s390: larl for Simode on 64-bit

[Jeff Law]

On 07/08/2015 03:05 PM, DJ Delorie wrote:
> In the TPF case, the software has to explicitly mark such pointers as
> SImode (such things happen only when structures that contain addresses
> can't change size, for backwards compatibility reasons[1]):
>
> int * __attribute__((mode(SImode))) ptr;
>
>    ptr = &some_var;
So in effect, we have two pointer sizes, 64 being the default, but we 
can also get a 32 bit pointer via the syntax above?  Wow, I'm surprised 
that works.

And the only time we'd be able to use larl is a dereference of a pointer 
declared with the syntax above.  Right

OK for the trunk with a simple testcase.  I think you can just scan the 
assembler output for the larl instruction.


>
> so I wouldn't consider this the "default" case for those apps, just
> *a* case that needs to be handled "well enough", and the user is
> already telling the compiler that they assume those addresses are
> 32-bit (that either the whole app, or at least the part with that
> object, will be linked below 4Gb).
>
> The majority of the addresses are handled as 64-bit.
>
>
> [1] /me refrains from commenting on the worth of such practices, just
>      that they exist and need to be (and have been) supported.
Understood, but we also need to make sure that we don't do something 
that breaks things.  Thus I needed to know the tidbit about explicitly 
declaring those pointers as SImode.



jeff