[PATCH, RFC] fma vs gcc 4.9

[Richard Henderson]

Ping?

On 07/03/2014 11:11 AM, Richard Henderson wrote:
> GCC 4.9 optimizes
> 
>   double a1 = z + m1;
>   double t1 = a1 - z;
>   double t2 = a1 - t1;
>   t1 = m1 - t1;
>   t2 = z - t2;
>   double a2 = t1 + t2;
>   feclearexcept (FE_INEXACT);
> 
>   if (a1 == 0 && m2 == 0)
>     ... return ...
> 
> into
> 
>   double a1 = z + m1;
>   feclearexcept (FE_INEXACT);
> 
>   if (a1 == 0 && m2 == 0)
>     ... return ...
> 
>   double t1 = a1 - z;
>   double t2 = a1 - t1;
>   t1 = m1 - t1;
>   t2 = z - t2;
>   double a2 = t1 + t2;
> 
> because the later computation is partially redundant for the path leading to
> the return.  I noticed this because, of course, the moved computation raises
> INEXACT, leading to incorrect results.
> 
> At first I was simply going to add a math_force_eval to make sure everything is
> complete before the feclearexcept, but on further reflection it seemed odd that
> most of the computation is in fact redundant.
> 
> It seems to me that there's a typo on that exact zero test: a2 should be used,
> not m2.  Correct, or have I mis-read the code?
> 
> 
> r~
>