" here, I think it avoided
placing the fake exit edges at the infinite loop headers as opposed
to the latch but I don't remember exactly.

[Bug tree-optimization/102711] [9/10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

--- Comment #6 from Martin Liška <marxin at gcc dot gnu.org> ---
> I see - probably makes sense since that subtly changes post-dominators
> for backwards unreachable regions which reflects back on control
> dependence.  But there's nothing "wrong" here, I think it avoided
> placing the fake exit edges at the infinite loop headers as opposed
> to the latch but I don't remember exactly.

Do we still need more bisection?

[Bug tree-optimization/102711] [10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |10.4

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 branch is being closed

[Bug tree-optimization/102711] [10/11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug tree-optimization/102711] [11 Regression] CDDCE removes condition that might lead to an infinite loop causing an unconditional infinite loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102711

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |11.5

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.