[Bug tree-optimization/108757] We do not simplify (a - (N*M)) / N + M -> a / N

[guojiufu at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108757

Jiu Fu Guo <guojiufu at gcc dot gnu.org> changed:

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

--- Comment #20 from Jiu Fu Guo <guojiufu at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #19)
> Note in the loop case we know it does not wrap because there is a check
> already:
>   <bb 2> [local count: 118111600]:
>   if (rows_8(D) > 3)
>     goto <bb 5>; [89.00%]
>   else
>     goto <bb 4>; [11.00%]
> 
>   <bb 5> [local count: 105119324]:
>   _13 = rows_8(D) + 18446744073709551612;
>   _15 = _13 / 4;
>   doloop.6_5 = _15 + 1;

Checking why below code does not work:
/* Simplify ((t + -N*M) / N + M) -> t / N: (t + -C) >> N + (C>>N) ==> t >> N */
(for div (trunc_div round_div)
 (simplify
  (plus (rshift (plus @0 INTEGER_CST@1) INTEGER_CST@2) INTEGER_CST@3)
  (if (ANY_INTEGRAL_TYPE_P (type) &&
       (wi::to_wide (@3) << wi::to_wide (@2)) == -wi::to_wide (@1))
   (if (TYPE_OVERFLOW_UNDEFINED (type))
    (div @0 @2)
#if GIMPLE
    (with
     {
       bool overflowed = true;
       value_range vr0;
       if (INTEGRAL_TYPE_P (type)
           && get_global_range_query ()->range_of_expr (vr0, @0)
           && !vr0.varying_p () && !vr0.undefined_p ())
         {
           wide_int wmin0 = vr0.lower_bound ();
           wide_int wmax0 = vr0.upper_bound ();
           wide_int w1 = -wi::to_wide (@1);
           wi::overflow_type min_ovf, max_ovf;
           wi::add (wmin0, w1, TYPE_SIGN (type), &min_ovf);
           wi::add (wmax0, w1, TYPE_SIGN (type), &max_ovf);
           if (min_ovf == wi::OVF_NONE && max_ovf == wi::OVF_NONE)
             overflowed = false;
         }
     }
    (if (!overflowed)
     (div @0 @2)))
#endif
   ))))


Interesting thing:
the VR is always VR_VARYING, even for the below simple case:

typedef unsigned long INT;
INT __attribute__ ((noinline)) foo (INT x)
{
  if (x < 4)
    return 0;
  INT a = x + 18446744073709551612ULL;
  INT b = a >> 2;
  return b + 1;
}