[Bug tree-optimization/104992] New: [missed optimization] x / y * y == x not optimized to x % y == 0

[Bug tree-optimization/104992] New: [missed optimization] x / y * y == x not optimized to x % y == 0

[tilkax at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

            Bug ID: 104992
           Summary: [missed optimization] x / y * y == x not optimized to
                    x % y == 0
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tilkax at gmail dot com
  Target Milestone: ---

This is especially helpful for constant y because checking
division-by-a-constant remainders against zero allows further optimization.

Repro case:

unsigned foo(unsigned x, unsigned y)
{
    return x / y * y == x;
}

GCC -O3:
        mov     eax, edi
        xor     edx, edx
        div     esi
        mov     eax, edi
        sub     eax, edx
        cmp     eax, edi
        sete    al
        movzx   eax, al
        ret

Clang -O3:
        mov     eax, edi
        xor     edx, edx
        div     esi
        xor     eax, eax
        test    edx, edx
        sete    al
        ret

[Bug tree-optimization/104992] [missed optimization] x / y * y == x not optimized to x % y == 0

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization

[Bug tree-optimization/104992] [missed optimization] x / y * y == x not optimized to x % y == 0

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-03-21
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

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

[Bug tree-optimization/104992] [missed optimization] x / y * y == x not optimized to x % y == 0

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

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

https://gcc.gnu.org/g:388fbbd895e72669909173c3003ae65c6483a3c2

commit r13-1916-g388fbbd895e72669909173c3003ae65c6483a3c2
Author: Sam Feifer <sfeifer@redhat.com>
Date:   Fri Jul 29 09:44:48 2022 -0400

    match.pd: Add new division pattern [PR104992]

    This patch fixes a missed optimization in match.pd. It takes the pattern,
    x / y * y == x, and optimizes it to x % y == 0. This produces fewer
    instructions. This simplification does not happen for complex types.

    This patch also adds tests for the optimization rule.

    Bootstrapped/regtested on x86_64-pc-linux-gnu.

            PR tree-optimization/104992

    gcc/ChangeLog:

            * match.pd (x / y * y == x): New simplification.

    gcc/testsuite/ChangeLog:

            * g++.dg/pr104992-1.C: New test.
            * gcc.dg/pr104992.c: New test.

[Bug tree-optimization/104992] [missed optimization] x / y * y == x not optimized to x % y == 0

[seurer at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

seurer at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |seurer at gcc dot gnu.org

--- Comment #3 from seurer at gcc dot gnu.org ---
The fix for this causes an issue on power 10.  See PR106516

[Bug tree-optimization/104992] [missed optimization] x / y * y == x not optimized to x % y == 0

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:8e69f2a6e91b7a01619dfd3b0788bfda4ad28941

commit r13-2018-g8e69f2a6e91b7a01619dfd3b0788bfda4ad28941
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Aug 11 10:26:32 2022 +0200

    testsuite: Fix up pr104992* tests on i686-linux [PR104992]

    These 2 tests were FAILing on i686-linux or e.g. with
    --target_board=unix/-m32/-mno-sse on x86_64-linux due to
    -Wpsabi warnings and also because dg-options in the latter
    test has been ignored due to missing space, so even -O2
    wasn't passed at all.

    2022-08-11  Jakub Jelinek  <jakub@redhat.com>

            PR tree-optimization/104992
            * gcc.dg/pr104992.c: Add -Wno-psabi to dg-options.
            * g++.dg/pr104992-1.C: Likewise.  Add space between " and } in
            dg-options.

[Bug tree-optimization/104992] [missed optimization] x / y * y == x not optimized to x % y == 0

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104992

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #5 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Hmm, shouldn't we close this as fixed now?