Re: s390 port

["Ulrich Weigand" <Ulrich.Weigand@de.ibm.com>]

"Paul Edwards" <mutazilah@gmail.com> wrote on 02.09.2021 22:05:39:
> > Is this about supporting a 4GB address space instead
> > of a 2GB space?
>
> Yes, correct.

OK, that makes things clearer.  This implies in particular:

- 4GB address space means you need to run in AMODE64

- AMODE64 means the native address size is 64 bits.  This
  implies that Pmode has to be DImode, since Pmode tells
  the compiler what the native address size is.

  Specifically, if you try to run AMODE64 with Pmode equals
  SImode, the compiler will not be aware that the hardware
  uses the high 32 bits of base and index registers, and
  will not necessarily keep them zero.  Also, the compiler
  will assume the base + index (+ displacement) arithmetic
  will operate in 32 bits -- I'm pretty sure this is
  actually the root cause of your "negative index" problem.

> > Is it about supporting a 32-bit pointer type in an
> > otherwise AM64 environment?  (This is already used
> > by the TPF target, but the 32-bit pointer will still
> > refer to a 2GB address space.)
> Yes, all pointers will be 32-bit – a normal 32-bit system.

Note that even if Pmode == DImode, you can still use 32-bit
*pointer* sizes.  This is exactly what e.g. the Intel x32
mode does (as was mentioned by Andreas).

> I’d like to approach the problem from the other
> direction – what modifications are required to
> be made to “-m31” so that it does “-m32” instead?
> I’m happy to simply retire “-m31”, but I don’t care
> if both exist.

If you want to go for an "x32" like mode, I think this
is wrong approach.  The right approach would be to
start from "-m64", and simply modify the pointer size
to be 32 bits.

This would work by setting POINTER_SIZE to 32, while
leaving everything else like for -m64.  I'm sure there
will be a few other places that need adaptation, but
it should be pretty straightforward.  You can also
check the Intel back-end where they're using the
TARGET_X32 macro.


We've thought about implementing this mode for Linux,
but decided not to do it, since it would only provide
marginal performance improvements, and has the drawback
of being another new ABI that would be incompatible to
the whole existing software ecosystem.

(The latter point may not be an issue for you if you're
looking into a completely new OS anyway.)

Bye,
Ulrich