Re: [2/2 ABI and toolchain part] Support Raspberry Pi [0-3] in big-endian mode

[Rin Okuyama <rokuyama.rk%gmail.com@localhost>]

Hi,

Let me explain again on what needs for earmv6{,hf}eb support for GCC.

ARMv6 supports both BE32 (deprecated) and BE8 (new) formats for
big-endian mode. While other OS's use BE8 on ARMv6, our code is
intended to use BE32 instead.

Therefore, we need to

(1) Do not emit --be8 linker flag for ARMv6 by default.

(2) Disable unaligned access for big-endian mode on ARMv6 by default.
    Even though ARMv6 supports unaligned access, it cannot be used with
    BE32, cf.,

    https://developer.arm.com/documentation/ddi0301/h/unaligned-and-mixed-endian-data-access-support/mixed-endian-access-support/armv6-support-for-mixed-endian-data

In the previous version, changes are made unconditionally. But, these
should be restricted to target NetBSD.

I updated the patch:
http://www.netbsd.org/~rin/rpi_eb_gcc_20201123.patch

For (1), our code uses its own BE8_LINK_SPEC. But this is incomplete;
new CPUs are not supported. Therefore, I define ARM_BE32_IF_POSSIBLE
macro in netbsd-eabi.h and stop emitting --be8 flag for ARMv6 if this
macro is defined.

For (2), disable -munaligned-access for big-endian mode on ARMv6, if
ARM_BE32_IF_POSSIBLE macro is defined.

I also note that we don't need worry about ABI changes. We don't have
working earmv6{,hf}eb userland yet. At the moment, our kernel does not
support earmv6{,hf}eb, and their userland does not work on other ARM
subarchs; due to lack of ARMv6 instructions on ARMv5 and prior, and
due to lack of BE32 support on ARMv7 and later.

Thoughts?

Thanks,
rin

On 2020/11/16 7:53, Rin Okuyama wrote:
> On 2020/11/16 6:28, matthew green wrote:
> > nice work.  i agree with your proposed changes in general.
>
> Thank you!
>
> > > Therefore,
> > > (1) configure GCC not to emit unaligned access for earmv6{,hf}eb.
> > >
> > > http://www.netbsd.org/~rin/rpi_eb_gcc_20201115.patch
> >
> > this part doesn't seem right:
> >
> > -                         && arm_arch6 && (arm_arch_notm || arm_arch7));
> > +          && arm_arch6 && ((arm_arch_notm && !BYTES_BIG_ENDIAN) || arm_arch7));
> >
> > i would have expected "&& !BYTES_BIG_ENDIAN" to have been
> > added to the "arm_arch6", not the "arm_arch_notm" section.
>
> This is not wrong;
>
>      arm_arch6 && ((arm_arch_notm && !BYTES_BIG_ENDIAN) || arm_arch7)
>
> is equivalent to
>
>      (arm_arch6 && arm_arch_notm && !BYTE_BIG_ENDIAN) || arm_arch7
>
> This is confusing, but I prioritized the original form.
>
> > since this is really a hardware issue, and this is a bug
> > all GCC arm, not netbsd target, it would be nice if you
> > attempted merging this upstream as well :)
>
> Unaligned access is possible for ARMv6 in BE8 big-endian mode, which
> seems to be used for OSes other than NetBSD. While it is not possible
> in BE32 big-endian mode for us.
>
> Hmm, I started thinking that the fix should be restricted to NetBSD
> target. But I don't know how to deal with this...
>
> FreeBSD uses SUBTARGET_OVERRIDE_INTERNAL_OPTIONS. But, at the moment
> when SUBTARGET_OVERRIDE_INTERNAL_OPTIONS is called,
> opts_set->x_unaligned_access is already modified by
> arm_option_override_internal().
>
> Any ideas? Define new macro, e.g.,
> SUBTARGET_OVERRIDE_UNALIGNED_ACCESS_OPTIONS in netbsd-eabi.h?
>
> Thanks,
> rin