Re: [Qemu-devel] How to add my implementation of the fmadds instruction to QEMU

[G 3 <programmingkidx@gmail.com>]

On Sep 27, 2016, at 7:43 AM, Peter Maydell wrote:

> On 26 September 2016 at 18:05, G 3 <programmingkidx@gmail.com> wrote:
>> I made my own experimental implementation of the fmadds  
>> instruction that I
>> would like to add to QEMU. How would I do this?
>>
>> My implementation would probably look like this:
>>
>> void fmadds(float *frD, float frA, float frC, float frB)
>> {
>>         *frD = frA * frC + frB;
>> }
>
> This isn't portable, because different host CPUs can have
> different implementations of floating point arithmetic
> with subtle differences (notably in corner cases like
> subnormals and also related to the floating point exception
> flags). This is why we use the softfloat library in fpu/,
> which (although slow) is guaranteed to give the right results.
> We did use to have a version of the fpu functions which
> used a "just do a C float or double operation", but we
> removed it many years ago for this reason.
>
> 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.

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?