Re: [PATCH] powerpc64le: Add single instruction math functions

[Rich Felker <dalias@libc.org>]

On Thu, Jun 29, 2017 at 09:49:34AM -0400, David Edelsohn wrote:
> On Sat, Jun 24, 2017 at 8:10 PM, Rich Felker <dalias@libc.org> wrote:
> > On Sat, Jun 24, 2017 at 06:57:23PM -0500, A. Wilcox wrote:
> >> Except Adélie, Sabotage, and anyone who is creating their own
> >> environment without using a distribution.  Or are you saying that GCC
> >> assumes LE with ELFv2?
> >>
> >> That is the primary reason I haven't shipped any PPC64 image yet.  In
> >> addition to the usual badness of porting an entire distro worth of
> >> packages to a platform nobody has really used yet (had a similar time
> >> with musl on MIPS64 and 32-bit PowerPC), I'm a bit uneasy on the
> >> toolchain itself being able to understand what Rich has said.  Since
> >> ELFv2 says that Power8 is the minimum ISA, gcc can do whatever it
> >> wants, and I'm not sure if -mcpu=power6 (specific lower ISA) or
> >> - -mcpu=powerpc64 (generic) will affect its code output when it sees
> >> - -mabi=elfv2.  So I'm going to need to put any PPC64 image through a
> >> much more rigorous test than I did any other platform.
> >
> > I don't see any reason GCC would introduce a problem here. It should
> > always honor -march, and the default -march for the
> > powerpc64-linux-musl (elfv2 of course) toolchain I just built seems to
> > be POWER4 according to the predefined macros.
> >
> >> > I added the macro tests for portability and completeness.
> >> >
> >> > The only ports of Musl that will function on existing, supported,
> >> > big-endian PowerPC systems are the 32 bit "powerpc" port and an
> >> > unimplemented PPC64 BE ELFv1 port.
> >>
> >>
> >> Except Rich specifically said that he did not want an ELFv1 port for
> >> 64-bit PowerPC when I asked him, so I don't think that's going to happen
> >
> > To clarify, my view is that it does not make sense to add a new port
> > that differs only in ABI, unless it's an ABI variant that's actually
> > necessary for reasonable support of some actual hardware (like
> > softfloat, fdpic for nommu, etc.). That is not the case here.
> 
> A colleague of mine reminded me that ELFv2 ABI specifies POWER8 as the
> minimum hardware (not little-endian).

This is a gratuitous requirement and has nothing to do with the
meaning of ELFv2 we're using (and likewise not with the gcc
--with-abi=elfv2).

> The implementation of ELFv2 can
> operate on earlier hardware, but binaries may not be forward
> compatible because of VSX.  Because of the calling convention of VSX
> registers in ELFv2, the stack may be corrupted if an application built
> without VSX support is linked with a library that does support VSX.
> One cannot mix and match musl libc built for POWER4 or PPC970 and musl
> libc built for POWER7.

I don't think this is accurate. If it is then it's a serious bug we
need to fix, and it should have been discussed at the time the port
was added...

Can you provide a citation for the usage of VSX registers in the
calling convention, and how you think that affects the stack?

Rich