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From: "guojiufu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108757] We do not simplify (a - (N*M)) / N +
 M -> a / N
Date: Fri, 12 May 2023 06:25:46 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108757

--- Comment #23 from Jiu Fu Guo <guojiufu at gcc dot gnu.org> ---
/* Simplify ((t + -N*M) / N + M) -> t / N: (t + -C) >> N + (C>>N) =3D=3D> t=
 >> N */
(for div (trunc_div exact_div)
 (simplify
  (plus (rshift (plus @0 INTEGER_CST@1) INTEGER_CST@2) INTEGER_CST@3)
  (if (INTEGRAL_TYPE_P (type) && TYPE_UNSIGNED (type) &&
       (wi::to_wide (@3) << wi::to_wide (@2)) =3D=3D -wi::to_wide (@1))
   (if (TYPE_OVERFLOW_UNDEFINED (type))
    (div @0 @2)
#if GIMPLE
    (with
     {
       bool overflowed =3D true;
       value_range vr0;
       if (get_range_query (cfun)->range_of_expr (vr0, @0)
           && !vr0.varying_p () && !vr0.undefined_p ())
         {
           wide_int wmin0 =3D vr0.lower_bound ();
           wide_int wmax0 =3D vr0.upper_bound ();
           wide_int w1 =3D -wi::to_wide (@1);
           wi::overflow_type min_ovf, max_ovf;
           wi::sub (wmin0, w1, TYPE_SIGN (type), &min_ovf);
           wi::sub (wmax0, w1, TYPE_SIGN (type), &max_ovf);
           if (min_ovf =3D=3D wi::OVF_NONE && max_ovf =3D=3D wi::OVF_NONE)
             overflowed =3D false;
         }
     }
    (if (!overflowed)
     (rshift @0 @2)))
#endif
   ))))

Got one match for the case.
Checking if it is safe(condition) or how to support other forms:
signed type, negative N, non-power2 N, negative M ...=