S390 should change the meaning of -m31

[Paul Edwards]

Hi Michael.

I have sort of (I used the i370 target of GCC 3.2.3 instead
of the s390 target) reproduced Jesus's work with z/PDOS
which can be obtained from here:

http://www.pdos.org/zpdos.zip

You can see that with standard Hercules 3.13:

18:02:24 Hercules Version 3.13
18:02:24 (c)Copyright 1999-2015 by Roger Bowler, Jan Jaeger, and others
18:02:24 Built on Sep 28 2017 at 01:35:43
18:02:24 Build information:
18:02:24   Windows (MSVC) build for AMD64

This memcpy:

memcpy((char *)0x7ffffffe, "\x01\x02\x03\x04", 4);

straddles the 2 GiB bar, and the memory is distinct from
low memory:

18:03:05 r 7ffffff0
18:03:05 R:000000007FFFFFF0:K:06=00000000 00000000 00000000 00000102 
................
18:03:05 R:0000000080000000:K:06=03040000 00000000 00000000 00000000 
................
18:03:05 R:0000000080000010:K:06=00000000 00000000 00000000 00000000 
................
18:03:05 R:0000000080000020:K:06=00000000 00000000 00000000 00000000 
................

18:03:13 r 00000000
18:03:13 R:0000000000000000:K:06=000C0000 80002000 00000000 00000000 
................
18:03:13 R:0000000000000010:K:06=0071D4B0 00000000 00000000 00000000 
..M.............
18:03:13 R:0000000000000020:K:06=000C0001 8050327A 00000000 00000000 
.....&.:........
18:03:13 R:0000000000000030:K:06=00000000 00000000 00000000 00000000 
................


BFN. Paul.




-----Original Message----- 
From: Paul Edwards
Sent: Friday, October 1, 2021 8:01 AM
To: gcc@gcc.gnu.org
Cc: matz@suse.de ; jesusantonio30122016@gmail.com
Subject: S390 should change the meaning of -m31

Hi Michael.

Thanks for picking up this issue. I have been working
with Jesus on this.

>> m31 is semantically the same as the m32 option.
>>
>>
>> The m31 option allows for 32 bit addressing and that is confusing since 
>> the m31 option in S390 would mean 2 GiB space addressing

> Indeed that's exactly what it means, and what it's supposed to mean.  On 
> s390, in AMODE(31) the toplevel bit of an (32bit) address is either 
> ignored or an indicator to switch back to 24bit addresses from the s360 
> times.  Either way that leaves 31 bits to generate the virtual address. On 
> s390 you indeed have a 2GB address space, not more.

He is using z/Arch and AM64.

But building with -m31.

>> Code used:
>>
>>     volatile uint64_t *gib_test = (volatile uint64_t *)0x7FFFFFFF;
>>     memset(gib_test, 1, 4096);
>>
>>
>> Hercules dump:
>>
>> r 0x7FFFFFFF-0x800001FF
>> R:000000007FFFFFFF:K:06=01 .

> I'm not sure what you believe to have demonstrated here.  The (virtual or 
> physical) address 0x7FFFFFFF is either (in AMODE(24)) equivalent to 
> 0x00ffffff or to 0xffffffff (in AMODE(31)), either way, the top byte of 
> the addressable range ...

I don't think that's what Hercules does for a real
memory display - it will instead say out of range.
But it doesn't matter, because we're using
64-bit Hercules with 4 GiB of memory and it doesn't
matter what the AMODE happens to be at any moment
in time, what is important is what is in that 4 GiB of
memory.

> The -mXX options are supposed to reflect the address space's size, not the 
> size of the general purpose registers.  An option that reflect AMODE(24) 
> would also be called -m24, despite the registers still being 32bit in 
> size.

The code generated by -m24 would be identical to
code generated by -m31 which would be identical to
code generated by -m32.

Why can't we just have a normal -m32 and accept -m31
and maybe -m24 too, and flag them as "obsolete"?

Thanks. Paul.