Re: s390 port

[Paul Edwards <mutazilah@gmail.com>]

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On Fri, 3 Sept 2021 at 20:12, Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
wrote:

> "Paul Edwards" <mutazilah@gmail.com> wrote on 03.09.2021 13:35:10:
> > >  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.
> > The compiler naturally keeps them zero. The
> > instructions that are used to load registers
> > do not pollute the high-order 32 bits.
>
> While this is true for most instructions, the compiler will not
> restrict itself to using only those.  (As just one obvious
> example, the compiler may use "lay" with a negative displacement,
> which will set the high bits of a GPR in AMODE64.)
>
> (And, b.t.w. not the -m31 DImode, which is a pair of 32-bit
> GPRs, but rather the -m64 DImode, which is a single 64-bit GPR.)
>

Hi all.

Turns out I have been asking the wrong question for several years.

I was going to generate a peephole (an idea from the author of
UDOS, now KinnowOS) to detect when a negative index was
being used, and force an addition instead of an index, when I
realized that it wasn't just literals that could use a negative
value.

That is when I realized that negative numbers were perfectly
valid/normal for indexing, and that it is the OS/hardware that
needs to adapt to this reality when transitioning from 32-bit
hardware to 64-bit hardware.

As such, I have updated z/PDOS-32 to use DAT to map the
4 GiB to 8 GiB region to 0 to 4 GiB, so that negative indexing
works fine.

You can download this from http://pdos.org (down the bottom).

So would it be possible now to update gcc to make -m32 and
-m31 and -m24 all work, as they all generate the exact same
code, regrardless of whether you are running as AM24 on
S/370, AM31 on S/390 or AM32 on Hercules/380 or AM64
with DAT set appropriately on z/Arch.

Thanks. Paul.