Re: RFC: Creating a more efficient sincos interface

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Sep 13, 2018 at 6:27 AM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> 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);

Is this an internal interface or public one?

> 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.
>
> LLVM already supports something similar for OSX using a struct of 2 floats.
> Using complex float is better since not all targets may support returning structures in
> floating point registers and GCC generates very inefficient code on targets that do
> (PR86145).
>
> What do people think? Ideally I'd like to support this in a generic way so all targets can
> benefit, but it's also feasible to enable it on a per-target basis. Also since not all libraries
> will support the new interface, there would have to be a flag or configure option to switch
> the new interface off if not supported (maybe automatically based on the math.h header).
>
> Wilco



-- 
H.J.