On 27 September 2016 at 07:33, G 3 <programmingk...@gmail.com> wrote: > > On Sep 27, 2016, at 7:43 AM, Peter Maydell wrote: >> In particular, for fmadds, it is important that there >> is no intermediate rounding done between the multiply >> and the addition. This means that you need to effectively >> do the multiply and the addition at a higher precision than >> the input arguments, so simple multiplication and addition >> of floats will give you wrong answers. > > > It sounds like I should change my argument types to double.
That will fix only a very tiny part of the problem. You will also need to get right floating point exception flags, handling of subnormal numbers, and various other implementation specifics of IEEE. > I still want to try implementing this function. I'm thinking > rewriting the helper_fmadd() function in target-ppc/fpu_helper.c. > Does that sound correct? That's the place that would need to be reimplemented. I recommend doing very thorough testing by feeding the instruction a lot of randomly selected input values and comparing the output value and the output floating point status/exception flags against the current implementation and/or real hardware. I really think you'll find that this is just impossible to implement correctly with the host compiler's floating point C expressions, though. thanks -- PMM
