i370 - negative indexes

["Paul Edwards" <mutazilah at gmail dot com>]

Hi.

On the i370 port of GCC 3.2.3, I am
getting the following issue.


This code:

C:\scratch\bug>type bug.c
const char *p;

void foo(void)
{
   printf("p-1 is %x\n", p[-1]);
}


generates:

...
L     2,=F'-1'
...
IC    4,0(2,3)

ie it is using a value of x’FFFFFFFF’ in R2 as an index.

This works fine in AM24 and AM31 environments, but
fails for AM64 where an address above 4 GiB is
computed.

Such code is very rare, so I would like to just have
a rule that the index must always be a positive
value, and for negative indexes like the above,
different code is generated to do an actual
subtract instead of trying to do everything via
the index.

Any idea how to achieve that? I can't see
anywhere in i370.md where I can put some
sort of constraint.

Note that I am producing 32-bit modules, but
setting them to AM64 so that they can use
the full 4 GiB on certain environments (like
MVS/380), instead of having a 2 GiB limit.

Thanks. Paul.


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