PPC64 libmvec sincos/sincosf ABI

[Wilco Dijkstra]

Hi,

> 1. What is the best vector ABI (best performance) for sincos on PPC64?  
> That may be a function of the particular vector instructions available on 
> PPC64; the best choice of ABI on PPC64 need not correspond to the best 
> choice on x86_64.

I don't think it is related to the target - the fastest ABI is one that avoids
unnecessary work. For example scalar sincos is slow due to the inefficient
ABI which forces the results through memory (fixing that gives a 50% speedup). 

Similarly for the vector ABI I think returning 2 vectors in registers will be the
fastest option in all cases. The actual vector instructions shouldn't affect the
ABI beyond the vector widths that can be supported.

Wilco