[Bug rtl-optimization/14851] [4.0 Regression] suboptimal fp division with -ffast-math

[uros at gcc dot gnu dot org]

------- Additional Comments From uros at gcc dot gnu dot org  2004-09-17 06:08 -------
This is caused by following code in expr.c, around line 7676:

--cut here--
    case RDIV_EXPR:
      /* Emit a/b as a*(1/b).  Later we may manage CSE the reciprocal saving
         expensive divide.  If not, combine will rebuild the original
         computation.  */
      if (flag_unsafe_math_optimizations && optimize && !optimize_size
	  && TREE_CODE (type) == REAL_TYPE
	  && !real_onep (TREE_OPERAND (exp, 0)))
        return expand_expr (build2 (MULT_EXPR, type, TREE_OPERAND (exp, 0),
				    build2 (RDIV_EXPR, type,
					    build_real (type, dconst1),
					    TREE_OPERAND (exp, 1))),
			    target, tmode, modifier);
--cut here--

It is obvious, that in current 4.0, combine is ignoring this construct.

BTW: gcc-3.3 has quite weird behaviour with testcases like the one above, when
numerator is constant. When dividing integer-like numerators (for example
123532.0 / x), combine converts expression back to original one, and for real
numerators (4241.432 / x), no conversion occurs.

I will try to fix mainline.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
      Known to fail|4.0                         |4.0.0
   Last reconfirmed|2004-07-09 03:49:33         |2004-09-17 06:08:21
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14851