[PATCH, RFC] fma vs gcc 4.9

[Joseph S. Myers]

On Thu, 3 Jul 2014, Richard Henderson wrote:

> 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?

The existing exact zero test seems correct to me.  The conditions for such 
an exact zero are that the result of the multiplication is exactly 
representable in 53 bits (i.e., m2 == 0, with m1 being the exact result of 
the multiplication), and that the result of the addition (of z to the high 
part of the multipliation result) is an exact zero (i.e. a1 == 0, which 
implies a2 == 0).

-- 
Joseph S. Myers
joseph@codesourcery.com