Re: Does ARMEL toolchain include NEON support?
On Wed, 2019-02-27 at 23:45 +0000, Steve McIntyre wrote:
> On Wed, Feb 27, 2019 at 06:30:36PM -0500, Jeffrey Walton wrote:
> > On Wed, Feb 27, 2019 at 5:46 PM Steve McIntyre <steve@einval.com> wrote:
> > > So, I've got to ask - what hardware are you likely targeting here
> > > where it matters to build stuff for armel yet also use NEON if it's
> > > available? Most people with hardware that *can* do NEON should be
> > > using armhf, surely?
> >
> > Yeah, I know what you are saying.
> >
> > The problem in practice with mainstream compilers is (1) ARM and the
> > ACLE defines are a mess, (2) -march=native does not work like on i686
> > or x86_64, and (3) RTFM does not work.
> >
> > For a regular user who wants to use Debian on ARM we need to figure
> > out how to build to the least capable machine (like ARMv5 or ARMv6)
> > while making more capable features (like NEON) available.
>
> So this is a place where the world is just *different* compared to x86
> - the different versions of the ARM architectures have signficantly
> different capabilities. If you're looking to build something that
> performs well on a modern v7 CPU, compilling for v5 is a
> mistake. You'll be using the wrong locking primitives, barriers,
> etc. Equally, the features you're going to be looking for (like SMP,
> NEON) just don't make sense / don't exist on v5 CPUs.
To spell it out: the gist of this is that it isn't possible to provide
a single arm binary which works well for both armel and armhf (which I
think is what Jeff is trying/wants to do?).
The advice here is to instead ship[0] two binaries -- one targetting v5
(no neon etc, aka armel in Debian) and another targetting v7 (w/
possible(? I forget what is optional) neon and other stuff aka armhf in
Debian and other distros).
Ian.
[0] and/or have the build system detect between.
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