Re: Optimization Question

[Stephen L Moshier <moshier at mediaone dot net>]

> language lawyer

Here is a quote from C9X.
GCC already conforms to this.  No change is required.

       [#14] EXAMPLE 5 Rearrangement for floating-point expressions
       is often restricted because of limitations in  precision  as
       well  as  range.   The implementation cannot generally apply
       the  mathematical  associative   rules   for   addition   or
       multiplication,   nor  the  distributive  rule,  because  of
       roundoff  error,  even  in  the  absence  of  overflow   and
       underflow.    Likewise,   implementations  cannot  generally
       replace decimal constants in order to rearrange expressions.
       In  the  following  fragment,  rearrangements  suggested  by
       mathematical rules for real numbers are often not valid (see
       F.8).

               double x, y, z;
               /* ... */
               x = (x * y) * z;  // not equivalent to x *= y * z;
               z = (x - y) + y ; // not equivalent to z = x;
               z = x + x * y;    // not equivalent to z = x * (1.0 + y);
               y = x / 5.0;      // not equivalent to y = x * 0.2;