[PATCH, PowerPC] ABI fixes / improvements for powerpc64-linux

[Ulrich Weigand]

Hal Finkel <hfinkel at anl.gov> wrote on 10.07.2014 00:08:57:

> > For example:
> >
> > struct test { long x; } __attribute__((aligned (128)));
> >
> > long callee (long x, struct test y)
> > {
> >   return y.x;
> > }
> >
> > This expects y.x to be in r5 with GCC, and with LLVM with my patch;
> > LLVM without the patch currently expects it in r4.
>
> As I noted in my last e-mail, the current LLVM implementation does
> not look like it skips registers when it adjusts the alignment
> offset. I'm happy to see a compatibility fix for that (although I'm
> unhappy that the current ABI requires it), but that's a separate issue.

Just a quick comment on this point for now: it is not just about
skipping registers; if you want to see the effect on in-memory
parameter, modify the example to something like:

struct pad { long x[11]; };
struct test { long x; } __attribute__((aligned (128)));

long callee (struct pad x, struct test y)
{
  return y.x;
}

In this case, GCC (and clang with patch) will expect y.x at 144(1),
while clang without patch will expect y.x at 136(1); in neither case
do we have 128-byte alignment.

[ As you say, LLVM apparently would handle over-aligned byval parameters,
but clang currently never actually passes any byval align value, so that
may be why that code doesn't trigger here.  ]

At this point, my intent with the clang patch I proposed was simply
to make clang compatible with GCC on the old ABI as-is.


The new ELFv2 ABI is really a separate issue, and I'm planning to post
patches to implement this soon; but as a general statement, an
implementation
of the new ABI is also already in the field, it's been in GCC since last
year
and full Linux distributions have been built with it; so again I don't see
much chance at this point to actually change the ABI, but simply to make
sure it is implemented in LLVM in a compatible way ...


Bye,
Ulrich