s390: SImode pointers vs LR

[Jeff Law]

On 05/29/2015 06:57 PM, DJ Delorie wrote:
> In config/s390/s390.c we accept addresses that are SImode:
>
>        if (!REG_P (base)
> 	  || (GET_MODE (base) != SImode
> 	      && GET_MODE (base) != Pmode))
> 	return false;
>
> However, there doesn't seem to be anything in the s390's opcodes that
> masks the top half of address registers in 64-bit mode, the SImode
> convention seems to just be a convention for addresses in the first
> 4Gb.
>
> So... what happens if gcc uses a subreg to load the lower half of a
> register (via LR), leaving the upper half with random bits in it, then
> uses that register as an address?  I could see no code that checked
> for this, and I have a fairly large and ungainly test case that says
> it breaks :-(
>
> My local solution was to just disallow "SImode" as an address in
> s390_decompose_address, which forces gcc to do an explicit SI->DI
> conversion to clear the upper bits, and it seems to work, but I wonder
> if it's the ideal solution...
Looks wrong to me -- but the s390 maintainers ought to chime in.

jeff