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 04:27:49PM -0400, Jeffrey Walton wrote:
> On Sat, Aug 28, 2021 at 3:53 PM Rich Felker <dalias@libc.org> wrote:
> >
> > 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.
> 
> Out of curiosity, does anyone use Hardware Capabilities
> (https://linux.die.net/man/8/ld-linux) nowadays? Something like a
> /usr/lib/musl, /usr/lib/sse/musl, /usr/lib/avx/musl,
> /usr/lib/neon/musl, etc?
> 
> The benefit to using it is no runtime switching. The switching occurs
> at load time, not runtime.

No, this is a really really bad way to do it. It's not compatible with
static linking and greatly increases startup overhead finding the
right libraries (leading to glibc recently redesigning how they do
it).

> I know of a handful of libraries that could benefit from the speed up
> on a critical path with an arch specific implementation. But I think
> most libraries don't need it.

If you really need this, assuming your distro/package installation
system knows what specific ISA extensions you want it could just
install suitable alt packages, *or* install both and edit the
ld-musl-$(ARCH).path file (or do boot-time symlink or bind mount
stuff, etc.) to make the desired ones available.

Rich