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

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

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

--- Comment #7 from CVS Commits  ---
The releases/gcc-11 branch has been updated by Richard Biener
:

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

commit r11-9586-g634c2bc38ff66f13164967a8e3b965e8255dd4d9
Author: Richard Biener 
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  

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] CDDCE doesn't eliminate conditional code in infinite loop

[rguenth at gcc dot gnu.org via Gcc-bugs]

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

Richard Biener  changed:

   What|Removed |Added

 Resolution|--- |FIXED
 Status|NEW |RESOLVED

--- Comment #6 from Richard Biener  ---
Fixed.

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

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

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

--- Comment #5 from CVS Commits  ---
The master branch has been updated by Richard Biener :

https://gcc.gnu.org/g:41439e1f6d2da1e86538c726f0603cffd5dd098e

commit r12-3182-g41439e1f6d2da1e86538c726f0603cffd5dd098e
Author: Richard Biener 
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  

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

[hubicka at ucw dot cz via Gcc-bugs]

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

--- Comment #4 from Jan Hubicka  ---
> 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

[rguenth at gcc dot gnu.org via Gcc-bugs]

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

Richard Biener  changed:

   What|Removed |Added

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

--- Comment #3 from Richard Biener  ---
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]

--- 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|-00-00 00:00:00 |2010-08-04 13:21:34
   date||


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