[Bug tree-optimization/107880] New: bool tautology missed optimisation

[Bug tree-optimization/107880] New: bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880

            Bug ID: 107880
           Summary: bool tautology missed optimisation
           Product: gcc
           Version: 13.0
            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: ---

From bug 91882 comment #1 (since the testcase in that was in bug 91882 comment
#0 is now fixed):

bool impl(bool a, bool b)
{
    return (!a || b);
}

// bad optimisation
bool always_true(bool a, bool b)
{
    return (impl(a,b) == impl(b,a)) == (a == b);
           // ( (a -> b) = (b -> a) ) = (a = b) tautology
}

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-11-26

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
We get in optimized:
  _9 = ~a_4(D);
  _10 = b_3(D) | _9;
  _7 = ~b_3(D);
  _8 = a_4(D) | _7;
  _1 = _8 == _10;
  _2 = b_3(D) == a_4(D);
  _6 = _1 == _2;

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=101807

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
One way of fixing this is first transform:
  _9 = ~a_4(D);
  _10 = b_3(D) | _9;
Into:

_10 = a_4(D) <= b_3(D);

And then we have:
  _8 = b_3(D) >= a_4(D);
  _7 = b_3(D) <= a_4(D);
  _1 = _7 == _8;
  _2 = b_3(D) == a_4(D);
  _6 = _1 == _2;

But  _1 is true iff a_4(D) == b_3(D) .
Let me file that second one as we don't optimize it also for int.

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=71762

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Turns out this was there before but was removed (r7-4801-g0eb078fe20d443e2)
because it caused some issues.
See PR 71762 .

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880
Bug 107880 depends on bug 107881, which changed state.

Bug 107881 Summary: (a <= b) == (b >= a) should be optimized to (a == b)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107881

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |DUPLICATE

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880
Bug 107880 depends on bug 107881, which changed state.

Bug 107881 Summary: (a <= b) == (b >= a) should be optimized to (a == b)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107881

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |ASSIGNED
         Resolution|DUPLICATE                   |---

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
With a patch I have for PR 95185

we get:
```
  _1 = b_2(D) == a_3(D);
  _10 = b_2(D) ^ a_3(D);
  _5 = _1 ^ _10;
```

Which is better than before.

One more improvement would be:
```
bool a(bool x, bool y)
{
        bool t = x == y;
        return t ^ x;
}
```

Into:
```
bool a0(bool x, bool y)
{
        bool t = (x ^ y);
        return t ^ x ^1; // ~y
}
```

So the 2 which are needed still:

/* (a == b) ^ a -> b^1 */
(simplify
 (bit_xor:c (eq:c zero_one_valued_p@0 zero_one_valued_p@1) @0)
 (bit_xor @1 { build_one_cst (type); })

/* (a == b) ^ (a^b) -> b^(b^1) or (b^b)^1 or rather 1 */
(simplify
 (bit_xor:c (eq:c zero_one_valued_p@0 zero_one_valued_p@1) (bit_xor:c @0 @1))
 { build_one_cst (type); })

So mine.

[Bug tree-optimization/107880] bool tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107880
Bug 107880 depends on bug 107881, which changed state.

Bug 107881 Summary: (a <= b) == (b >= a) should be optimized to (a == b)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107881

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