Re: [PATCH v3 5/7] math: Remove powerpc e_hypot

[Wilco Dijkstra via Libc-alpha <libc-alpha@sourceware.org>]

Hi Adhemerval,

> On 10/11/2021 11:34, Wilco Dijkstra wrote:
>> I think the new algorithm will always be slower due to the dependent sqrt and
>> division. So it's hard to improve unless we only use it for special cases (eg. when
>> ax and ay are close). Returning sqrt (fma (ax, ax, ay * ay)) is about twice as fast
>> and gives just over 1 ULP, so we're losing a lot of performance for a small ULP
>> improvement.
>
> My main drive for this change is remove the arch-specific implementation in
> favor of an implementation that might be optimized better by the compiler
> without the need to extra hacks by arch-specific hooks (as I did for power7).

I'm all for having a single optimized generic implementation like we did for other
math functions. In general there is little scope for compiler optimizations due to
conservative FP settings - it is all down to highly optimizing both the algorithm
and implementation.

> Another option is to use the powerpc implementation which favor FP over integer
> as the default one.

That is the fastest implementation. It is less accurate though (~1.04ULP with FMA
and ~1.21ULP without FMA), so I'm not sure that would be acceptable.

I did some quick optimizations on the new algorithm, on Neoverse N1 my fastest
version is less than 10% slower than the powerpc version, and has ~0.94 ULP error.

Cheers,
Wilco