[Bug target/116815] Make better use of overflow flags in codegen of min/max(a, add/sub(a, b))

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D116815

--- Comment #17 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Kyrylo Tkachov <[email protected]>:

https://gcc.gnu.org/g:c62176c0ea0d8139161ebb82fabfa0a02e493e2e

commit r17-2923-gc62176c0ea0d8139161ebb82fabfa0a02e493e2e
Author: Kyrylo Tkachov <[email protected]>
Date:   Thu Jul 30 10:36:40 2026 -0700

    aarch64: Add missing CC clobber to max/min-of-add/sub patterns [PR11681=
5]

    *aarch64_plus_within_<optab><mode>3_<ovf_commutate> and
    *aarch64_minus_within_<optab><mode>3 split into a flag-setting ADDS or =
SUBS
    followed by a CSEL, but their insn patterns do not say that they write =
the
    condition codes.

    For

      unsigned f (unsigned a, unsigned b, unsigned c, unsigned d)
      {
        unsigned s =3D a + b;
        unsigned m =3D s > a ? s : a;
        return (c < d && a < b) ? m : d;
      }

    combine produces

      (parallel [(set (reg:SI 0 x0)
                      (umax:SI (plus:SI (reg:SI 107) (reg:SI 108))
                               (reg:SI 107)))
                 (clobber (scratch:SI))])

    which claims to leave the flags alone.  The compare feeding the enclosi=
ng
    CCMP chain is therefore treated as still live and is removed, and split1
    then emits an ADDS that overwrites the flags the outer CSEL reads:

      adds  w1, w0, w1
      csel  w1, w1, w0, cc
      csel  w0, w1, w3, cc

    so f (5, 7, 9, 2) returns 12 rather than 2.

    Add the (clobber (reg:CC CC_REGNUM)) that the neighbouring
    *aarch64_minmax_plus pattern already carries.  The comparison is then k=
ept:

      cmp   w2, w3
      ccmp  w0, w1, 2, cc
      bcs   .L2
      adds  w1, w0, w1
      csel  w3, w1, w0, cc

    Bootstrapped and tested on aarch64-none-linux-gnu.

    gcc/ChangeLog:

            PR middle-end/116815
            * config/aarch64/aarch64.md
            (*aarch64_plus_within_<optab><mode>3_<ovf_commutate>): Add a
            clobber of CC_REGNUM.
            (*aarch64_minus_within_<optab><mode>3): Likewise.

    gcc/testsuite/ChangeLog:

            PR middle-end/116815
            * gcc.target/aarch64/pr116815-4.c: New test.

    Signed-off-by: Kyrylo Tkachov <[email protected]>=