[Bug tree-optimization/109957] New: Missing loop PHI optimization

[Bug tree-optimization/109957] New: Missing loop PHI optimization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109957

            Bug ID: 109957
           Summary: Missing loop PHI optimization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
void foo();
int main() {
  _Bool c = 0;
  _Bool e = 1;
  int i;
  for (i = 0; i < 10000; i++)
  {
    c |= (e!=0);
    e = 0;
  }
  if (c == 0)
    foo();
  return 0;
}
```

This should be just optimized to just `return 0`.
The reason is once c is 1, it will always stay 1.
But currently we don't notice that.

Note this code is reduced from PR 108352 testcase after a phiopt improvement
that provided the above form and ran into a testcase failure because of that.

[Bug tree-optimization/109957] Missing loop PHI optimization

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109957

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-05-25
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think we have some code doing final value replacement of bit ops now, I
suppose that could be enhanced to cover this.  Yes, we handle &= ...
but not |= ...

[Bug tree-optimization/109957] Missing loop PHI optimization

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109957

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Andrew Pinski <pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:64d90d06d2db43538c8a45adbb3d74842f7868ae

commit r14-1597-g64d90d06d2db43538c8a45adbb3d74842f7868ae
Author: Andrew Pinski <apinski@marvell.com>
Date:   Wed May 24 07:08:45 2023 +0000

    Add match patterns for `a ? onezero : onezero` where one of the two
operands are constant

    This adds a match pattern that are for boolean values
    that optimizes `a ? onezero : 0` to `a & onezero` and
    `a ? 1 : onezero` to `a | onezero`.

    This was reported a few times and I thought I would finally
    add the match pattern for this.

    This hits a few times in GCC itself too.

    Notes on the testcases:
    * phi-opt-2.c: This now is optimized to `a & b` in phiopt rather than
ifcombine
    * phi-opt-25b.c: The test part that was failing was parity which now gets
`x & y` treatment.
    * ssa-thread-21.c: there is no longer a threading opportunity, so need to
disable phiopt.
      Note PR 109957 is filed for the now missing optimization in that testcase
too.

    gcc/ChangeLog:

            PR tree-optimization/89263
            PR tree-optimization/99069
            PR tree-optimization/20083
            PR tree-optimization/94898
            * match.pd: Add patterns to optimize `a ? onezero : onezero` with
            one of the operands are constant.

    gcc/testsuite/ChangeLog:

            * gcc.dg/tree-ssa/phi-opt-2.c: Adjust the testcase.
            * gcc.dg/tree-ssa/phi-opt-25b.c: Adjust the testcase.
            * gcc.dg/tree-ssa/ssa-thread-21.c: Disable phiopt.
            * gcc.dg/tree-ssa/phi-opt-27.c: New test.
            * gcc.dg/tree-ssa/phi-opt-28.c: New test.
            * gcc.dg/tree-ssa/phi-opt-29.c: New test.
            * gcc.dg/tree-ssa/phi-opt-30.c: New test.
            * gcc.dg/tree-ssa/phi-opt-31.c: New test.
            * gcc.dg/tree-ssa/phi-opt-32.c: New test.