RFC: Creating a more efficient sincos interface

[Florian Weimer]

On 09/13/2018 03:27 PM, Wilco Dijkstra wrote:
> Hi,
> 
> The existing sincos functions use 2 pointers to return the sine and cosine result. In
> most cases 4 memory accesses are necessary per call. This is inefficient and often
> significantly slower than returning values in registers. I ran a few experiments on the
> new optimized sincosf implementation in GLIBC using the following interface:
> 
> __complex__ float sincosf2 (float);
> 
> This has 50% higher throughput and a 25% reduction in latency on Cortex-A72 for
> random inputs in the range +-PI/4. Larger inputs take longer and thus have lower
> gains, but there is still a 5% gain on the (rarely used) path with full range reduction.
> Given sincos is used in various HPC applications this can give a worthwile speedup.

I think this is totally fine if you call it expif or something like that 
(and put the sine in the imaginary part, of course).

In general, I would object to using complex numbers for arbitrary pairs, 
but this doesn't apply to this case.

Thanks,
Florian