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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/111780] Missed optimization of '(t*4)/(t*2)
 -> 2'
Date: Thu, 12 Oct 2023 08:34:44 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111780

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-10-12
     Ever confirmed|0                           |1

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

int foo (int a, int b, int c)
{
  return 2*c*(a*b) / (a*b);
}

note when we cannot remove the division like for

 c*a / d*a

we have to watch out for -INT_MIN / -1, but I think the only way
this would not invoke undefined behavior before the transform is
when the factor is equal to -1 but then c*a cannot be -INT_MIN so
it should be safe in general, not only when m and n are constants?

Note we cannot re-associate for the transform.

We only have

/* Simplify (t * 2) / 2) -> t.  */
(for div (trunc_div ceil_div floor_div round_div exact_div)
 (simplify
  (div (mult:c @0 @1) @1)
  (if (ANY_INTEGRAL_TYPE_P (type))
   (if (TYPE_OVERFLOW_UNDEFINED (type))
    @0
#if GIMPLE
    (with {value_range vr0, vr1;}
     (if (INTEGRAL_TYPE_P (type)
          && get_range_query (cfun)->range_of_expr (vr0, @0)
          && get_range_query (cfun)->range_of_expr (vr1, @1)
          && range_op_handler (MULT_EXPR).overflow_free_p (vr0, vr1))
      @0))
#endif
   ))))

but not the case with two multiplies.=