[Bug tree-optimization/96779] New: Failure to optimize comparison of negative version of self

[Bug tree-optimization/96779] New: Failure to optimize comparison of negative version of self

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

            Bug ID: 96779
           Summary: Failure to optimize comparison of negative version of
                    self
           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(int a)
{
    return -a == a;
}

This can be optimized to `return !a;`. This transformation is done by LLVM, but
not by GCC.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-08-25
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Keywords|                            |easyhack
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is simple, I will take.
(for cmp (eq ne)
 (simplify
  (cmp:c @0 (negate @0))
   (if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0)))
    (cmp:c @0 { build_zero_cst (TREE_TYPE(@0)); })))))

Should be enough.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> This is simple, I will take.
> (for cmp (eq ne)
>  (simplify
>   (cmp:c @0 (negate @0))
>    (if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0)))
>     (cmp:c @0 { build_zero_cst (TREE_TYPE(@0)); })))))
> 
> Should be enough.

But it is wrong for wrapping types.

For wrapping types:
-a == a -> (a << 1) == a or (a + a) == 0
But this is worse on targets which have a pattern for -a CMP a (hint aarch64).
So for wrapping types keep the -a == a is the best I think.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[navidrahimi at microsoft dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

navidrahimi <navidrahimi at microsoft dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |navidrahimi at microsoft dot com

--- Comment #4 from navidrahimi <navidrahimi at microsoft dot com> ---
Hi Andrew,

I just used your code and added a check to check whether the type is wrapping
type:


(for cmp (eq ne)
 (simplify
  (cmp:c @0 (negate @0))
   (if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0))
        && !TYPE_OVERFLOW_WRAPS (type))
    (cmp:c @0 { build_zero_cst (TREE_TYPE(@0)); })))))

This should work.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[navidrahimi at microsoft dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

--- Comment #5 from navidrahimi <navidrahimi at microsoft dot com> ---
And this is the behavior of different compilers for this optimization:

https://compiler-explorer.com/z/ahdEzxxTv

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jeff Law <law@gcc.gnu.org>:

https://gcc.gnu.org/g:e888bea2384a0d8d29a6545c4f57f41cb49df0a6

commit r12-5458-ge888bea2384a0d8d29a6545c4f57f41cb49df0a6
Author: Navid Rahimi <navidrahimi@microsoft.com>
Date:   Mon Nov 22 19:46:17 2021 -0500

    Re: [PATCH] PR tree-optimization/96779 Adding a missing pattern to match.pd

            PR tree-optimization/96779
    gcc/
            * match.pd (-x == x) -> (x == 0): New optimization.

    gcc/testsuite
            * gcc.dg/tree-ssa/pr96779.c: Testcase for this optimization.
            * gcc.dg/tree-ssa/pr96779-disabled.c: Testcase for this
optimization
            when -fwrapv passed.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
                 CC|                            |law at gcc dot gnu.org
             Status|ASSIGNED                    |RESOLVED

--- Comment #7 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Should be fixed by Navid's patch on the trunk.

[Bug tree-optimization/96779] Failure to optimize comparison of negative version of self

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96779

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0