[4/9] Add a dedicated rtx union member for REGs

[Jeff Law]

On 05/19/2015 01:00 AM, Richard Sandiford wrote:
>> Just to confirm, this doesn't change the size of a REG object, right?
>> If it doesn't change the size, then it's OK.
>
> It doesn't change the size for LP64 hosts.  It makes it 32 bits bigger
> for ILP32 hosts.  See https://gcc.gnu.org/ml/gcc-patches/2015-05/msg01598.html
> for discussion about that.
>
> If we make the opposite call this time and say that it's better to have
> two alternative rtx layouts rather than grow an rtx for ILP32 hosts,
> then there are things we could do.  E.g. I doubt an ILP32 host has
> enough addressable memory to cope with 1<<24 registers (and associated
> instructions), so for ILP32 we could split original_regno into an 8/24
> bitfield, using the :8 for the number of registers and the :24 for the
> original_regno.  original_regno is so rarely used that an extra shift
> or mask shouldn't matter.
>
> Or we could simply leave ILP32 hosts accessing hard_regno_nregs each
> time.  The problem then would be that an incorrectly-cached number
> of registers would only show up on one type of host.  It would also mean
> that REG_NREGS would need to be defined in regs.h for ILP32 at least,
> since hard_regno_nregs isn't visible inside rtl.h.
I can live with increasing on ilp32 hosts with lp64 staying the same -- 
ilp32 hosts are less and less important every day.

jeff