What is acceptable for -ffast-math? (Was: associative law incombine)

[Stephen L Moshier]

> > Floating point arithmetic, IEEE or otherwise, does not obey the
> > associative or distributive laws.
> 
> This is YOUR opinion.

That is not just an opinion, it is simple arithmetic.

Further, GCC is supposed to be standards compliant by default.  You
seem to be advocating that GCC should, without warning, generate
programs that violate the standard and are mathematically wrong.
The C99 standard says the following, among other things, about floating
point transformations:


       [#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;