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

[Rich Felker <dalias@libc.org>]

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. 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?

Generally we don't have any significant asm implementations that
depend on non-baseline extensions to the ISA. The same is true for x86
where no sse/avx is used.

The asm files we have for arm are already way too large and complex,
with all the high level flow gratuitously written in asm. Ideally at
some point we will refactor that to have all the high level logic in C
and just core block copy primitives provided by the archs. Whether
this would cleanly admit using methods only known to be available at
runtime I'm not sure.

Do you know what performance difference you're missing out on by not
having neon? At what block sizes does it matter?

Rich