Re: Re: musl libc for PPC64

[Rich Felker <dalias@libc.org>]

On Mon, Feb 08, 2016 at 03:30:51PM -0500, David Edelsohn wrote:
> On Mon, Feb 8, 2016 at 3:18 PM, Rich Felker <dalias@libc.org> wrote:
> > On Mon, Feb 08, 2016 at 12:48:48PM -0500, David Edelsohn wrote:
> >> On Mon, Feb 8, 2016 at 11:51 AM, Rich Felker <dalias@libc.org> wrote:
> >> > On Mon, Feb 08, 2016 at 05:17:58PM +0100, Szabolcs Nagy wrote:
> >> >> * David Edelsohn <dje.gcc@gmail.com> [2016-02-08 09:43:08 -0500]:
> >> >> > What work is necessary to enable basic musl libc support for PPC64
> >> >> > Linux Little Endian?
> >> >>
> >> >> once the abi is clear (is long double ieee128?) the arch specific
> >> >> parts of musl need to be ported for ARCH=powerpc64.
> >> >
> >> > IIRC one of the powerpc64 ABIs uses function descriptors rather than
> >> > normal function pointers. If so that may affect a few details in the
> >> > dynamic linker and may require some changes to non-arch-specific code,
> >> > but since we have SH/FDPIC all the basic framework for function
> >> > descriptors should be there already.
> >>
> >> PPC64LE Linux is little endian and does not use function descriptors.
> >
> > OK. I've been looking more and this is what's called the "ELFv2 ABI"
> > for ppc64, I believe. Is it possible to use ELFv2 ABI with big endian
> > targets? It looks like support is there on the toolchain side but I'm
> > not sure if we would run into problems. The reason I'm asking is that
> > there's probably also interest in older BE ppc64 systems and I think
> > it makes more sense to use the same musl port/ABI for both
> > endiannesses if at all possible rather than doing two separate ones.
> 
> The future direction for Linux on PPC64 is little endian.  I was
> trying to simplify things because of your comments about multiple
> ABIs.
> 
> The original PPC64 ABI could (and has) operated in little endian mode,
> and the new PPC64 ELFv2 ABI could operate in big endian mode.  But the
> entire PowerPC software ecosystem and all Linux distros are
> distributed with PPC64 big endian ABI using function descriptors and
> little endian ABI using ELFv2.

Yes. For glibc and other systems with existing ecosystems, the old
ecosystem was based on the v1 ABI and was BE, and the new ecosystem is
based on the v2 ABI and is LE. But since musl doesn't have an existing
ppc64 ecosystem (and it's not ABI-compatible with glibc on ppc anyway
because of double-double and other issues) I think it makes sense to
use the v2 ABI for both LE and BE (if anyone even cares to do BE at
all). Does that make sense to you?

Of course either way, to begin with we can just start with the modern
LE ABI.

BTW one thing I'm not clear on -- is there separate LE/BE hardware, or
does the same hardware just have a switchable mode? In the latter case
BE seems rather uninteresting to support at all, but maybe people want
to use it for routers or something?

> >> > Also to clarify what you asked about long double ABI, if ieee128
> >> > (quad) is not used, the compiler needs to be built to use plain double
> >> > (ieee64 double) for long double instead of using ibm double-double.
> >>
> >> GCC can use IEEE64 or IEEE128 for long double.
> >
> > OK. Then the choice of which to use depends mainly on whether there's
> > current or future hardware that would do IEEE128 natively. If so we
> > should probably choose an ABI that supports it even if in the
> > short-term (or for the baseline ISA) it requires soft-float code to be
> > linked; that's how AArch64 is doing it. But if native IEEE128 support
> > is not available and not planned for future hardware, doing it as
> > soft-float "just because" is probably not as good idea.
> 
> IBM has announced that the next generation of the processor will
> support native IEEE128 floating point in hardware.  There will be
> software emulation for the current processors.  Support is included in
> the forthcoming GCC 6.

OK. Does the existing software emulation properly update the hardware
floating point status (exception flags) and use the rounding mode?

Rich