FCP-100: armv7 plan

[Ian Lepore]

On Sat, 2017-09-09 at 21:25 +0200, Jan Beich wrote:
> Warner Losh <imp at bsdimp.com> writes:
> 
> > 
> > On Sat, Sep 9, 2017 at 1:05 PM, Jan Beich <jbeich at vfemail.net>
> > wrote:
> > 
> > > 
> > > Warner Losh <imp at bsdimp.com> writes:
> > > 
> > > > 
> > > > Greetings,
> > > > 
> > > > This will serve as 'Last Call' for any objections to the plan
> > > > to create
> > > an
> > > > 
> > > > armv7 MACHINE_ARCH in FreeBSD, as documented in FCP-0100.
> > > [...]
> > > 
> > > Some ports want NEON support but FCP-0100 is vague about FreeBSD-
> > > specific
> > > differences between armv6 and armv7. Clang appears to enable NEON
> > > for all
> > > *-gnueabi* targets but I have no clue about GCC. Apparently,
> > > Android and
> > > Debian don't assume NEON on armv7.
> > > 
> > > related: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221898
> > > 
> > Yes. We are vague about it on purpose. Just like we're vague about
> > MMX,
> > MMX2, etc on x86 because different processors can/want use
> > different
> > things.
> Do you mean similar to how FreeBSD i386 is vague about not supporting
> real i386, only i486 or later?
> 
> > 
> > The goal, if it doesn't work already, is for NEON to work if used,
> > but not be required, just like all the other optional features of a
> > CPU.
> FreeBSD doesn't support detecting NEON at runtime unlike Linux.

We should fix that.  In fact, we should probably fix it in exactly the
way linux does it (does it today, I think it's their 3rd scheme so
far), using getauxval() and elf AT_HWCAP.

Adding the AT_HWCAP stuff would be relatively easy.  I'm not as sure
what to do about getauxval().  To be maximally linux-compatible (which
would be good for ports) we'd put getauxval() in libc and make it work
just like the linux one.  That's a bit at odds with the support we have
now, which is procstat_getauxv() in libprocstat.  It's not very
compatible with how linux getauxval() works, so using just that might
lead to a lot of patches in ports.

-- Ian

>  Do you
> mean at compile time? If so then the following probably needs to
> change
> 
> $ cc -target armv7-unknown-freebsd12.0-gnueabihf -dM -E -</dev/null
> |& fgrep -i neon
> #define __ARM_NEON 1
> #define __ARM_NEON_FP 0x4
> #define __ARM_NEON__ 1
> _______________________________________________