Easiest way to use FMA instruction

[Ben Jones]

On Thursday, 9 January 2020 at 20:57:10 UTC, Ben Jones wrote:
> What's the easiest way to use the FMA instruction (fused 
> multiply add that has nice rounding properties)?  The FMA 
> function in Phobos just does a*b +c  which will round twice.
>
> Do any of the intrinsics libraries include this?  Should I 
> write my own inline ASM?

This seems to work with DMD, but seems fragile:

`
///returns round(a*b + c)  -- computed as if in infinite 
precision, rounded at the end
double fma(double a, double b, double c) @safe pure @nogc nothrow{
     asm @safe pure @nogc nothrow {
         naked;
         vfmadd231sd XMM0, XMM1, XMM2;
         ret;
     }
}
`