Re: [musl] Why the musl libc did not support neon simd acceleartor officially on mem* operations?

[Markus Wichmann <nullplan@gmx.net>]

On Sat, Aug 28, 2021 at 04:01:40PM +0800, tugouxp wrote:
> HI guys:
>   I found that the current implmention of musl arm port memcpy.S and
>   other mem*.S operations did not use arm  neon instructions, this
>   seems differenct with other counterparts like newlibc, glibc and
>   bonic libc, which all impl. the neon version of mem* operations.

I am not an expert on this or indeed anything, but it seems like there
are two plausible explanations for this:

1.) Neon support is not architectural, and not subarchitectural. It
appears Neon comes with a new register set the kernel has to switch out
on context switch, so Neon support is predicated on kernel support. That
makes it a runtime thing. Leaving aside where the kernel declares such
support, having a runtime switch in memcpy() is going to be a slowdown
of some kind for all existing users. I don't know how much of a
slowdown, but a slowdown nonetheless.

My guess is the other libcs ignore the issue of the user compiling a
userspace that won't run on the target platform and allow the user to
shoot themselves in the foot.

Adverting to the competition is a poor choice for this mailing list, by
the way, because part of musl's success comes from looking at them and
saying: No, we are not going to do that.

2.) The code does not exist because it has not been written yet. You are
welcome to submit a patch. I mean, you found the mailing list easily
enough.


> so could you tell me why? is there and concern about on this in musl?
> if i want to imple my self imple. how to do this, is there any matual
> pathches to use?

None that I am aware of, so they have not been submitted to this list
for sure.

Ciao,
Markus