s390 port

[Paul Edwards]

>> >  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.

>> Where is this logic please? Can I do a #if 0 or similar
>> to disable it?

> This is not in one single place, but spread throughout the
> compiler, both common code and back-end.  I do not think it will
> be possible to get the compiler to generate correct code if
> you do not specify the address size correctly.
1. Is there any way to put a constraint on index
registers, to say that a particular machine can
only index in the range of –512 to +512 or some
other arbitrary set? If so, I can do 0 to 2 GiB.
2. Is there a way of saying a machine doesn’t
support indexing at all?
>> > 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.
>  
>> That will generate 64-bit z/Arch instructions.
>> I wish to generate ESA/390 instructions.

> Why? AMODE64 exists only in z/Arch, so of course there
> will be z/Arch instructions available ...

For the same reason people constructed Babbage’s
invention, I wish to demonstrate the minor changes
that would have been required to the S/360 so that
we would never have arrived at a 31-bit black hole,
and we could have in fact had the perfect 32-bit
machine. Almost identical to the 31-bit machine.
A S/360+, a S/370+ and a S/390+. 

>> > 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.
>> Shouldn’t the end user be able to decide this
>> for themselves?

> It's open source, of course everybode can decide what they
> want to work on themselves.  But we decide what we spend
> our own time on based on we think is useful ...

Sure.

>> No-one at all is interested in 32-bit mainframes?

> Not any more, at least not in Linux.  Linux is pretty much
> 64-bit only at this point.

I think z/OS is pretty much still 31-bit only,
as far as apps are concerned, right? I’d like to
bump that up to 32-bit.

BFN. Paul.