[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|TREE                        |
   Last reconfirmed|2021-07-26 00:00:00         |2021-8-26

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So no, that's not the problem at all.  The only loop is the infinite while (1)
one, the problem is that because we cannot prove finiteness of the loop we're
doing

      for (auto loop : loops_list (cfun, 0))
        if (!finite_loop_p (loop))
          {
            if (dump_file)
              fprintf (dump_file, "cannot prove finiteness of loop %i\n",
loop->num);
            mark_control_dependent_edges_necessary (loop->latch, false);
          }

and that marks a condition that has nothing to do with loop control.  I suppose
we can elide this when the loop has no exit (we are already marking backedges
of irreducible loops).

But I never actually understood this particular part of CD-DCE.

The desired transform is that we end up with just

int main()
{
  while (1)
    ;
}

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[hubicka at ucw dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

--- Comment #4 from Jan Hubicka <hubicka at ucw dot cz> ---
> and that marks a condition that has nothing to do with loop control.  I suppose
> we can elide this when the loop has no exit (we are already marking backedges
> of irreducible loops).
> 
> But I never actually understood this particular part of CD-DCE.
Skipping this for loops with no exit edges should work - we will end up
removing some control flow inside but the logic finding path to next
live stmt can probably be told to chose any path in empty fully
infinite loop.

Honza

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

--- Comment #5 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:41439e1f6d2da1e86538c726f0603cffd5dd098e

commit r12-3182-g41439e1f6d2da1e86538c726f0603cffd5dd098e
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Aug 27 09:47:49 2021 +0200

    tree-optimization/45178 - DCE of dead control flow in infinite loop

    This fixes DCE to be able to elide dead control flow in an
    infinite loop without an exit edge.  This special situation is
    handled well by the code finding an edge to preserve since there's
    no chance it will find the exit edge and make the loop finite.

    2021-08-27  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/45178
            * tree-ssa-dce.c (find_obviously_necessary_stmts): For
            infinite loops without exit do not mark control dependent
            edges of the latch necessary.

            * gcc.dg/tree-ssa/ssa-dce-3.c: Adjust testcase.

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

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

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

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed.

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Richard Biener
<rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:634c2bc38ff66f13164967a8e3b965e8255dd4d9

commit r11-9586-g634c2bc38ff66f13164967a8e3b965e8255dd4d9
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Aug 27 09:47:49 2021 +0200

    tree-optimization/45178 - DCE of dead control flow in infinite loop

    This fixes DCE to be able to elide dead control flow in an
    infinite loop without an exit edge.  This special situation is
    handled well by the code finding an edge to preserve since there's
    no chance it will find the exit edge and make the loop finite.

    2021-08-27  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/45178
            * tree-ssa-dce.c (find_obviously_necessary_stmts): For
            infinite loops without exit do not mark control dependent
            edges of the latch necessary.

            * gcc.dg/tree-ssa/ssa-dce-3.c: Adjust testcase.

    (cherry picked from commit 41439e1f6d2da1e86538c726f0603cffd5dd098e)

[Bug tree-optimization/45178] New: CDDCE doesn't eliminate conditional code in infinite loop

[rguenth at gcc dot gnu dot org]

Reduced from gcc.dg/tree-ssa/ssa-dce-3.c:

int main(void)
{
  unsigned j = 0;
  while (1)
    {
      j += 500;
      if (j % 7)
        j++;
      else
        j--;
    }
  return 0;
}


-- 
           Summary: CDDCE doesn't eliminate conditional code in infinite
                    loop
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[hubicka at ucw dot cz]

------- Comment #1 from hubicka at ucw dot cz  2010-08-04 13:05 -------
Subject: Re:   New: CDDCE doesn't eliminate
        conditional code in infinite loop

Hmm, so the problem is that we produce two alternating loops and both with
unknown number of iterations?
We might teach loop discovery to preffer single infinite loop over alternation
I guess.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178

[Bug tree-optimization/45178] CDDCE doesn't eliminate conditional code in infinite loop

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2010-08-04 13:21 -------
(In reply to comment #1)
> Subject: Re:   New: CDDCE doesn't eliminate
>         conditional code in infinite loop
> 
> Hmm, so the problem is that we produce two alternating loops and both with
> unknown number of iterations?
> We might teach loop discovery to preffer single infinite loop over alternation
> I guess.

Yeah, that might help.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-08-04 13:21:34
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45178