[Bug tree-optimization/95926] New: Failure to optimize xor pattern when

[Bug tree-optimization/95926] New: Failure to optimize xor pattern when

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95926

            Bug ID: 95926
           Summary: Failure to optimize xor pattern when
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool f(bool a, bool b)
{
    bool tmp = a & b;
    return (tmp ^ b) | (tmp ^ a);
}

This code does not optimize as well as this code :

bool f(bool a, bool b)
{
    return ((a & b) ^ b) | ((a & b) ^ a);
}

which optimizes to `return a ^ b;`. This seems rather odd considering that they
are equivalent and the first example is just a simplification of the second
(i.e. made more readable).

[Bug tree-optimization/95926] Failure to optimize xor pattern when using temporary variable

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95926

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
It is different to gcc because in the first case, tmp is used twice, while in
the second case, each a&b is only used once, and gcc only transforms (a&b)^b to
b&~a if this is the only use of a&b. Yes, this heuristic often backfires, but
as long as we consider &~ as 2 operations, not restricting the transformation
could generate worse code in some cases.

[Bug tree-optimization/95926] Failure to optimize xor pattern when using temporary variable

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95926

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-06-29
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note :s tries to overcome this for "single" result operations like a ^ b
but single_use () checks make that stronger (and fail).

[Bug tree-optimization/95926] Failure to optimize xor pattern when using temporary variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95926

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2020-06-29 00:00:00         |2021-8-14
           Severity|normal                      |enhancement