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Message-ID: <YTN7Bmcux8OtRcwi@pirotess>
Date: Sat, 4 Sep 2021 15:56:22 +0200
From: Ismael Luceno <ismael@...ev.co.uk>
To: musl@...ts.openwall.com
Cc: Jeffrey Walton <noloader@...il.com>
Subject: Re: Why the musl libc did not support neon simd acceleartor
officially on mem* operations?
On 28/Aug/2021 16:34, Rich Felker wrote:
> 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@...c.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
It might be interesting to use the dynamic linker to implement this, by
switching the symbols depending on some link-time test, which could be
provided by the library in a special function symbol (is __init too late?).
I'm interested in rewriting these functions...
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